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A mechanistic overview of abscission signaling is presented to provide an easy entry point into the exciting field of research on how plants control shedding of organs. Abstract Abscission is a process in plants for shedding unwanted organs such as leaves, flowers, fruits, or floral organs. Shedding of leaves in the fall is the most visually obvious display of abscission in nature. The very shape plants take is forged by the processes of growth and abscission. Mankind manipulates abscission in modern agriculture to do things such as prevent pre-harvest fruit drop prior to mechanical harvesting in orchards. Abscission occurs specifically at abscission zones that are laid down as the organ that will one day abscise is developed. A sophisticated signaling network initiates abscission when it is time to shed the unwanted organ. In this article, we review recent advances in understanding the signaling mechanisms that activate abscission. Physiological advances and roles for hormones in abscission are also addressed. Finally, we discuss current avenues for basic abscission research and potentially lucrative future directions for its application to modern agriculture.

The periclinal walls of cambial cells in neighboring lineages (rows) may not be parallel when viewed in their radial aspect. This lack of longitudinal parallelism may be so extensive that in active cambium pairs of cells from neighboring rows may be in contact only along restricted segments. This means that the initial cells, rather than farming a continuous layer, may be arranged in an irregular network pattern from which some parts project inward or outward from the layer of their mutual cantacts. The longitudinal nonparallelism of cambial cells becomes more pronounced during symplasitic radial growth. Unequal periclinal divisions counteract this, and in initial cells abscission of the parts projecting from the layer of mutual contact occurs. When the cambium passes from a period of activity to a Period of rest a continuous layer of initials is reestabhshed. This involves elongation by intrusive growth of those cells previously shortened as the result of irregular periclinal divisions. The division walls in cambial cells may be warped, that is they change their orientation along the longitudinal direction perhaps even similar to an aircraft propeller. A division wall may thus be periclinal in one part of the cell and anticlinal in another.

Abscission is a developmental process with important implications for agricultural practices. Ethylene has long been considered as a key regulator of the abscission process. The existence of an ethylene-independent abscission pathway, controlled by the complex of INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) peptide and the HAESA (HAE) and HAESA-like2 (HSL2) kinases, has been proposed, based mainly on observations that organ abscission in ethylene- insensitive mutants was delayed but not inhibited. A recent review on plant organ abscission signaling high-lighted the IDA–HAE–HSL2 components as the regulators of organ abscission, while the role of auxin and ethylene in this process was hardly addressed. After a careful analysis of the relevant abscission literature, we propose that the IDA–HAE–HSL2 pathway is essential for the final stages of organ abscission, while ethylene plays a major role in its initiation and progression. We discuss the view that the IDA–HAE–HSL2 pathway is ethylene independent, and present recent evidence showing that ethylene activates the IDA–HAE–HSL2 complex. We conclude that the ability of an organ to abscise is tightly linked to cell turgidity in the abscission zone, and suggest that lack of cell turgidity might contribute to the failure of floral organ abscission in the ida mutants.

Abscission is a cell separation process by which plants can shed organs such as fruits, leaves, or flowers. The process takes place in specific locations termed abscission zones. In fruit crops like citrus, fruit abscission represents a high percentage of annual yield losses. Thus, understanding the molecular regulation of abscission is of capital relevance to control production. To identify genes preferentially expressed within the citrus fruit abscission zone (AZ-C), we performed a comparative transcriptomics assay at the cell type resolution level between the AZ-C and adjacent fruit rind cells (non-abscising tissue) during ethylene-promoted abscission. Our strategy combined laser microdissection with microarray analysis. Cell wall modification-related gene families displayed prominent representation in the AZ-C. Phylogenetic analyses of such gene families revealed a link between phylogenetic proximity and expression pattern during abscission suggesting highly conserved roles for specific members of these families in abscission. Our transcriptomic data was validated with (and strongly supported by) a parallel approach consisting on anatomical, histochemical and biochemical analyses on the AZ-C during fruit abscission. Our work identifies genes potentially involved in organ abscission and provides relevant data for future biotechnology approaches aimed at controlling such crucial process for citrus yield.

The HAESA (HAE/HSLs) family of leucine-rich repeat receptor-like kinases (LRR-RLKs) represents a central signaling hub and showed high conservation in multiple plants, integrating developmental cues and environmental stresses. Initially characterized for their pivotal role in floral organ abscission through the IDA-HAE/HSL2 ligand–receptor module, subsequent studies have revealed broader functions in root development, seed longevity, stomatal regulation, and responses to both biotic and abiotic stresses. Structural and genetic analyses have uncovered complex regulatory layers, including membrane trafficking, endoplasmic reticulum output control, co-receptor recruitment, and MAPK cascade activation, all of which ensure precise spatiotemporal control of abscission and related processes. Importantly, the multi-functionality of the HAE family proteins reflects a dynamic trade-off between growth and defense, highlighting its role in balancing resource allocation under complex environments. Overall, the HAESA family not only provides fundamental insights into plant signaling networks but also holds potential as a key target for future crop improvement and sustainable agriculture.

The activity of transglutaminase (TGase), an enzyme responsible for polyamine conjugation to proteins, was analyzed in relationship to developmental cell death (DCD) during the flower life span stages of the tobacco (Nicotiana tabacum) corolla. As the DCD exhibits an acropetal gradient, TGase was studied in corolla proximal, medial, and distal parts. TGase was immunorecognized by three TGase antibodies; the main 58-kD band decreased during corolla life, whereas a 38-kD band localized progressively from basal to distal parts. The former was present in the soluble, microsomal, plastidial (together with the 38-kD band), and cell wall fractions. The endogenous TGase activity increased during DCD reaching a maximum soon after the corolla opening. The activity maximum shifted from proximal to distal part, preceding the DCD acropetal pattern. A similar activity increase was observed by the exogenous TGase substrate (histidine(6)-Xpr-green fluorescent protein). Subcellular activities were detected in (1) the microsomes, where TGase activity is in general higher in the proximal part, peaking at the corolla opening; (2) the soluble fraction, where it is present only in the proximal part at senescence; (3) the plastids, where it shows an increasing trend; and (4) cell walls, prevailing in the distal part and progressively increasing. These data suggest a relationship between DCD and TGase; the latter, possibly released in the cell wall through the Golgi vesicles, could cooperate to cell wall strengthening, especially at the abscission zone and possibly during corolla shape change. The plastid TGase, stabilizing the photosystems, could sustain the energy requirements for the senescence progression.

Key messageRbIDL1 and RbIDL4 are up-regulated in an ethylene-responsive manner during rose petal abscission and restored the Arabidopsis ida-2 mutant abscission defect suggesting functional conservation of the IDA pathway in rose.Abscission is an ethylene-regulated developmental process wherein plants shed unwanted organs in a controlled manner. The INFLORESCENCE DEFICIENT IN ABSCISSION family has been identified as a key regulator of abscission in Arabidopsis, encoding peptides that interact with receptor-like kinases to activate abscission. Loss of function ida mutants show abscission deficiency in Arabidopsis. Functional conservation of the IDA pathway in other plant abscission processes is a matter of interest given the discovery of these genes in several plants. We have identified four members of the INFLORESCENCE DEFICIENT IN ABSCISSION-LIKE family from the ethylene-sensitive, early-abscising fragrant rose, Rosa bourboniana. All four are conserved in sequence and possess well-defined PIP, mIDa and EPIP motifs. Three of these, RbIDL1, RbIDL2 and RbIDL4 show a three–fourfold increase in transcript levels in petal abscission zones (AZ) during ethylene-induced petal abscission as well as natural abscission. The genes are also expressed in other floral tissues but respond differently to ethylene in these tissues. RbIDL1 and RbIDL4, the more prominently expressed IDL genes in rose, can complement the abscission defect of the Arabidopsis ida-2 mutant; while, promoters of both genes can drive AZ-specific expression in an ethylene-responsive manner even in Arabidopsis silique AZs indicating recognition of AZ-specific and ethylene-responsive cis elements in their promoters by the abscission machinery of rose as well as Arabidopsis.

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.

Plant organ abscission is a fundamental biological process. and is a stress and evolutionary mechanism formed to adapt to environmental changes due to external signal stimulation or age development. Flower, leaf and fruit abscission are common in production, not only in vivo plants but also in in vitro culture. Breakthroughs have been made in organ abscission studies for model plants and crops in vivo plants, but little is known about organ abscission in in vitro culture, which is a complex biological process. This paper reviews the organ abscission mechanism from the perspectives of cell histology, physiological biochemistry and molecular biology and looks forward to organ abscission research, which aims to fully clarify the plant organ abscission mechanism and provide theoretical and technical guidance for the normal/abnormal abscission of plant organs in actual production.