Kim Joonyup, Sundaresan Srivignesh, Philosoph-Hadas Sonia, Yang Ronghui, Meir Shimon, Tucker Mark L
Soybean Genomics and Improvement Lab, Agricultural Research Service, United States Department of Agriculture Beltsville, MD, USA.
Department of Postharvest Science of Fresh Produce, Agricultural Research Organization, The Volcani Center Bet-Dagan, Israel ; The Robert H. Smith Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem Rehovot, Israel.
Front Plant Sci. 2015 Dec 15;6:1109. doi: 10.3389/fpls.2015.01109. eCollection 2015.
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.
离区（AZ）的发育以及脱落（植物器官分离）的进程大致可分为四个阶段：第一，AZ分化；第二，对脱落信号作出响应的能力；第三，脱落激活；第四，保护层的形成以及脱落后的转分化。第三阶段，即脱落激活阶段，此时细胞壁和细胞外基质会发生变化以支持器官的成功分离。大多数脱落研究聚焦于在离区内分解细胞壁的酶的基因表达，以及调节这些酶表达的植物激素和其他信号事件的变化。在此，对大豆、番茄和拟南芥的转录组数据进行了研究，并进行了比较，不仅关注与细胞壁分解相关的基因，还关注与新细胞外基质生物合成相关的基因表达。与细胞壁分解相关的基因，包括纤维素酶（β-1,4-内切葡聚糖酶，CELs）、多聚半乳糖醛酸酶（PGs）和扩展蛋白（EXPs）在AZ中特异性上调，这正如预期；然而，奇怪的是，木葡聚糖内转糖基酶/水解酶（XTHs）的表达变化在大豆中并非AZ特异性的。出乎意料的是，我们发现了蜡质样角质层合成相关基因表达的早期增加。基于表达数据，我们提出，大量小病程相关（PR）基因的早期上调与分离细胞细胞外基质的结构变化更为密切相关，而非在病原体抗性中起酶促作用。此外，这些观察结果使我们提出，除了细胞壁松弛酶外，脱落还需要（或因）小蛋白质（15 - 25 kDa）和蜡质的生物合成与分泌，这些物质在分离细胞表面形成可扩展的细胞外基质和边界层。边界层的合成先于通常与断裂平面上保护性疤痕的脱落后合成相关的过程。关于这种脱落模型的修改如何影响我们对多种脱落信号作用的解释，本文进行了讨论。
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