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Leaf senescence represents a key developmental process through which resources trapped in the photosynthetic organ are degraded in an organized manner and transported away to sustain the growth of other organs including newly forming leaves, roots, seeds, and fruits. The optimal timing of the initiation and progression of senescence are thus prerequisites for controlled plant growth, biomass accumulation, and evolutionary success through seed dispersal. Recent research has uncovered a multitude of regulatory factors including transcription factors, micro-RNAs, protein kinases, and others that constitute the molecular networks that regulate senescence in plants. The timing of senescence is affected by environmental conditions and abiotic or biotic stresses typically trigger a faster senescence. Various phytohormones, including for example ethylene, abscisic acid, and salicylic acid, promote senescence, whereas cytokinins delay it. Recently, several reports have indicated an involvement of auxin in the control of senescence, however, its mode of action and point of interference with senescence control mechanisms remain vaguely defined at present and contrasting observations regarding the effect of auxin on senescence have so far hindered the establishment of a coherent model. Here, we summarize recent studies on auxin-related genes that affect senescence in plants and highlight how these findings might be integrated into current molecular-regulatory models of senescence.
Cobalt (Co2+) inhibits vegetative growth of Lemna minor gradually from 1 mu M to 100 mu M. Fronds accumulated up to 21 mg Co2+ g(-1) dry weight at 10 mu M external Co2+ indicating hyperaccumulation. Interestingly, accumulation of Co2+ did not decrease the iron (Fe) content in fronds, highlighting L. minor as a suitable system for studying effects of Co2+ undisturbed by Fe deficiency symptoms unlike most other plants. Digital image analysis revealed the size distribution of fronds after Co2+ treatment and also a reduction in pigmentation of newly formed daughter fronds unlike the mother fronds during the 7-day treatment. Neither chlorophyll nor photosystem II fluorescence changed significantly during the initial 4 d, indicating effective photosynthesis. During the later phase of the 7-day treatment, however, chlorophyll content and photosynthetic efficiency decreased in the Co2+-treated daughter fronds, indicating that Co2+ inhibits the biosynthesis of chlorophyll rather than leading to the destruction of pre-existing pigment molecules. In addition, during the first 4 d of Co2+ treatment starch accumulated in the fronds and led to the transition of chloroplasts to chloro-amyloplasts and amylo-chloroplasts, while starch levels strongly decreased thereafter. (C) 2015 Elsevier Ltd. All rights reserved.