ESCRT function in cytokinesis: location, dynamics and regulation by mitotic kinases

Mammalian cytokinesis proceeds by constriction of an actomyosin ring and furrow ingression, resulting in the formation of the midbody bridge connecting two daughter cells. At the centre of the midbody resides the Flemming body, a dense proteinaceous ring surrounding the interlocking ends of anti-parallel microtubule arrays. Abscission, the terminal step of cytokinesis, occurs near the Flemming body. A series of broad processes govern abscission: the initiation and stabilisation of the abscission zone, followed by microtubule severing and membrane scission—The latter mediated by the endosomal sorting complex required for transport (ESCRT) proteins. A key goal of cell and developmental biologists is to develop a clear understanding of the mechanisms that underpin abscission, and how the spatiotemporal coordination of these events with previous stages in cell division is accomplished. This article will focus on the function and dynamics of the ESCRT proteins in abscission and will review recent work, which has begun to explore how these complex protein assemblies are regulated by the cell cycle machinery

Anatomy of ethylene-induced floral-organ abscission in Chamelaucium uncinatum (Myrtaceae)

Postharvest abscission of Geraldton waxflower (Chamelaucium uncinatum Schauer) flower buds and flowers is ethylene-mediated. Exposure of floral organs to exogenous ethylene (1 mu L L-1) for 6 h at 20 degrees C induced separation at a morphologically and anatomically distinct abscission zone between the pedicel and. oral tube. Flower buds with opening petals and flowers with a nectiferous hypanthium were generally more responsive to exogenous ethylene than were flower buds enclosed in shiny bracteoles and aged (senescing) flowers. The anatomy of abscission-zone cells did not change at sequential stages of floral development from immature buds to aged flowers. The zone comprised a layer of small, laterally elongated-to-rounded, closely packed and highly protoplasmic parenchyma cells. Abscission occurred at a two- to four-cell-wide separation layer within the abscission zone. The process involved degradation of the middle lamella between separation layer cells. Following abscission, cells on both the proximal and distal faces of the separation layer became spherical, loosely packed and contained degenerating protoplasm. Central vascular tissues within the surrounding band of separation layer cells became torn and fractured. For flower buds, bracteoles that enclose the immature floral tube also separated at an abscission zone. However, this secondary abscission zone appeared less sensitive to ethylene than the primary ( central). oral-tube abscission zone as bracteoles generally only completely abscised when exposed to 10 mu L L-1 ethylene for the longer period of 24 h at 20 degrees C. The smooth surfaces of abscised separation-layer cells suggest that hydrolase enzymes degrade the middle lamella between adjacent cell walls

The role of ethylene in fruit and petal abscission in the red raspberry (Rubus idaeus L. cv. Glen Clova)

Weakening of fruit and petal abscission zones in Rubus idaeus L. cv. Glen Clova was accompanied by increased rates of ethylene production. Both processes were accelerated by a supply of exogenous ethylene. In the ripe fruit natural ethylene levels were saturating. The rise in ethylene production clearly preceded petal abscission but in fruit the increase virtually coincided with the start of weakening. Raspberry fruit of other varieties and blackberries clearly showed the abscission zone weakening could precede any increase in ethylene production. The internal ethylene concentrations of Glen Clova fruit at the mottled stage reached those levels which had to be added to stimulate abscission (ie 0.25 to 0.5 ppm). This is the very stage at which abscission zone weakening was first noticeable. Both fruit and petal abscission was retarded by the application of inhibitors of ethylene production ( AVG, Co 2+) or action (Ag+ ). Likewise a reduction in the internal ethylene under hypobaric pressure also retarded fruit abscission. None of these treatments were totally capable of preventing abscission. In fruit abscission the receptacle appears to have an important role. The increase in receptacle ethylene production precedes that of the drupelets. The enlargement and swelling of the receptacle tissues are important in both abscission zone weakening and ethylene production. This receptacle development may in turn be controlled by the development of fertilised drupelets. The ethylene production in both fruit and petal abscission is limited initially by the supply of ACC. In both cases endogenous ACC levels increase in step with ethylene production. Ethylene's role as a coordinating/accelerating agent in fruit and petal abscission is discussed

Peranan Auksin dalam Usaha Menekan Kelayuan Buah Muda Kakao (Theobroma Kakao L.)

In the cocoa tree, the genes for producing an abscission zone are in pedicles. The process is hormonally driven by ethylene whilst auxins apparently decrease the sensitivity to ethylene. When the level of auxin declones, a special layer of cells (the abscission layer) is formed at the base of petiole or fruit stalk. Ethylene stimulates the production of enzimes that degrade the middle lamella beetween cells in the abscission zone. The concept development so far is correct that should be possible to delay abscission by aplication of sinthetic auxins. Aplication of compound enhance levels of auxins or decrease absicic acid and /or ethylene. Such conditions may inhibit young fruit abscission and promote fruit development

Ethylene-induced differential gene expression during abscission of citrus leaves

The main objective of this work was to identify and classify genes involved in the process of leaf abscission in Clementina de Nules (Citrus clementina Hort. Ex Tan.). A 7 K unigene citrus cDNA microarray containing 12 K spots was used to characterize the transcriptome of the ethylene-induced abscission process in laminar abscission zone-enriched tissues and the petiole of debladed leaf explants. In these conditions, ethylene induced 100% leaf explant abscission in 72 h while, in air-treated samples, the abscission period started later and took 240 h. Gene expression monitored during the first 36 h of ethylene treatment showed that out of the 12 672 cDNA microarray probes, ethylene differentially induced 725 probes distributed as follows: 216 (29.8%) probes in the laminar abscission zone and 509 (70.2%) in the petiole. Functional MIPS classification and manual annotation of differentially expressed genes highlighted key processes regulating the activation and progress of the cell separation that brings about abscission. These included cell-wall modification, lipid transport, protein biosynthesis and degradation, and differential activation of signal transduction and transcription control pathways. Expression data associated with the petiole indicated the occurrence of a double defensive strategy mediated by the activation of a biochemical programme including scavenging ROS, defence and PR genes, and a physical response mostly based on lignin biosynthesis and deposition. This work identifies new genes probably involved in the onset and development of the leaf abscission process and suggests a different but co-ordinated and complementary role for the laminar abscission zone and the petiole during the process of abscission

A petal breakstrength meter for Arabidopsis abscission studies

BACKGROUND: Abscission is the regulated dropping of plant organs, such as leaves or flower petals. This process involves a break down of the cell wall between layers of cells in the abscission zone, causing the organ to become detached. The model plant Arabidopsis thaliana undergoes floral organ abscission. Various experimental methods have been used to study Arabidopsis floral organ abscission, including measuring the petal breakstrength, or the amount of force required to pull a petal from the receptacle. Petal breakstrength provides a quantitative insight into the physical integrity of the petal abscission zone. RESULTS: We developed a petal breakstrength meter that allows rapid data acquisition on a personal computer. We present the design of the device and show its utility in measuring Arabidopsis petal breakstrength for abscission studies. CONCLUSION: This petal breakstrength meter should enable researchers to perform the petal breakstrength assay as a routine part of the characterization of environmental and genetic factors affecting abscission

Light and hormonal control of leaf abscission.

The mechanism of light control of leaf abscission was studied in coleus (Coleus blumei Benth.) and mung bean (Vigna radiata (L.) Wilczek) cuttings by evaluating the roles of IAA, carbohydrate, and cellulase in the process of abscission. Treatment of the cuttings with red light maintained high break strength in the abscission zone in both tested plants, whereas treatment with far-red light initiated a dramatic increase in abscission. Leaf diffusate collected from red light treated leaves contained 3 times more IAA than that collected from leaves maintained in the dark or treated with far-red light. Treatment of the cuttings with 2,3,5-triiodobenzoic acid (TIBA), an auxin transport inhibitor, eliminated the inhibitory effect of red light on abscission. No significant difference in IAA oxidase activity was detected among the light treatments. The rate of decarboxylation of IAA in leaf tissue treated with red light, however, was higher than in the leaves receiving dark or far-red light. Conversion of tryptophan to IAA was higher in red light-treated leaf blades than in leaves maintained in dark or treated with far-red light. Treatment of mung bean leaf cuttings with red or far-red light, produced no significant differences in the content of fructose 6-phosphate, glucose plus fructose, sucrose, total soluble sugars, or starch. A higher level of glucose 6-phosphate was detected in red light-treated leaves. Loading the tissue with sucrose, a sugar reported to affect organ abscission, did not change the effect of light on abscission. The differences in the rate of IAA synthesis observed in leaf tissue under various light treatments is proposed as the controlling mechanism for light regulation of abscission. Total sugar or starch content in the leaf blade was not related to leaf abscission, indicating that photoassimilate in leaf tissue was not involved in light regulation of abscission. While cellulase activity in the abscission zone accounts for the decrease in break strength of abscission zone, the enzyme per se was not affected by light

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

Distribution of xylem hydraulic resistance in fruiting truss of tomato influenced by water stress

In this study xylem hydraulic resistances of peduncles (truss stalk), pedicels (fruit stalk) and the future abscission zone (AZ) halfway along the pedicel of tomato (Lycopersicon esculentum L.) plants were directly measured at different stages of fruit development, in plants grown under two levels of water availability in the root environment. The xylem hydraulic connection between shoot and fruits has previously been investigated, but contradictory conclusions were drawn about the presence of a flow resistance barrier in the pedicel. These conclusions were all based on indirect functional measurements and anatomical observations of water-conducting tissue in the pedicel. In the present study, by far the largest resistances were measured in the AZ where most individual vessels ended. Plants grown at low water availability in the root environment had xylem with higher hydraulic resistances in the peduncle and pedicel segments on both sides of the AZ, while the largest increase in hydraulic resistance was measured in the AZ. During fruit development hydraulic resistances in peduncle and pedicel segments decreased on both sides of the AZ, but tended to increase in the AZ. The overall xylem hydraulic resistance between the shoot and fruit tended to increase with fruit development because of the dominating role of the hydraulic resistance in the AZ. It is discussed whether the xylem hydraulic resistance in the AZ of tomato pedicels in response to water stress and during fruit development contributes to the hydraulic isolation of fruits from diurnal cycles of water stress in the shoot