We have examined the possibility that modulation of [Ca2+]cyt may represent a signal which induces senescence processes in leaves, through triggering of lipid hydrolysis leading to the cascade of detriorative events. Characterization of the signal transduction components operating during leaf senescence was gained by studying various Ca2+-dependent activities of parsley and chrysanthemum leaves, in relation to several senescence functions, and in response to senescence-modulating hormones (ethylene,ABA, BA and IAA). Some innovative findings regarding the control of senescence processes by [Ca2+]cyt were established: Several Ca2+-or CaM-related compounds were shown to modulate [Ca2+]cyt and action, thereby affecting whole leaf senescence. The involvement of [Ca2+]cyt in mediating the effects of senescence-modulating hormones has been demonstrated. Loss of energized Ca2+-transport capability of PM was found to an early event in leaf senescence, which occurs before changes in senescence parameters are observed, and while other PM ATPase enzymes still retain about 50% of their activities. A general pattern of increased phosphorylation of PM proteins with advanced senescence, which could be modified by plant hormones applied in vivo (BA) or in vitro (ABA), sa found. Taken together, all this indirect evidence indicate that [Ca2+]cyt is elevated due to the senescence-induced decrease in the ability to extrude Ca2+, which results particularly from reduced PM Ca2++-transport capability rather than increased operation of Ca2+ channels or elevated Ins(1,4,5)P3 levels. The direct proof for such a senescence-related elevation in [Ca2+]cyt was provided for the first time by the Ca2+ imaging measures with fura-2, showing a rise in [Ca2+]cyt of mesophyll cells upon senescence induction, which preceeded changes in typical senescence characteristics. This research provides strong evidence for regarding the rise in [Ca2+]cyt as a primary event in induction of the senescence syndrome in detached leaves. The findings have also broad implications for postharvest handling of leafy crops and ornamentals, and open new avenues for employing Ca2+-related inhibitors to delay leaf senescence.
Signal Transduction in Leaf Senescence: Progress and Perspective