No abstract

The abscission process is initiated by changes in the auxin gradient across the abscission zone (AZ) and is triggered by ethylene. Although changes in gene expression have been correlated with the ethylene-mediated execution of abscission, there is almost no information on the molecular and biochemical basis of the increased AZ sensitivity to ethylene. We examined transcriptome changes in the tomato (Solanum lycopersicum 'Shiran 1335') flower AZ during the rapid acquisition of ethylene sensitivity following flower removal, which depletes the AZ from auxin, with or without preexposure to 1-methylcyclopropene or application of indole-3-acetic acid after flower removal. Microarray analysis using the Affymetrix Tomato GeneChip revealed changes in expression, occurring prior to and during pedicel abscission, of many genes with possible regulatory functions. They included a range of auxin-and ethylene-related transcription factors, other transcription factors and regulatory genes that are transiently induced early, 2 h after flower removal, and a set of novel AZ-specific genes. All gene expressions initiated by flower removal and leading to pedicel abscission were inhibited by indole-3-acetic acid application, while 1-methylcyclopropene pretreatment inhibited only the ethylene-induced expressions, including those induced by woundassociated ethylene signals. These results confirm our hypothesis that acquisition of ethylene sensitivity in the AZ is associated with altered expression of auxin-regulated genes resulting from auxin depletion. Our results shed light on the regulatory control of abscission at the molecular level and further expand our knowledge of auxin-ethylene cross talk during the initial controlling stages of the process.

Abscission occurs specifically in the abscission zone (AZ) tissue as a natural stage of plant development. Previously, we observed delay of tomato (Solanum lycopersicum) leaf abscission when the LX ribonuclease (LX) was inhibited. The known association between LX expression and programmed cell death (PCD) suggested involvement of PCD in abscission. In this study, hallmarks of PCD were identified in the tomato leaf and flower AZs during the late stage of abscission. These included loss of cell viability, altered nuclear morphology, DNA fragmentation, elevated levels of reactive oxygen species and enzymatic activities, and expression of PCD-associated genes. Overexpression of antiapoptotic proteins resulted in retarded abscission, indicating PCD requirement. PCD, LX, and nuclease gene expression were visualized primarily in the AZ distal tissue, demonstrating an asymmetry between the two AZ sides. Asymmetric expression was observed for genes associated with cell wall hydrolysis, leading to AZ, or associated with ethylene biosynthesis, which induces abscission. These results suggest that different abscission-related processes occur asymmetrically between the AZ proximal and distal sides. Taken together, our findings identify PCD as a key mechanism that occurs asymmetrically during normal progression of abscission and suggest an important role for LX in this PCD process.

Abscission zone (AZ) development and the progression of abscission (detachment of plant organs) have been roughly separated into four stages: first, AZ differentiation; second, competence to respond to abscission signals; third, activation of abscission; and fourth, formation of a protective layer and post-abscission trans-differentiation. Stage three, activation of abscission, is when changes in the cell wall and extracellular matrix occur to support successful organ separation. Most abscission research has focused on gene expression for enzymes that disassemble the cell wall within the AZ and changes in phytohormones and other signaling events that regulate their expression. Here, transcriptome data for soybean, tomato and Arabidopsis were examined and compared with a focus not only on genes associated with disassembly of the cell wall but also on gene expression linked to the biosynthesis of a new extracellular matrix. AZ-specific up-regulation of genes associated with cell wall disassembly including cellulases (beta-1,4-endoglucanases, CELs), polygalacturonases (PGs), and expansins (EXPs) were much as expected; however, curiously, changes in expression of xyloglucan endotransglucosylase/hydrolases (XTHs) were not AZ-specific in soybean. Unexpectedly, we identified an early increase in the expression of genes underlying the synthesis of a waxy-like cuticle. Based on the expression data, we propose that the early up-regulation of an abundance of small pathogenesis-related (PR) genes is more closely linked to structural changes in the extracellular matrix of separating cells than an enzymatic role in pathogen resistance. Furthermore, these observations led us to propose that, in addition to cell wall loosening enzymes, abscission requires (or is enhanced by) biosynthesis and secretion of small proteins (15–25 kDa) and waxes that form an extensible extracellular matrix and boundary layer on the surface of separating cells. The synthesis of the boundary layer precedes what is typically associated with the post-abscission synthesis of a protective scar over the fracture plane. This modification in the abscission model is discussed in regard to how it influences our interpretation of the role of multiple abscission signals.

Number of figures: 1 Word count: Re-evaluation of the ethylene-dependent and-independent pathways in the regulation of organ abscission focusing on model plants of flower abscission Highlights: Ethylene is the key regulator of organ abscission, while the IDA-HAE-HSL2 pathway acts downstream of ethylene signaling. The involvement of the turgor pressure in the execution of abscission is suggested. .

A newly developed rapid and convenient method was used for fractionation and analysis of fluorescent compounds (FCs) formed during lipid peroxidation in parsley (Petroselinum crispum Mill.) leaves. These lipofuscin-like FCs [which arise in vivo from reaction of malondialdehyde (MDA) with amino acids] were found to increase during the senescence of detached parsley leaves, following the commencement of chlorophyll degradation and proteolysis. However, accumulation of FCs in response to exogenous ethylene coincided with the onset of chlorophyll loss and proteolysis on day 2 and was accelerated markedly later. Unlike FC accumulation, levels of aldehydes and MDA in control leaves increased more drastically during senescence, but were not affected significantly by exogenous ethylene. The results suggest that the accumulation of FCs in detached parsley leaves exposed to exogenous ethylene is an early senescence-associated process.

ABSTRACISince 14malonylamino)cyclopropane-l-carboxylic acid (MACC), the major conjugate of 1-aminocyclopropane-l-carboxylic acid (ACC) in plant tissues, is a poor ethylene producer, it is generally thought that MACC is a biologically inactive end product of ACC. In the present study we ACC2 is the immediate precursor of ethylene, which is synthesized via the following sequence in higher plants: Methionine -* SAM --+ ACC --ethylene (1, 24). In addition to serving as a precursor for ethylene, ACC can also be widely metabolized to the stable conjugate, MACC (3,6,7,11,13,16,19,22,23). This conversion is thought to participate in the regulation of ethylene biosynthesis by removing excess ACC (24). Since MACC is a poor ethylene precursor and the conjugation of ACC to MACC is essentially irreversible, MACC is generally thought to be a biologically inactive end product of ACC rather than a storage pool for ACC and hence for ethylene (3,7,8,25). MACC was found to accumulate when high rates of ACC synthesis were induced by water deficit (8) or other factors (3,6,20,23). However, a decrease in MACC content was reported so far only in two systems: in the second node of pea plants (6) (15). Seedling leaves from various ages were used. In one experiment leaf discs from lettuce (Lactuca sativa L.) and soybean (Glycine max L.), root segments of radish (Raphanus sativus L.) and carrot (Daucus carota L.) and fruit plugs of banana (Musa acuminata Colla) and chinese gooseberry (Actinidia chinensis) were used. All fruits and vegetables were purchased from local market.

Subjecting some chilling-sensitive commodities to various moderate stresses can cause them to acquire resistance to chilling injury. Jasrnonates have been implicated in playing an integral role in the signal transduction cascade that operates in plants to induce responses to stress. Therefore, it was hypothesized that exogenous application of jasrnonic acid or methyl jasmonate to chilling-sensitive commodities might replace the moderate stress treatment and reduce their chilling symptoms. To test this hypothesis, three chilling-susceptible fruits, avocado (Persea americana Mill., cvs. Hass, Etinger, and Fuerte), grapefruit (Citrus paradisi cv. Marsh seedless), and red bell pepper (Capsicum annum cv. Maor), were treated with various concentrations of methyl jasmonate prior to 4-10 weeks of storage at 2°C. Results show that methyl jasmonate dipping (for 30 s) of avocado 'Fuerte' and 'Hass' (2.5 pM), avocado 'Etinger' or grapefruit (10 pM), and red bell pepper fruits (25 pM) significantly reduced both the severity of their chilling injury symptoms and the percentage of injured fruits. Application of methyl jasrnonate by gassing for 24 h was similarly effective. In avocado 'Etinger', methyl jasmonate dipping was effective in alleviating chilling injury either after immediate storage at 2OC or after the fruits had been held for 1 day at 20°C prior to their storage at 2OC. The results suggest that methyl jasmonate, which is receiving renewed biological interest as a potentially important signaling molecule in plants, might mediate the plant's natural response to chilling stress, and by its application might provide a simple means to reduce chilling injuries in chilling-susceptible commodities. Risumi: Le fait de soumettre certaines denrtes sensibles au froid B divers stress modtrCs peut les arnener B dCvelopper une rtsistance aux dommages par le froid. Les auteurs ont formulC l'hypothkse qu'une application exogene d'acide jasmonique, ou de jasmonate de rnCthyl, B des denrCes sensibles aux dommages par le froid, pourrait remplacer le traitement par des stress modtrts et rCduire les syrnptBrnes de refroidissement. Afin de vtrifier cette hypothese, trois fruits sensibles au froid, l'avocado (Persea americana Mill., cvs. Hass, Etinger et Fuerte), le pamplemousse (Citrus paradisi cv. Marsh seedless), et le pirnent doux rouge (Capsicum annum cv. Maor), ont Ctt traitCs avec diverses concentrations de jasmonate de mtthyl, avant de les conserver pendant 4-10 sernaines B 2°C. Les rtsultats montrent que l'imrnersion dans le jasmonate de mtthyl (pendant 30 s) des fruits d'avocados 'Fuerte' et 'Haas' (2,5 pM), d'avocado 'Etinger' ou de pamplemousse (10 pM), et de pirnent doux rouge (25 pM) rtduit significativement la gravitC des symptBmes de refroidissement et le pourcentage des fruits endommagks. L'application du jasmonate de rntthyl sous forme gazeuse pendant 24 h est Cgalement efficace. Chez l'avocado 'Etinger' l'imrnersion dans le jasmonate de methyl est efficace B prCvenir les dommages par le froid, qu'elle soit suivie irnrntd...