This practice prevents fruits from dropping to the ground before mechanical harvesters can collect them. It should be noted that fruit growers also spray orchards with chemicals to thin excess flowers earlier in the season to prevent over-cropping Byers and Carbaugh, Tomatoes used in the canning industry are bred with the jointless mutation, which results in plants with no pedicel abscission zone Mao et al.
When jointless tomatoes are picked, they leave their calyx and stem behind on the plant. This results in less puncture damage to other tomatoes when they are placed together in a harvesting container Zahara and Scheuerman, The future looks bright for further agricultural improvements resulting from altering abscission.
For instance, many crop plants are overly sensitive to periods of mild drought common in the agricultural setting. It may be possible to increase yield in beans by preventing flowers from abscising in response to mild drought conditions Pandey et al.
Abscission occurs specifically at a specialized region of cells called the abscission zone AZ. AZs are laid down early in development and often have a band-like appearance. The cells in an AZ are smaller than the surrounding cells and have a densely packed cytoplasm.
Once abscission is triggered, AZ cells expand and the middle lamella the pectin layer that glues two cells together is dissolved via hydrolytic enzymes, allowing cell separation. Dehiscence shattering is an abscission-related process that allows plants to spread their mature dry seeds. Shattering occurs once the seed pod and seeds dry out, and can occur explosively in some plant species.
Humans selected against seed pod shattering and seed shattering very early in the domestication of grain crops to prevent large losses in yield due to seeds falling on the ground prior to harvest Dong and Wang, The dehiscence zone is the specialized region of cells that allow the shattering process to occur once the plant material becomes dry. Currently, most of the genetic knowledge regarding shattering almost exclusively relates to dehiscence zone development.
Mutations in various transcription factors account for the overwhelming majority of non-shattering plants Dong and Wang, Importantly, shattering and abscission are not governed by the same genes in Arabidopsis. Loosening of cells in the abscission or dehiscence zone with polygalacturonases prior to cell separation is one area of overlap between the two processes Ogawa et al.
This review will not cover dehiscence zone development in detail since previous reviews do a good job describing what is known about this process Lewis et al. For years, Arabidopsis floral organ abscission has served as the premier model for understanding abscission at the molecular genetic level. Forward and reverse genetic approaches have revealed a number of components that are necessary for abscission, and recently biochemistry has led to some mechanistic insights.
Abscission can be divided into four phases to simplify its explanation. First, abscission zones must develop. Second, abscission signaling is activated. Finally, the abscission scar further differentiates and seals itself Kim, The hydrolysis phase, which is controlled by cell wall-modifying enzymes such as polygalcturonases, has been thoroughly reviewed and will not be discussed in detail here see Niederhuth et al.
A model of the physiological phases of abscission is shown in Fig. Physiological model of abscission. In phase 1, abscission zones develop. In phase 2, the abscission signaling loop is activated and the indicated proteins are all required. The hormones ethylene and jasmonic acid positively regulate abscission, while auxin negatively regulates it by reducing the effect of ethylene. At this step, abscission zone cells begin to enlarge and the pH of their cytosol becomes alkaline. These same enzymes, particularly expansins, are likely involved in the cell expansion that occurs before cell separation.
Finally, in phase 4, the abscission zone scar is sealed with a protective layer and the pH of the abscission zone cells returns to neutral. Abscission zone cells are depicted in color where green represents neutral cytosolic pH and blue represents alkaline pH. This figure is adapted and updated from Kim J.
Four shades of detachment: regulation of floral organ abscission. AZ development begins very early in the development of the organ that will later be able to abscise. Some genetics is known about AZ development in Arabidopsis. While MADS domain transcription factors regulate abscission activation in Arabidopsis, they have not been shown to regulate floral organ or leaf AZ development in Arabidopsis Fernandez et al.
There is a broad understanding of the mechanisms that result in the activation of abscission signaling in Arabidopsis. Signaling events in the abscission activation phase initiate the expression of a mixture of cell wall-modifying enzymes that dissolve the pectin-rich middle lamella of the AZ. No known Arabidopsis mutants abscise but then fail to further differentiate their AZs after abscission.
However, it is likely that signaling components of the abscission activation phase play a role in the final differentiation of the AZ scar. A peptide released from IDA by subtilisin-like serine proteinase processing is also required for abscission Butenko et al. BRI1 is the receptor that perceives brassinosteroid hormones that are responsible for cell expansion and elongation He et al. It will be interesting to see if new intricacies of the ligand—receptor complex can be uncovered in AZs. A recent study overcame functional redundancy of subtilisin-like proteinases SBTs by expressing tissue-specific proteinase inhibitors to show that SBTs are both required for maturation of the IDA peptide and floral organ abscission Schardon et al.
SBTs 4. Interestingly, the in vitro , mesophyll protoplast, and N. In contrast, studies involving AZs from Arabidopsis required no such modification for complementation of abscission defects of ida mutants or plants blocked in SBT activity Stenvik et al. This difference between findings in AZs and other non-AZ systems represents an opportunity to further refine our understanding of the abscission signaling mechanism. At the moment, it is not clear which MAP triple kinase functions in the abscission pathway, nor is it clear whether there are other intermediates between the HAE receptor complex and the MAPK cascade.
A study in floral receptacles revealed that under native protein levels AGL15 binds the HAE promoter, keeping HAE from being expressed prior to the activation of abscission. Newly synthesized HAE completes a positive feedback loop once it takes its place in the plasma membrane. The positive feedback loop and the MAPK cascade both serve to greatly amplify the starting signal to abscise, which explains how HAE expression is increased fold during the process of floral organ abscission.
While AGL15 appears to be a major transcription factor regulating abscission, it certainly cannot be the only one. Other transcription factors that block abscission when overexpressed have been reviewed previously Niederhuth et al. Currently, it is not entirely clear how these other transcription factors fit into the aforementioned positive feedback loop. However, this story is anything but simple.
Mutations in three different secondary genes can partially restore vesicle trafficking and restore abscission in nev mutants. From a molecular mechanistic standpoint, it is not clear how mutating the co-receptor of HAE could restore abscission in nev mutants. The triple-mutant serk1 serk2 bak1 actually has a mild floral organ abscission defect Meng et al.
This observation suggests that players involved in the abscission activation phase also function in the final differentiation of the AZ scar. In addition to being shuttled by vesicles, HAE passes through an error-checking mechanism in the endoplasmic reticulum as well. When the ERAD system is defective, alleles of HAE that generate a partially functional protein can still make it to the plasma membrane and transduce the abscission signal.
A model of the abscission activation signaling pathway is shown in Fig. Model of the abscission activation signaling pathway. Newly synthesized HAE is then shuttled back to the plasma membrane in endosome vesicles with the assistance of NEV, completing a positive feedback loop.
Numbers in blue indicate additional components that are located at the plasma membrane 1 , cytoplasm 2 , and nucleus 3 that cannot be precisely placed on the diagram. The names of these additional proteins are listed in Table 1 , and they have been reviewed previously in detail by Niederhuth et al. Additional abscission signaling components that cannot be precisely placed on Fig.
At first glance, the molecular mechanisms regulating abscission that are described above seem relatively straightforward and logical. However, their depiction is overly simplified since there are several points that are only partially congruent with physiological observations. Therefore, a great many opportunities exist to connect our understanding of molecular mechanisms regulating abscission with the actual physiological changes that occur in AZs during abscission.
In our opinion, a glaring issue that could be clarified is what exactly does IDA do at the physiological level? Recent literature refers to the IDA peptide as a hormone, which it is likely to be Santiago et al.
However, hormones are typically defined as molecules produced in one tissue that exert an effect in another tissue. Currently, no effort has been made to determine where the IDA peptide acting on a given AZ cell is coming from: the same cell, the immediately adjacent cells, or from more distant cells? Also, why is there a peptide signal at all? Does it help synchronize the abscission process? As drawn in model diagrams, all the molecular components necessary for signaling abscission are produced in each individual cell.
In vitro experiments show that IDA peptide in agar plates can enter the pedicel of detached flowers and complement ida mutants Stenvik et al. In short, there could be many future breakthroughs at the interface of molecular signaling and the physiology of complex AZ tissue. A number of more classical plant hormones exert influence over abscission. Ethylene is broadly necessary for abscission in Arabidopsis and crop plants.
Arabidopsis mutants defective in ethylene perception, ethylene response 1 etr1 , and ethylene signaling, ethylene insensitive 2 ein2 , have delayed floral organ abscission Patterson and Bleecker, Overexpression of FYF results in delayed abscission Chen et al. Auxin is generally thought to negatively regulate abscission by making tissue insensitive to ethylene Sexton and Roberts, As mentioned above, a cocktail of ethylene blocker aminoethoxyvinylglycine HCl and synthetic auxin 2,4-Dichlorophenoxyacetic acid are used to prevent pre-harvest fruit drop in Citrus and apple tree Anthony and Coggins Jr.
Jasmonic acid positively regulates floral organ abscission in Arabidopsis. Mutations in the jasmonic acid receptor, coronatine insensitive 1 coi1 , result in delayed floral organ abscission in Arabidopsis Kim et al. Hormones other than ethylene, auxin, or jasmonic acid are also likely to influence abscission.
Salicylic acid may also regulate abscission. Salicylic acid has a well-established role in regulating senescence, and both floral organs and cauline leaves appear to senesce before they abscise Guiboileau et al. Many interesting yet relatively unexplained physiological events occur in AZs once abscission is activated.
As abscission advances, the cytosol of AZ cells becomes more alkaline. Treatments that slow abscission, such as ethylene blockers, prevent this cytosolic alkalization Sundaresan et al.
Additionally, cytosols of ethylene-insensitive mutants in Arabidopsis do not alkalize, while mutants with overly active ethylene signaling are already alkalized before abscission begins. The cytosolic alkalization of AZ cells, associated with abscission, has been shown to occur in Arabidopsis, tomato, and wild rocket Sundaresan et al.
One hypothesis is that alkaline pH may be optimal for some abscission enzymes. Abscission zone cells also enlarge as abscission occurs. Arabidopsis cauline leaf AZ cells can clearly be seen to begin enlarging slightly prior to abscission. Thus, AZ middle lamella hydrolysis and AZ cell enlargement overlap in timing. In drought-triggered abscission, the final size of AZ cells that have a sealed scar are not noticeably larger than AZ cells at the moment of first cell separation Patharkar and Walker, Previous reviews suggest that floral organ AZ cell enlargement only happens after cell separation is complete Kim, This notion is likely due to the fact that floral organ AZ cells cannot be visualized non-destructively prior to abscission because sepals and petals cover the AZs.
Scanning electron micrographs of stamen AZs reveal that floral organ AZ cells also enlarge prior to abscission Cai and Lashbrook, AZ cells on both the leaving organ and the main body of the plant enlarge prior to abscission; however, cells on the leaving organ dry out shortly after abscission occurs Tucker and Kim, It should be noted that early in the 20th century scientists believed that the mechanical shearing force from AZ cell enlargement was the primary driving force for abscission Fitting, ; Sexton and Roberts, Currently, no mutants of Arabidopsis abscise but fail to enlarge their AZ cells, suggesting that AZ enlargement is necessary for abscission in Arabidopsis.
The most detailed explanation of abscission signaling has come from studying Arabidopsis floral organ abscission. The Arabidopsis floral organ system has been used to work out a number of molecular mechanisms regulating abscission signaling described above that would have been more difficult to solve in less genetically tractable systems.
Floral organ abscission occurs after fertilization. Since Arabidopsis is self-pollinating, abscission basically occurs in a developmentally timed manner. Recently, it has become clear that cauline leaf abscission triggered by drought requires the same core abscission signaling mechanism as floral organ abscission Patharkar and Walker, This is an interesting finding because cauline leaf abscission is not set on a developmental clock but rather occurs conditionally.
Drought to the point of wilting activates HAE expression and then, once plants are rewatered, leaf abscission occurs Patharkar and Walker, HAE expression is also activated in the vestigial pedicel abscission zone in Arabidopsis prior to partial abscission. This observation suggests that fruit abscission may also utilize the same signaling pathway as leaves and floral organs Patharkar and Walker, The cauline leaf abscission system in Arabidopsis has two distinct features from the floral organ abscission system that will likely aid researchers in further unraveling the process of abscission.
First, the cauline leaf AZ can be non-destructively observed from development through abscission because no tissue obscures its view. Second, abscission is not triggered by a developmental stage, rather it is triggered by environmental conditions. This allows separation of abscission events from developmental events. Currently, drought is known to trigger cauline leaf abscission; however, other environmental stimuli may also initiate it Patharkar and Walker, How conserved is the Arabidopsis abscission signaling module in other species?
Recent research indicates that it probably extends far past Arabidopsis. Taken together, these findings provide strong evidence that the Arabidopsis abscission signaling module works in other and distantly related species.
In our opinion, two areas of basic abscission research stand out as particularly likely to pay big dividends in the future. First, a literature cross-reference analysis can provide a number of easy-to-test leads to extend the existing abscission signaling pathway. Since abscission components overlap with defense and brassinosteroid fields, and drought-triggered abscission has been characterized, mining those fields for connections relevant to abscission could be fruitful. The power of cross-reference analysis will grow as more network hub components are added to the abscission pathway.
The ability to work in both floral organ and cauline leaf abscission zones also gives researchers options. Of course, they will have to take care in interpreting non-AZ data and applying it to AZs. The AZ is a unique tissue that does not behave like other parts of the plant. A second promising area is research focused on precisely connecting molecular data to the physiology of abscission zones.
At the moment, molecular signaling mechanisms have very vague outputs. For the most part, all that can be said for the output of the molecular pathway is that it triggers abscission.
However, abscission can be broken down into a number of physiological events, such as AZ cell enlargement, AZ cell cytosol alkalization, and middle lamella hydrolysis.
How does the molecular pathway affect these different physiological aspects? It is unlikely that all AZ cells behave the same. Using modern physiological methods to break down the AZ into functional groups of cells and events will yield a tangible understanding of what actually happens at the cellular level to allow abscission.
Finally, there is also promise in using our basic knowledge of abscission signaling to do applied research that could benefit agriculture. For example, preventing soybeans from shedding flowers in response to mild drought conditions might result in more seed pods come harvest time. Alternatively, yield trials could be conducted where chemicals that delay abscission are sprayed on plants prior to drought treatment. While there is no guarantee that the proposed manipulations to soybean would result in increased grain yield, the mechanism for grain yield improvement is simple and therefore straightforward to assay.
Basically, more flowers on the plant could equate to more seed pods later on. In summary, there are more opportunities in basic and applied abscission research right now than ever before. The Plant Journal 50 , — Google Scholar. Addicott FT. Google Preview. Anthony MF , Coggins Jr. The efficacy of five forms of 2,4-D in controlling preharvest fruit drop in citrus.
Scientia Horticulturae 81 , — Disrupting ER-associated protein degradation suppresses the abscission defect of a weak hae hsl2 mutant in Arabidopsis. Journal of Experimental Botany 67 , — Several auxin transporters were down-regulated during abscission and inhibition of polar auxin transport delayed leaf shedding. Ethylene signaling was not involved in the regulation of these auxin transporters and in the formation of an abscission zone, but was required for the expression of hydrolytic enzymes associated with cell separation.
Since exogenous auxin delayed abscission in absence of ethylene signaling auxin likely acts independently of ethylene signaling on cell separation. In contrast to animal cells, most plant cells are tightly glued together by the middle lamella, such that even subtle positional changes to neighboring cells are impeded.
During the life cycle of plants, however, many developmental processes require cell separation, e. Although the enzymatic activities required for the hydrolysis of middle lamellae and cell walls in various separation phenomena are thought to be similar, the triggers and therefore the upstream signaling are obviously diverse Taylor and Whitelaw, ; Roberts et al.
While stress-induced and developmentally regulated organ abscission is relatively well studied little attention has been paid to the understanding of how seasonal cues are integrated to trigger the separation of organs from the plant body. In temperate climates the most remarkable cell separation process is the autumnal shedding of leaves from trees in fall. Despite of the paramount importance of leaf shedding as an adaptation to freezing and for nutrient cycling in forest ecosystems Aponte et al.
Important roles in timing of abscission have been assigned to the plant hormones ethylene and auxin. La Rue showed that removal of the leaf blade induces abscission; but when auxin is applied to the site of removal, abscission is inhibited. Addicott and Lynch , Addicott et al. They found that not the absolute concentration of auxin but the ratio between distal and proximal auxin was relevant for the timing of abscission Addicott et al.
Lower auxin concentrations on the distal than on the proximal side of the abscission zone favored abscission, whereas relatively higher auxin concentrations on the distal side delayed abscission Louie and Addicott, From these experiments, it was concluded that an auxin gradient spans the abscission zone and regulates the induction of abscission Addicott et al.
Although testing of the auxin gradient model has been proven difficult in absence of highly resolved auxin concentration measurements or appropriate auxin response reporters the importance of auxin has further been strengthened by genetic evidence from the model plant Arabidopsis.
Since petioles and pedicels Arabidopsis do not develop functional abscission zones Patterson, , research has focused on abscission of floral organs and dehiscence. Ethylene has been shown to play an antagonistic role to auxin in abscission of various organs.
In the ethylene-insensitive Arabidopsis mutants ein2 and etr abscission is delayed Bleecker and Patterson, ; Patterson and Bleecker, , while application of ethylene hastens abscission in various organs and species.
In line with a promotive role of ethylene in cell separation, ethylene levels often increase shortly before organ separation and ethylene is sufficient to induce the expression of a polygalacturonase required for cell separation in tomato petioles Hong et al.
Interestingly, the same polygalacturonase is inhibited by the exogenous application of auxin, underlying the suggested antagonistic effects of auxin and ethylene in abscission Hong et al. A broadly accepted model of hormonal interaction during organ separation suggests that a depletion of auxin levels in the abscission zone renders cells more sensitive to ethylene, which in turn induces secretion of middle lamellae hydrolyzing enzymes Estornell et al. Although the physiology and transcriptional changes related to autumnal leaf abscission in trees have been subject to intense studies Chen et al.
Here, we describe an experimental system to induce leaf abscission in Populus synchronously and report that auxin is a plausible long-range signal regulating abscission that acts independently of ethylene signaling. The in vitro growth conditions were according to Love et al. Briefly, trees were grown in clear polypropylene containers height, 14 cm; diameter, 10 cm with OSODS gas-exchange spore filters Combiness and cultured on Murashige and Skoog medium 2.
The 3 to 4-weeks-old in vitro grown transgenic trees height, 10—12 cm were transferred in 2. Teichmann et al. These lines were used for the exogenous auxin and auxin transport inhibitor applications Figures 2 and 4.
All the other experiments were conducted in the T89 background. Leaf blades were covered with aluminum bags under standard greenhouse conditions. Control samples were bagged in transparent plastic bags of the same weight. Each bag was labeled with a unique code referring to the genotype, tree replicate, leaf number, and treatment.
Trees were gently shaken once per day, the dropped bags collected and the identifiers recorded. The resulting plasmids were transformed into Agrobacterium tumefaciens strain GV pMP90, pSoup and used for stable transformations of the hybrid aspen clone T89 according to Nilsson et al. For all constructs at least five lines were selected and tested for GUS expression.
Three lines each were analyzed in detail and representative GUS expression patterns are shown in Figure 5. Data analysis has been performed with the help of the Robin software package Lohse et al.
GUS expression patterns were determined in 3-mm-thick longitudinal median sections of leaf axils. Samples were infiltrated employing vacuum for 30 min with GUS buffer 50 mM sodium phosphate buffer pH 7.
In Populus , gradual reduction of day length and temperature induces leaf abscission; whereas over-expression of phytochrome A is sufficient to prevent abscission under these conditions Olsen et al. However, it remains unclear if shorter day length is sensed by individual leaf blades, apices or if it is rather a response of the whole plant to reduced photoperiod. Therefore, we tested if shading of the leaf blade is sufficient to induce abscission in greenhouse-grown Populus trees.
To this end we covered blades but not petioles of fully expanded leaves with aluminum foil bags. Shading induced cell divisions in the leaf axil and the formation of typical abscission zones 6 and 9 days, respectively, after the treatment started. Although the petioles were not shaded strong de-greening of the petiole was observed Figure 1.
By contrast, leaves in transparent bags of the same tree did not develop an abscission zone, were not shed and their petioles did not de-greened during the same period of observation Figures 1A,E. Taken together, these results suggest that reduced photoperiod is sensed in the leaf blade and a mobile signal transports this information from the blade to the abscission zone, where it induces the development of an abscission zone.
A mobile signal from the leaf blade induces leaf abscission. A—D Toluidine blue stained longitudinal median section of leaf axils.
A No abscission zone visible in axil of non-shaded leaf blade. Blade was bagged in a transparent plastic bag for 9 days. B Mature abscission zone in an axil of a shaded leaf blade. Blade was covered for 9 days in aluminum foil. C,D Abscission zones 6 and 9 days after shading started, respectively. Yellow arrowheads point to dividing cells.
E Leaf axil of a non-shaded blade. F Leaf axil of a blade shaded for 12 days. Chlorophyll distal to the abscission zone is degraded although leaf petiole was not shaded. Previous work in explants of annual species indicated that auxin could contribute to a leaf blade-derived abscission inhibiting signal Louie and Addicott, After 6 days of shading, this activity became weaker but at the lower abaxial side of the petiole a new auxin response maximum emerged, which gradually expanded to the upper adaxial, after 9 days side of the petiole preceding the formation and maturation of the abscission zone Figures 2B—E.
A new auxin response maximum is established prior to the formation of an abscission zone. White arrow heads point to the abscission zones. E Mature abscission zone appears in brown; GUS precipitate in blue. Scale bars correspond to approximately 1 mm A—C ; 0,5 mm D,E. Black arrowheads point to the forming D and mature abscission zone E. We then applied auxin indoleacetic acid, IAA in lanoline paste directly to the axils of intact leaves in order to test if auxin can delay abscission of shaded leaves Figure 3.
Auxin application to the very proximal end of the petiole delayed dark-induced leaf abscission highly significantly by approximately 1 day. In order to examine if auxin can also work as a long-distance signal, which is transported from the leaf blade to the axil, we applied auxin IAA to the most distal end of the petiole.
Also in this case, abscission was significantly delayed supporting the idea that auxin not only acts locally but has the potential to be a long-distance signal in leaf abscission.
Exogenous auxin can act over short and long distance to delay leaf abscission. Box plot, center lines show the medians; box limits indicate the 25th and 75th percentiles; whiskers extend to 5th and 95th percentiles, outliers are represented by dots; crosses represent sample means. Number of observations indicated above the x-axis.
The polar auxin transport inhibitor morphactin 9-hydroxyfluorenecarboxylic acid, CF has previously been show to delay leaf abscission in citrus Goren et al. Local application of morphactin to the axil of shaded leaves delayed separation by approximately 7 days, to an even stronger extent as observed for the application of the auxin influx carrier substrate 2,4- D Figure 4A.
We then tested whether morphactin modulates auxin response in leaf axils. In mock-treated axils, 6 days after shading started, a new local GHGUS maximum appeared at the lower side of the petiole.
After 9 days, in partly separated petioles, most of the auxin response reporter activity occurred on the proximal side of the abscission zone Figures 4B—D. By contrast, in presence of morphactin formation of the new auxin response maxium as well as of the abscission zone were delayed and only became visible 18 days after shading started Figures 4E—G.
Hence, morphactin likely acts on the formation of the abscission zone by preventing the establishment of a new auxin response maximum. The auxin transport inhibitor morphactin delays abscission.
A Box plot, center lines show the medians; box limits indicate the 25th and 75th percentiles; whiskers extend to 5th and 95th percentiles, outliers are represented by dots; crosses represent sample means. B—G Formation of new auxin response maximum and abscission zone is delayed by morphactin.
B—D Mock treatment, lanolin paste locally applied to abscission zone. D,G Arrowheads point to the abscission zone. In order to identify auxin transporters, which could be involved in the formation of a new auxin response maximum during abscission we performed a microarray experiment. Total RNA was isolated from dissected leaf axils 9 days after shading started and from axils covered with transparent plastic bags. Among the approximately differentially expressed genes auxin related transcripts were strongly overrepresented Supplemental Dataset.
By contrast, none of the known putative auxin transporters were up-regulated. Expression of all reporter gene constructs was inducible by exogenous application of auxin Supplementary Figure S1. Similarly, expression of the intracellular auxin transporters was most prominent in vascular tissues of axils from non-shaded leaf blades Figures 5B,C.
These findings underline the importance of auxin transport in the formation of a new auxin maximum and the formation of an abscission zone. A—C axils of non-shaded leaf blades; D—F 9 days after shading started. Red arrowheads point to mature abscission zone. Scale bars, 1 mm. TABLE 1. Genes involved in auxin transport and response are down-regulated in axils of shaded leaves. We then wondered if auxin acts through ethylene signaling on leaf abscission as it has been suggested previously for review, Estornell et al.
We first analyzed abscission in 35S::etr ethylene insensitive trees Love et al. Expression of the dominant Arabidopsis etr allele under the control of the 35S promoter renders Populus trees insensitive to ethylene. We then tested if the expression of auxin transporter genes is reduced after shading in 35S::etr trees to a similar extent than in wild type.
Together these findings suggest that the regulation of auxin transporters and the formation of an abscission zone are independent of ethylene signaling. In order to test if auxin acts on leaf abscission solely upstream of ethylene or in an independent parallel pathway we tested if auxin has an effect on the timing of abscission in 35S::etr axils of shaded leaves Figure 6C. Exogenous auxin further delayed abscission and the number of leaves, which did not abscise, in an additive manner, as compared to mock treated 35S::etr axils, suggesting that auxin acts in an independent pathway, parallel to ethylene signaling, on leaf abscission.
Auxin acts independently of ethylene on leaf abscission. White arrowheads point to mature abscission zones. Normalized to ACT1 expression. D Gene expression of auxin transporters. E Gene expression of pectin modifying enzymes. Based on earlier work, an auxin gradient spanning the abscission zone has been proposed to regulate timing of organ separation Addicott et al.
Similar spatially discrete auxin maxima or gradients have been proposed to regulate the positioning of leaf and lateral root primordia as well as the zonation patterning in the cambium Dubrovsky et al. In the cambium high auxin concentrations correlate with meristematic activity and cell division of protoplasts requires the presence of auxin Bhalerao and Fischer, Intriguingly, formation of the abscission zone not only involves specialization of cells but also in many cases cell proliferation.
In contrast to an auxin response maximum, which precedes the formation of an abscission zone, Sorefan et al. At the site of reduced auxin response secondary walls are deposited, which are instrumental during the separation process. Interestingly, a strong auxin response is detected in the replum, the tissue between valve margins, coincidentally, overlapping with the region where cell divisions occur during the formation of an abscission zone Wu et al.
Thus, the mode of auxin action might be similar in abscission zones as suggested for the cambium, high concentrations permit cell division, below a certain threshold cell expansion and differentiation are favored. Recently, an auxin response maximum has been observed in the abscission zone of Arabidopsis petals Basu et al.
In auxin influx carrier mutants, petal break strength is increased and therefore AUX1 and its paralogs might be involved in the formation of an auxin concentration maximum in the abscission zone Basu et al. In wild-type, the outermost tier of the root cap is continuously shed from the root. The pin4 phenotype is either a consequence of over-proliferation of root cap cells or, more likely, of impaired cell separation.
Our findings of delayed auxin maximum formation and abscission upon inhibition of polar auxin transport and the strong reduction of auxin carriers in dark-induced leaf axils are in support with the idea that auxin transport is instrumental for the establishment of an instructive auxin response maximum during abscission. Interestingly, expression of iaa L, which conjugates auxin, under the control of an abscission zone specific polygalacturonase promoter results in reduced auxin response and increased petal break strength Basu et al.
Polygalacturonases are integral parts of cell wall remodeling during the separation process Roongsattham et al. Consequently, relatively low auxin concentrations during the separation phase are sufficient to hasten abscission.
Hence, auxin is likely to have a dual function during abscission in first providing positional information for the formation of the abscission zone and secondly as a signal, which regulates temporal aspects of the separation. Such a mode-of-action ensures that under high auxin concentrations an abscission zone can differentiate but separation cannot be initiated. As a consequence premature abscission, which can be deleterious for plant performance, might be prevented.
0コメント