Plant senescence

About: Plant senescence is a research topic. Over the lifetime, 401 publications have been published within this topic receiving 17189 citations. The topic is also known as: senescence in plants.

Grain filling of cereals under soil drying.

Abstract: Monocarpic plants require the initiation of whole-plant senescence to remobilize and transfer assimilates pre-stored in vegetative tissues to grains. Delayed whole-plant senescence caused by either heavy use of nitrogen fertilizer or adoption of lodging-resistant cultivars/hybrids that remain green when the grains are due to ripen results in a low harvest index with much nonstructural carbohydrate (NSC) left in the straw. Usually, water stress during the grain-filling period induces early senescence, reduces photosynthesis, and shortens the grain-filling period; however, it increases the remobilization of NSC from the vegetative tissues to the grain. If mild soil drying is properly controlled during the later grain-filling period in rice (Oryza sativa) and wheat (Triticum aestivum), it can enhance whole-plant senescence, lead to faster and better remobilization of carbon from vegetative tissues to grains, and accelerate the grain-filling rate. In cases where plant senescence is unfavorably delayed, such as by heavy use of nitrogen and the introduction of hybrids with strong heterosis, the gain from the enhanced remobilization and accelerated grain-filling rate can outweigh the loss of reduced photosynthesis and the shortened grain-filling period, leading to an increased grain yield, better harvest index and higher water-use efficiency.

The role of free radicals in senescence and wounding.

Abstract: Reactions involving free radicals are an inherent feature of plant senescence and appear to contribute to a process of oxidative deterioration that leads ultimately to cell death. Radical species derived from molecular oxygen are the primary mediators of this oxidative damage, but non-radical excited states of oxygen, specifically singlet oxygen, may also be involved. Several lines of evidence suggest that degradation of lipids in senescing membranes and the ensuing release of free fatty acids initiate oxidative deterioration by providing substrate for lipoxygenase. In some tissues, lipoxygenase activity increases with advancing senescence in a pattern that is consistent with its putative role in promoting oxidative damage. However, there are important exceptions to this which may be explained by the fact that the timing and extent of peroxidative reactions initiated by lipoxygenase are likely to be determined more by the availability of substrate for the enzyme than by changes in its activity. There are both membranous and cytosolic forms of lipoxygenase in senescing tissues, and peroxidation of membrane lipids appears to be initiated by the membranous enzyme once the appropriate fatty acid substrates, linoleic acid and linolenic acid, become available. Since lipid peroxidation is known to form alkoxy and peroxy radicals as well as singlet oxygen, these reactions in membrane bilayers are probably a major source of activated oxygen species in senescing tissues. Further-more, there are indications that activated oxygen from the lipoxygenase reaction can become substrate for the cytosolic form of the enzyme which, in turn, may raise the titre of activated oxygen during senescence. Additional possible sources of increased free radical production in senescing tissues include peroxidase, which shows greatly increased activity with advancing age, leakage of electrons from electron transport systems to oxygen, in particular from the photosynthetic electron transport system, and decompartmentalization of iron, which would facilitate formation of the highly reactive hydroxyl radical from the less reactive superoxide anion. A variety of macromolecules can be damaged by activated oxygen. Unsaturated fatty acids are especially prone to attack, and this implies that membranes are primary targets of free radical damage. The manifestations of this damage in senescing tissues range from altered membrane fluidity and phase properties to leakiness that can be attributed to a destabilized and highly perturbed membrane bilayer. There is also a progressive breakdown of cellular protein with advancing senescence. Free radicals can inactivate proteins by reacting with specific amino acid residues, and a number of in zitro studies have indicated that such alteration renders the proteins more prone to hydrolysis by proteases. Thus, although there is no direct evidence linking enhanced proteolysis during senescence to free radical damage, there is reason to believe that this may be a contributing factor. Wounding of certain plant tissues also initiates a series of reactions that revolve around the breakdown of membrane lipids and their peroxidation. Indeed, as in the case of senescence, membrane deterioration follokving wounding appears to be facilitated by a self-perpetuating wave of free radical production emanating from peroxidation within the lipid bilayer. There is also recent evidence for activation of an O2- -producing NADPH oxidase in plant tissues following fungal infection that may be analogous to the well-characterized O2- -generating NADPH oxidase associated with the plasma membrane of polymorphonuclear leukocytes. This raises the interesting possibility that plants and animals share a common defence response to invading organisms. Contents Summary 317 I. Introduction 318 II. Species of activated oxygen 319 III. Sites of activated oxygen production 319 IV. Free radical production during senescence 323 V. Targets of free radical damage in senescing tissues 330 VI. The role of free radicals in seed ageing 336 VII. The role of free radicals in wounding 337 VIII. Concluding remarks 338 Acknowledgement 338 References 338.

Targets of AtWRKY6 regulation during plant senescence and pathogen defense

Abstract: In Arabidopsis, WRKY factors comprise a large gene family of plant-specific transcriptional regulators controlling several types of plant stress responses. To understand the regulatory role of WRKY proteins during such processes, we identified targets of the senescence- and defense-associated WRKY6 factor. WRKY6 was found to suppress its own promoter activity as well as that of a closely related WRKY family member, indicating negative autoregulation. On the other hand, WRKY6 positively influenced the senescence- and pathogen defense-associated PR1 promoter activity, most likely involving NPR1 function. One novel identified target gene, SIRK, encodes a receptor-like protein kinase, whose developmental expression is strongly induced specifically during leaf senescence. The transcriptional activation of SIRK is dependent on WRKY6 function. Senescing leaves of wrky6 knockout mutants showed a drastic reduction, and green leaves of WRKY6 overexpression lines showed clearly elevated SIRK transcript levels. Furthermore, the SIRK gene promoter was specifically activated by WRKY6 in vivo, functioning very likely through direct W-box interactions.

Plant senescence and crop productivity

Abstract: Senescence is a developmental process which in annual crop plants overlaps with the reproductive phase. Senescence might reduce crop yield when it is induced prematurely under adverse environmental conditions. This review covers the role of senescence for the productivity of crop plants. With the aim to enhance productivity, a number of functional stay-green cultivars have been selected by conventional breeding, in particular of sorghum and maize. In many cases, a positive correlation between leaf area duration and yield has been observed, although in a number of other cases, stay-green cultivars do not display significant effects with regards to productivity. In several crops, the stay-green phenotype is observed to be associated with a higher drought resistance and a better performance under low nitrogen conditions. Among the approaches used to achieve stay-green phenotypes in transgenic plants, the expression of the IPT gene under control of senescence-associated promoters has been the most successful. The promoters employed for senescence-regulated expression contain cis-elements for binding of WRKY transcription factors and factors controlled by abscisic acid. In most crops transformed with such constructs the stay-green character has led to increased biomass, but only in few cases to increased seed yield. A coincidence of drought stress resistance and stay-green trait is observed in many transgenic plants.

A gene regulatory network controlled by the NAC transcription factor ANAC092/AtNAC2/ORE1 during salt-promoted senescence.

Abstract: The onset and progression of senescence are under genetic and environmental control. The Arabidopsis thaliana NAC transcription factor ANAC092 (also called AtNAC2 and ORE1) has recently been shown to control age-dependent senescence, but its mode of action has not been analysed yet. To explore the regulatory network administered by ANAC092 we performed microarray-based expression profiling using estradiol-inducible ANAC092 overexpression lines. Approximately 46% of the 170 genes up-regulated upon ANAC092 induction are known senescence-associated genes, suggesting that the NAC factor exerts its role in senescence through a regulatory network that includes many of the genes previously reported to be senescence regulated. We selected 39 candidate genes and confirmed their time-dependent response to enhanced ANAC092 expression by quantitative RT-PCR. We also found that the majority of them (24 genes) are up-regulated by salt stress, a major promoter of plant senescence, in a manner similar to that of ANAC092, which itself is salt responsive. Furthermore, 24 genes like ANAC092 turned out to be stage-dependently expressed during seed growth with low expression at early and elevated expression at late stages of seed development. Disruption of ANAC092 increased the rate of seed germination under saline conditions, whereas the opposite occurred in respective overexpression plants. We also detected a delay of salinity-induced chlorophyll loss in detached anac092-1 mutant leaves. Promoter-reporter (GUS) studies revealed transcriptional control of ANAC092 expression during leaf and flower ageing and in response to salt stress. We conclude that ANAC092 exerts its functions during senescence and seed germination through partly overlapping target gene sets.