Uzelac Branka, Janošević Dušica, Simonović Ana, Motyka Václav, Dobrev Petre I, Budimir Snežana
Institute for Biological Research "Siniša Stanković", University of Belgrade, Bulevar despota Stefana 142, Belgrade, Serbia.
Institute of Botany and Botanical Garden "Jevremovac", Faculty of Biology, University of Belgrade, Takovska 43, Belgrade, Serbia.
Protoplasma. 2016 Mar;253(2):259-75. doi: 10.1007/s00709-015-0802-9. Epub 2015 Apr 3.
Leaf senescence is a highly regulated final phase of leaf development preceding massive cell death. It results in the coordinated degradation of macromolecules and the subsequent nutrient relocation to other plant parts. Very little is still known about early stages of leaf senescence during normal leaf ontogeny that is not triggered by stress factors. This paper comprises an integrated study of natural leaf senescence in tobacco plants grown in vitro, using molecular, structural, and physiological information. We determined the time sequence of ultrastructural changes in mesophyll cells during leaf senescence, showing that the degradation of chloroplast ultrastructure fully correlated with changes in chlorophyll content. The earliest degenerative changes in chloroplast ultrastructure coinciding with early chromatin condensation were observed already in mature green leaves. A continuum of degradative changes in chloroplast ultrastructure, chromatin condensation and aggregation, along with progressive decrease in cytoplasm organization and electron density were observed in the course of mesophyll cells ageing. Although the total amounts of endogenous cytokinins gradually increased during leaf ontogenesis, the proportion of bioactive cytokinin forms, as well as their phosphate precursors, in total cytokinin content rapidly declined with ageing. Endogenous indole-3-acetic acid (IAA) levels were strongly reduced in senescent leaves, and a decreasing tendency was also observed for abscisic acid (ABA) levels. Senescence-associated tobacco cysteine proteases (CP, E.C. 3.4.22) CP1 and CP23 genes were induced in the initial phase of senescence. Genes encoding glutamate dehydrogenase (GDH, E.C. 1.4.1.2) and one isoform of cytosolic glutamine synthetase (GS1, E.C. 6.3.1.2) were induced in the late stage of senescence, while chloroplastic GS (GS2) gene showed a continuous decrease with leaf ageing.
叶片衰老 是叶片发育过程中在大量细胞死亡之前受到高度调控的最后阶段。它导致大分子的协同降解以及随后营养物质向植物其他部位的重新分配。对于正常叶片个体发育过程中未受胁迫因素触发的叶片衰老早期阶段，人们仍然知之甚少。本文利用分子、结构和生理信息，对体外培养的烟草植株中的自然叶片衰老进行了综合研究。我们确定了叶片衰老过程中叶肉细胞超微结构变化的时间顺序，表明叶绿体超微结构的降解与叶绿素含量的变化完全相关。在成熟绿叶中已经观察到与早期染色质浓缩同时出现的叶绿体超微结构最早的退行性变化。在叶肉细胞衰老过程中，观察到叶绿体超微结构、染色质浓缩和聚集的一系列降解变化，以及细胞质组织和电子密度的逐渐降低。尽管内源细胞分裂素的总量在叶片个体发育过程中逐渐增加，但生物活性细胞分裂素形式及其磷酸前体在总细胞分裂素含量中的比例随着衰老而迅速下降。衰老叶片中内源吲哚 - 3 - 乙酸（IAA）水平大幅降低，脱落酸（ABA）水平也呈现下降趋势。衰老相关的烟草半胱氨酸蛋白酶（CP，E.C. 3.4.22）CP1和CP23基因在衰老初期被诱导。编码谷氨酸脱氢酶（GDH，E.C. 1.4.1.2）和一种胞质谷氨酰胺合成酶同工型（GS1，E.C. 6.3.1.2）的基因在衰老后期被诱导，而叶绿体GS（GS2）基因随着叶片衰老持续下降。
