Organ Abscission Research Articles

Crosstalk between Hydrogen Sulfide and Other Signal Molecules Regulates Plant Growth and Development.

Hydrogen sulfide (H2S), once recognized only as a poisonous gas, is now considered the third endogenous gaseous transmitter, along with nitric oxide (NO) and carbon monoxide (CO). Multiple lines of emerging evidence suggest that H2S plays positive roles in plant growth and development when at appropriate concentrations, including seed germination, root development, photosynthesis, stomatal movement, and organ abscission under both normal and stress conditions. H2S influences these processes by altering gene expression and enzyme activities, as well as regulating the contents of some secondary metabolites. In its regulatory roles, H2S always interacts with either plant hormones, other gasotransmitters, or ionic signals, such as abscisic acid (ABA), ethylene, auxin, CO, NO, and Ca2+. Remarkably, H2S also contributes to the post-translational modification of proteins to affect protein activities, structures, and sub-cellular localization. Here, we review the functions of H2S at different stages of plant development, focusing on the S-sulfhydration of proteins mediated by H2S and the crosstalk between H2S and other signaling molecules.

Cotton production during the tropical monsoon season. II – Biomass accumulation, partitioning and RUE in response to boll loss and compensation

Abstract Growth and reproductive structure retention in cotton (Gossypium hirsutum) is dependent on light intensity, which is often irregular in tropical environments due to shading from cloudiness associated with the monsoon. The objective of this study was to evaluate the potential effects that cloudiness-induced reproductive organ abscission and compensatory responses might have on cotton growth, partitioning and conversion of radiation to biomass (radiation use efficiency RUE) as this was unknown for the monsoon tropics. Cotton cultivars representing different morphology were sown in field experiments at closely timed intervals during the early monsoon over 5 seasons to expose crops to varying amounts of cloud-related shading, creating a matrix of crop responses. In paper I, final boll number was strongly linked with lint yield, and boll retention on the early and late flowering sympodia and yield were correlated with the radiation environment during flowering. Grouping sowing time treatments that had a similar intra-canopy distribution of bolls at maturity provided a mechanism to compare yield recovery post boll loss and consider causal factors. Maximum crop biomass ranged from 1232−1933 g m−2. Lint yield was correlated with the fraction of total biomass present as bolls (P Differences in biomass partitioning between cultivars were minor. When cloud cover abated, extended concurrent vegetative and reproductive growth enabled yield recovery. Greater LAI was a consequence of compensatory processes following fruit loss. Faster development of LAI in response to the monsoon during early flowering could further shade lower canopy fruiting sites, exacerbating fruit abscission and impacting yield compensation. RUEg (glucose equivalent) of 2.35 & 2.63 g MJ−1 PAR was comparable with cotton grown in temperate environments but declined when lower canopy fruit abscission followed by extended concurrent growth altered the ratio of vegetative vs reproductive biomass partitioning. Cloudiness poses a dichotomy of challenges for crop management during the tropical monsoon season. After cloudiness-induced fruit loss, extended concurrent growth when the radiation environment improves must be encouraged to enable the initiation of new fruiting sites for yield recovery. However, expansionary canopy growth in response to cloud and fruit loss may exacerbate detrimental shading of the lower canopy and detract from later yield recovery. Therefore management tactics including the use of irrigation, nutrient and growth regulant inputs must achieve an appropriate balance between vegetative and reproductive components during compensatory growth.

LcEIL2/3 are involved in fruitlet abscission via activating genes related to ethylene biosynthesis and cell wall remodeling in litchi.

Fruit crops are subject to precocious fruit abscission, during which the phytohormone ethylene (ET) acts as a major positive regulator. However, the molecular basis of ET-induced fruit abscission remains poorly understood. Here, we show that two ETHYLENE INSENSITIVE 3-like (EIL) homologs in litchi, LcEIL2 and LcEIL3, play a role in ET-activated fruitlet abscission. LcEIL2/3 were significantly upregulated in the fruit abscission zone (AZ) during the ET-induced fruitlet abscission in litchi. The presence of LcEIL2/3 in wild-type Arabidopsis and ein3 eil1 mutants can accelerate the floral organ abscission. Moreover, the electrophoretic mobility shift assay and dual luciferase reporter analysis illustrated that LcEIL2/3 directly interacted with the gene promoters to activate the expression of cell wall remodeling genes LcCEL2/8 and LcPG1/2, and ET biosynthetic genes LcACS1/4/7 and LcACO2/3. Furthermore, we showed that LcPG1/2 were expressed in the floral abscission zone of Arabidopsis, and constitutive expression of LcPG2 in Arabidopsis promoted the floral organ abscission. In conclusion, we propose that LcEIL2/3 are involved in ET-induced fruitlet abscission via controlling expression of genes related to ET biosynthesis and cell wall remodeling in litchi.

Transcription factor CDF4 promotes leaf senescence and floral organ abscission by regulating abscisic acid and reactive oxygen species pathways in Arabidopsis.

Leaf senescence is a highly complex developmental process that is tightly controlled by multiple layers of regulation. Abscisic acid (ABA) and reactive oxygen species (ROS) are two well-known factors that promote leaf senescence. We show here that the transcription factor CDF4 positively regulates leaf senescence. Constitutive and inducible overexpression of CDF4 accelerates leaf senescence, while knockdown of CDF4 delays it. CDF4 increases endogenous ABA levels by upregulating the transcription of the ABA biosynthesis genes 9-cis-epoxycarotenoid dioxygenase 2, 3 (NCED2, 3) and suppresses H2 O2 scavenging by repressing expression of the catalase2 (CAT2) gene. NCED2, 3 knockout and CAT2 overexpression partially rescue premature leaf senescence caused by CDF4 overexpression. We also show that CDF4 promotes floral organ abscission by activating the polygalacturonase PGAZAT gene. Based on these results, we propose that the levels of CDF4, ABA, and ROS undergo a gradual increase driven by their interlinking positive feedback loops during the leaf senescence and floral organ abscission processes.

Plant Biology: Distinct New Players in Processing Peptide Hormones during Abscission

Flower organ abscission in Arabidopsis is regulated by a peptide hormone that is released from its precursor by a network of redundant subtilases. An exciting new study describes how drought-induced flower abscission in tomato is regulated similarly, but distinctly via a single, different subtilase that releases a very different peptide hormone.

Disruption of the Auxin Gradient in the Abscission Zone Area Evokes Asymmetrical Changes Leading to Flower Separation in Yellow Lupine.

How auxin transport regulates organ abscission is a long-standing and intriguing question. Polar auxin transport across the abscission zone (AZ) plays a more important role in the regulation of abscission than a local concentration of this hormone. We recently reported the existence of a spatiotemporal sequential pattern of the indole-3-acetic acid (IAA) localization in the area of the yellow lupine AZ, which is a place of flower detachment. In this study, we performed analyses of AZ following treatment with an inhibitor of polar auxin transport (2,3,5-triiodobenzoic acid (TIBA)). Once we applied TIBA directly onto the AZ, we observed a strong response as demonstrated by enhanced flower abscission. To elucidate the molecular events caused by the inhibition of auxin movement, we divided the AZ into the distal and proximal part. TIBA triggered the formation of the IAA gradient between these two parts. The AZ-marker genes, which encode the downstream molecular components of the inflorescence deficient in abscission (IDA)-signaling system executing the abscission, were expressed in the distal part. The accumulation of IAA in the proximal area accelerated the biosynthesis of abscisic acid and ethylene (stimulators of flower separation), which was also reflected at the transcriptional level. Accumulated IAA up-regulated reactive oxygen species (ROS) detoxification mechanisms. Collectively, we provide new information regarding auxin-regulated processes operating in specific areas of the AZ.

LuluDB-The Database Created Based on Small RNA, Transcriptome, and Degradome Sequencing Shows the Wide Landscape of Non-coding and Coding RNA in Yellow Lupine (Lupinus luteus L.) Flowers and Pods.

Yellow lupine (Lupinus luteus L.) belongs to a legume family that benefits from symbiosis with nitrogen-fixing bacteria. Its seeds are rich in protein, which makes it a valuable food source for animals and humans. Yellow lupine is also the model plant for basic research on nodulation or abscission of organs. Nevertheless, the knowledge about the molecular regulatory mechanisms of its generative development is still incomplete. The RNA-Seq technique is becoming more prominent in high-throughput identification and expression profiling of both coding and non-coding RNA sequences. However, the huge amount of data generated with this method may discourage other scientific groups from making full use of them. To overcome this inconvenience, we have created a database containing analysis-ready information about non-coding and coding L. luteus RNA sequences (LuluDB). LuluDB was created on the basis of RNA-Seq analysis of small RNA, transcriptome, and degradome libraries obtained from yellow lupine cv. Taper flowers, pod walls, and seeds in various stages of development, flower pedicels, and pods undergoing abscission or maintained on the plant. It contains sequences of miRNAs and phased siRNAs identified in L. luteus, information about their expression in individual samples, and their target sequences. LuluDB also contains identified lncRNAs and protein-coding RNA sequences with their organ expression and annotations to widely used databases like GO, KEGG, NCBI, Rfam, Pfam, etc. The database also provides sequence homology search by BLAST using, e.g., an unknown sequence as a query. To present the full capabilities offered by our database, we performed a case study concerning transcripts annotated as DCL 1–4 (DICER LIKE 1–4) homologs involved in small non-coding RNA biogenesis and identified miRNAs that most likely regulate DCL1 and DCL2 expression in yellow lupine. LuluDB is available at http://luluseqdb.umk.pl/basic/web/index.php.

Cotton production during the tropical monsoon season. I – The influence of variable radiation on boll loss, compensation and yield

Growth and reproductive structure development in cotton (Gossypium hirsutum) is dependent on light intensity, which is often irregular in tropical environments due to shading from cloudiness associated with the monsoon. The objective of this study was to evaluate the effects of cloudiness on reproductive organ abscission and how subsequent compensatory organ replacement, with the provision of irrigation, might recover yield in the semi-arid monsoon tropics. Cotton cultivars representing different morphology were sown in field experiments at closely timed intervals during the early monsoon over five seasons to expose crops to varying amounts of cloud-related shading, creating a matrix of crop responses. A wide range for lint yield (1099 to 2900 kg ha−1) was symptomatic of the treatments’ exposure to highly varied environmental conditions. Lint yield was moderately correlated (P < 0.01; Adj R2 = 0.409) with monsoon cloudiness, reducing 235 kg ha−1 per MJ m−2 decrease in the average daily incident radiation during flowering. Final boll number rather than boll size was the key yield component (P < 0.001), with the number of bolls retained and their location within the canopy correlated with the solar radiation pattern spanning flowering. Boll number m−2 decreased linearly in proportion to mean daily incident solar radiation, by 5.02 and 4.39 bolls MJ m−2 during early (P = 0.007) and late flowering (P = 0.02) respectively. Cloudiness-induced boll loss from the early flowering canopy produced a negative correlation with late boll numbers, increasing by 7.2 bolls per MJ m-2 d−1 decrease in mean incident radiation during early flowering (P < 0.001) as lower canopy boll loss enabled surplus assimilates for later set bolls. Fibre quality parameters were not negatively affected by reduced incident solar radiation during flowering and boll filling.The clustering of sowing times into groups according to similarities in the number and final distribution of bolls within the crop canopy provided a mechanism to identify sowing time treatments that shared similar compensation responses. The highest group yields occurred when minimal cloudiness during flowering allowed uninterrupted boll accumulation or alternatively when moderate boll losses from the lower canopy were compensated via additional later set bolls when radiation improved. Yield recovery was restricted for groups where cloudiness was ongoing or increased during late flowering or when canopy factors detracted from efficient use of available radiation. Extrapolation of the linear relationship between boll number and incident radiation showed boll retention was maximised at a mean daily value of 24.2 MJ m−2 (Rmax). Quantification of Rmax was a key finding as it could be applied to calculate a cumulative radiation deficit (RD) = ∑ (if daily radiation is < Rmax then daily RD = Rmax-daily radiation) and provided an alternative independent variable for quantifying the impact of cloudiness during flowering on boll survival. Wider application of this concept requires research in larger solar radiation and temperature regimes than observed here.Yield compensation from cloud induced fruit abscission in the semi-arid tropics will be assisted by management that can extend flowering to create new fruiting sites whilst balancing the partitioning of canopy growth.

The new insight of auxin functions: transition from seed dormancy to germination and floral opening in plants

In angiosperms, plants go through different transitions to complete a life cycle. These different transition stages are crucial for plant development. External and internal stimuli affect these transitions, including environmental condition and phytohormones. Auxin is one of the classical phytohormones and crucial to plant morphogenesis establishment and various physiological processes, like phototropism, cell differentiation, cell expansion, floral opening, organ abscission and seed germination. Abscisic acid (ABA) and gibberellic acid (GA) are widely known to work antagonistically in seed germination. Apart from GA and ABA, recent studies have demonstrated that auxin is also involved in the transition from seed dormancy to germination and floral opening. Auxin promotes seed dormancy and inhibits seed germination. Moreover, functions of auxin during floral opening is promising for complementing hormonal crosstalk during this process. In this review, we focus on auxin–ABA crosstalk in regulation of seed germination and dormancy mainly in model plant Arabidopsis thaliana and summarize the knowledge about auxin-mediated flowering in several different species. The systematical knowledge of hormonal signaling provides a guidance for uncovering largely unknown functions of auxin.

Hydrogen sulfide inhibits ethylene-induced petiole abscission in tomato (Solanum lycopersicum L.)

Abscission is a dynamic physiological process that is ubiquitous in plants and can also be an essential agronomic trait in crops, thus attracting attention from plant growers and breeders. In general, the process of plant organ abscission can be divided into four steps, among which the step to obtain the competence to respond to abscission signals (step 2) is the most complex; however, the molecular mechanism underlying this process remains unclear. In this study, we found that hydrogen sulfide (H2S) inhibited the abscission of the tomato petiole in a dose-dependent manner, and the abscission of the petiole was accelerated when an H2S scavenger was applied. Further enzymatic activity and gene expression analyses showed that H2S suppressed the activity of enzymes capable of modifying the cell wall by inhibiting the usual upregulation of the transcription of the corresponding genes during the abscission process but not by affecting the activities of these enzymes by direct posttranslational modification. H2S treatment upregulated the expression levels of SlIAA3 and SlIAA4 but downregulated the transcription of ILR-L3 and ILR-L4 in the earlier stages of the abscission process, indicating that H2S probably functioned in the second step of the abscission process by preventing the abscission zone cells from obtaining the competence to respond to abscission signals by modulating the content of the bioactive-free auxin in these cells. Moreover, similar H2S inhibitory effects were also demonstrated in the process of floral organ abscission and anther dehiscence in other plant species, suggesting a ubiquitous role for H2S in cell separation processes.

Identification and Characterization of HAESA-Like Genes Involved in the Fruitlet Abscission in Litchi.

Regulation of abscission is an important agricultural concern since precocious abscission can reduce crop yield. INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) peptide and its receptors the HAESA (HAE) and HAESA-like2 (HSL2) kinases have been revealed to be core components controlling floral organ abscission in the model plant Arabidopsis. However, it is still unclear whether the homologs of IDA-HAE/HSL2 in non-model plants are correlated to abscission. Previously, we found LcIDL1, a homolog of IDA from litchi, has a similar role to AtIDA in control of floral organ abscission in Arabidopsis. Here, we further isolated an HAESA-like homolog, LcHSL2, which is likely involved in the fruitlet abscission in litchi. Ectopic expression of LcHSL2 in wild type Arabidopsis has no effect on the floral organ abscission. However, its presence in the hae hsl2 mutant background completely rescued the floral organ abscission deficiency. LcHSL2 is localized in the cell membrane and the LcHSL2 gene is expressed at the pedicel abscission zone (AZ) of litchi and floral AZ of Arabidopsis. Real-time PCR analysis showed that the expression level of LcHSL2 was increased during ethephon-induced fruitlet abscission in litchi. Taken together, our findings suggest that HSL2 homologs have functional conservation in Arabidopsis and litchi, and LcHSL2 might play a critical role in regulation of fruitlet abscission in litchi.

Involvement of HD-ZIP I transcription factors LcHB2 and LcHB3 in fruitlet abscission by promoting transcription of genes related to the biosynthesis of ethylene and ABA in litchi.

Abnormal fruitlet abscission is a limiting factor in the production of litchi, an economically important fruit in Southern Asia. Both ethylene and abscisic acid (ABA) induce organ abscission in plants. Although ACS/ACO and NCED genes are known to encode key enzymes required for ethylene and ABA biosynthesis, respectively, the transcriptional regulation of these genes is unclear in the process of plant organ shedding. Here, two polygalacturonase (PG) genes (LcPG1 and LcPG2) and two novel homeodomain-leucine zipper I transcription factors genes (LcHB2 and LcHB3) were identified as key genes associated with the fruitlet abscission in litchi. The expression of LcPG1 and LcPG2 was strongly associated with litchi fruitlet abscission, consistent with enhanced PG activity and reduced homogalacturonan content in fruitlet abscission zones (FAZs). The promoter activities of LcPG1/2 were enhanced by ethephon and ABA. In addition, the production of ethylene and ABA in fruitlets was significantly increased during fruit abscission. Consistently, expression of five genes (LcACO2, LcACO3, LcACS1, LcACS4 and LcACS7) related to ethylene biosynthesis and one gene (LcNCED3) related to ABA biosynthesis in FAZs were activated. Further, electrophoretic mobility shift assays and transient expression experiments demonstrated that both LcHB2 and LcHB3 could directly bind to the promoter of LcACO2/3, LcACS1/4/7 and LcNCED3 genes and activate their expression. Collectively, we propose that LcHB2/3 are involved in the litchi fruitlet abscission through positive regulation of ethylene and ABA biosynthesis.

Critical Review of Plant Cell Wall Matrix Polysaccharide Glycosyltransferase Activities Verified by Heterologous Protein Expression.

The life cycle and development of plants requires the biosynthesis, deposition, and degradation of cell wall matrix polysaccharides. The structures of the diverse cell wall matrix polysaccharides influence commercially important properties of plant cells, including growth, biomass recalcitrance, organ abscission, and the shelf life of fruits. This review is a comprehensive summary of the matrix polysaccharide glycosyltransferase (GT) activities that have been verified using in vitro assays following heterologous GT protein expression. Plant cell wall (PCW) biosynthetic GTs are primarily integral transmembrane proteins localized to the endoplasmic reticulum and Golgi of the plant secretory system. The low abundance of these enzymes in plant tissues makes them particularly difficult to purify from native plant membranes in quantities sufficient for enzymatic characterization, which is essential to study the functions of the different GTs. Numerous activities in the synthesis of the major cell wall matrix glycans, including pectins, xylans, xyloglucan, mannans, mixed-linkage glucans (MLGs), and arabinogalactan components of AGP proteoglycans have been mapped to specific genes and multi-gene families. Cell wall GTs include those that synthesize the polymer backbones, those that elongate side branches with extended glycosyl chains, and those that add single monosaccharide linkages onto polysaccharide backbones and/or side branches. Three main strategies have been used to identify genes encoding GTs that synthesize cell wall linkages: analysis of membrane fractions enriched for cell wall biosynthetic activities, mutational genetics approaches investigating cell wall compositional phenotypes, and omics-directed identification of putative GTs from sequenced plant genomes. Here we compare the heterologous expression systems used to produce, purify, and study the enzyme activities of PCW GTs, with an emphasis on the eukaryotic systems Nicotiana benthamiana, Pichia pastoris, and human embryonic kidney (HEK293) cells. We discuss the enzymatic properties of GTs including kinetic rates, the chain lengths of polysaccharide products, acceptor oligosaccharide preferences, elongation mechanisms for the synthesis of long-chain polymers, and the formation of GT complexes. Future directions in the study of matrix polysaccharide biosynthesis are proposed.

Control of Organ Abscission and Other Cell Separation Processes by Evolutionary Conserved Peptide Signaling.

Plants both generate and shed organs throughout their lifetime. Cell separation is in function during opening of anthers to release pollen; floral organs are detached after pollination when they have served their purpose; unfertilized flowers are shed; fruits and seeds are abscised from the mother plant to secure the propagation of new generations. Organ abscission takes place in specialized abscission zone (AZ) cells where the middle lamella between adjacent cell files is broken down. The plant hormone ethylene has a well-documented promoting effect on abscission, but mutation in ethylene receptor genes in Arabidopsis thaliana only delays the abscission process. Microarray and RNA sequencing have identified a large number of genes differentially expressed in the AZs, especially genes encoding enzymes involved in cell wall remodelling and disassembly. Mutations in such genes rarely give a phenotype, most likely due to functional redundancy. In contrast, mutation in the INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) blocks floral organ abscission in Arabidopsis. IDA encodes a small peptide that signals through the leucine-rich repeat receptor-like kinases HAESA (HAE) and HAE-LIKE2 (HSL2) to control floral organ abscission and facilitate lateral root emergence. Untimely abscission is a severe problem in many crops, and in a more applied perspective, it is of interest to investigate whether IDA-HAE/HSL2 is involved in other cell separation processes and other species. Genes encoding IDA and HSL2 orthologues have been identified in all orders of flowering plants. Angiosperms have had enormous success, with species adapted to all kinds of environments, adaptations which include variation with respect to which organs they shed. Here we review, from an evolutionary perspective, the properties of the IDA-HAE/HSL2 signaling module and the evidence for its hypothesized involvement in various cell separation processes in angiosperms.

Rosa hybrida RhERF1 and RhERF4 mediate ethylene- and auxin-regulated petal abscission by influencing pectin degradation.

The timing of plant organ abscission is modulated by the balance of two hormones, ethylene and auxin, while the mechanism of organ shedding depends on the loss of middle lamella pectin in the abscission zone (AZ). However, the mechanisms involved in sensing the balance of auxin and ethylene and that affect pectin degradation during abscission are not well understood. In this study, we identified two members of the APETALA2/ethylene-responsive factor (AP2/ERF) transcription factor family in rose (Rosa hybrida), RhERF1 and RhERF4 which play a role in petal abscission. The expression of RhERF1 and RhERF4 was influenced by ethylene and auxin, respectively. Reduced expression of RhERF1 or RhERF4 was observed to accelerate petal abscission. Global expression analysis and real-time PCR assays revealed that RhERF1 and RhERF4 modulate the expression of genes encoding pectin-metabolizing enzymes. A reduction in the abundance of pectin epitopes was detected in the AZs of RhERF1 and RhERF4-silenced plants by immunofluorescence microscopy analysis. In addition, RhERF1 and RhERF4 were shown to bind to the promoter of the pectin-metabolizing gene β-GALACTOSIDASE 1 (RhBGLA1), and reduced expression of RhBGLA1 delayed petal abscission. We conclude that during petal abscission, RhERF1 and RhERF4 integrate and coordinate ethylene and auxin signals to modulate pectin metabolism, in part by regulating the expression of RhBGLA1.

Transcriptional Regulation of Abscission Zones.

Precise and timely regulation of organ separation from the parent plant (abscission) is consequential to improvement of crop productivity as it influences both the timing of harvest and fruit quality. Abscission is tightly associated with plant fitness as unwanted organs (petals, sepals, filaments) are shed after fertilization while seeds, fruits, and leaves are cast off as means of reproductive success or in response to abiotic/biotic stresses. Floral organ abscission in Arabidopsis has been a useful model to elucidate the molecular mechanisms that underlie the separation processes, and multiple abscission signals associated with the activation and downstream pathways have been uncovered. Concomitantly, large-scale analyses of omics studies in diverse abscission systems of various plants have added valuable insights into the abscission process. The results suggest that there are common molecular events linked to the biosynthesis of a new extracellular matrix as well as cell wall disassembly. Comparative analysis between Arabidopsis and soybean abscission systems has revealed shared and yet disparate regulatory modules that affect the separation processes. In this review, we discuss our current understanding of the transcriptional regulation of abscission in several different plants that has improved on the previously proposed four-phased model of organ separation.

Genome-wide identification and expression analysis of citrus fruitlet abscission-related polygalacturonase genes.

Polygalacturonases (PGs) encoded by a relatively large gene family are involved in plant organ abscission, but few data is available in citrus. Here, to explore the role of PGs in citrus fruitlet abscission (CFA), we have obtained 38 citrus PG (CitPG) members, based on the citrus genome sequences. The ORF length varied from 378 to 2418bp, encoding proteins with theoretical pI and molecular mass ranging from 4.83 to 9.92 and from 13,951.71 to 85,542.28, respectively. Most CitPGs contained no less than 3 introns, suggesting a high probability of alternative splicing. Phylogenetic tree revealed that all PGs could be divided into three groups, in which 9 CitPGs, including CitPG2, CitPG3, CitPG10, CitPG24, CitPG27, CitPG29, CitPG30, CitPG33 and CitPG34 possessed a close relationship with abscission-associated PGs, indicating their role in CFA. Expression analysis further demonstrated that CitPG2, CitPG29 and CitPG34 might be involved in CFA, the expression levels of which could be induced by ethylene, inhibited by IAA and increased during CFA. The findings in this study have provided a foundation for future studies to elucidate the roles of CitPGs in CFA.

The PIP Peptide of INFLORESCENCE DEFICIENT IN ABSCISSION Enhances Populus Leaf and Elaeis guineensis Fruit Abscission.

The programmed loss of a plant organ is called abscission, which is an important cell separation process that occurs with different organs throughout the life of a plant. The use of floral organ abscission in Arabidopsis thaliana as a model has allowed greater understanding of the complexities of organ abscission, but whether the regulatory pathways are conserved throughout the plant kingdom and for all organ abscission types is unknown. One important pathway that has attracted much attention involves a peptide ligand-receptor signalling system that consists of the secreted peptide IDA (INFLORESCENCE DEFICIENT IN ABSCISSION) and at least two leucine-rich repeat (LRR) receptor-like kinases (RLK), HAESA (HAE) and HAESA-LIKE2 (HSL2). In the current study we examine the bioactive potential of IDA peptides in two different abscission processes, leaf abscission in Populus and ripe fruit abscission in oil palm, and find in both cases treatment with IDA peptides enhances cell separation and abscission of both organ types. Our results provide evidence to suggest that the IDA–HAE–HSL2 pathway is conserved and functions in these phylogenetically divergent dicot and monocot species during both leaf and fruit abscission, respectively.

Hypermorphic SERK1 Mutations Function via a SOBIR1 Pathway to Activate Floral Abscission Signaling.

In Arabidopsis (Arabidopsis thaliana), the abscission of floral organs is regulated by two related receptor-like protein kinases, HAESA (HAE) and HAESA-LIKE2 (HSL2). In complex with members of the SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE (SERK) family of coreceptor protein kinases, HAE and HSL2 are activated when bound by INFLORESCENCE DEFICIENT IN ABSICSSION, a proteolytically processed peptide ligand, activating the expression of genes encoding secreted cell wall remodeling and hydrolase enzymes. hae hsl2 mutants fail to induce expression of these genes and retain floral organs indefinitely. Here, we report identification of an allelic series of hae hsl2 suppressor mutations in the SERK1 coreceptor protein kinase gene. Genetic and transcriptomic evidence indicates that these alleles represent a novel class of gain-of-function mutations that activate signaling independently of HAE/HSL2. We show that, surprisingly, the suppression effect does not rely on the protein kinase activity of SERK1 and that activation of signaling relies on the receptor-like kinase gene SUPPRESSOR OF BIR1 (SOBIR1). The effect of these mutations can be mimicked by loss of function of BAK1-INTERACTING RECEPTOR-LIKE KINASE1 (BIR1), a known negative regulator of SERK-SOBIR1 signaling. These results suggest that BIR1 negatively regulates SERK-SOBIR1 signaling during abscission and that the identified SERK1 mutations likely interfere with this negative regulation.

Genome-wide characterization of the auxin response factor (ARF) gene family of litchi (Litchi chinensisSonn.): phylogenetic analysis, miRNA regulation and expression changes during fruit abscission

Auxin response factors (ARFs) play fundamental roles in modulating various biological processes including fruit development and abscission via regulating the expression of auxin response genes. Currently, little is known about roles of ARFs in litchi (Litchi chinensis Sonn.), an economically important subtropical fruit tree whose production is suffering from fruit abscission. In this study, a genome-wide analysis of ARFs was conducted for litchi, 39 ARF genes (LcARFs) were identified. Conserved domain analysis showed that all the LcARFs identified have the signature B3 DNA-binding (B3) and ARF (Aux_rep) domains, with only 23 members having the dimerization domain (Aux_IAA). The number of exons in LcARF genes ranges from 2 to 16, suggesting a large variation for the gene structure of LcARFs. Phylogenetic analysis showed that the 39 LcARFs could be divided into three main groups: class I, II, and III. In total, 23 LcARFs were found to be potential targets of small RNAs, with three conserved and one novel miRNA-ARF (miRN43-ARF9) regulatory pathways discovered in litchi. Expression patterns were used to evaluate candidate LcARFs involved in various developmental processes, especially in flower formation and organ abscission. The results revealed that most ARF genes likely acted as repressors in litchi fruit abscission, that is, ARF2D/2E, 7A/7B, 9A/9B, 16A/16B, while a few LcARFs, such as LcARF5A/B, might be positively involved in this process. These findings provide useful information and resources for further studies on the roles of ARF genes in litchi growth and development, especially in the process of fruit abscission.