Regulation of Senescence of Excised Leaves of Some C3 and C4 Species by Endogenous H2O2

Hydrogen peroxide as a signalling molecule in plants and its crosstalk with other plant growth regulators under heavy metal stress

Hydrogen peroxide (H₂O₂) acts as a significant regulatory component interrelated with signal transduction in plants. The positive role of H₂O₂ in plants subjected to myriad of abiotic factors has led us to comprehend that it is not only a free radical, generated as a product of oxidative stress, but also helpful in the maintenance of cellular homeostasis in crop plants. Studies over the last two centuries has indicated that H₂O₂ is a key molecule which regulate photosynthesis, stomatal movement, pollen growth, fruit and flower development and leaf senescence. Exogenously-sourced H₂O₂ at nanomolar levels functions as a signalling molecule, facilitates seed germination, chlorophyll content, stomatal opening, and delays senescence, while at elevated levels, it triggers oxidative burst to organic molecules, which could lead to cell death. Furthermore, H₂O₂ is also known to interplay synergistically or antagonistically with other plant growth regulators such as auxins, gibberellins, cytokinins, abscisic acid, jasmonic acid, ethylene and salicylic acid, nitric oxide and Ca²⁺ (as signalling molecules), and brassinosteroids (steroidal PGRs) under myriad of environmental stresses and thus, mediate plant growth and development and reactions to abiotic factors. The purpose of this review is to specify accessible knowledge on the role and dynamic mechanisms of H₂O₂ in mediating growth responses and plant resilience to HM stresses, and its crosstalk with other significant PGRs in controlling various processes. More recently, signal transduction by mitogen activated protein kinases and other transcription factors which attenuate HM stresses in plants have also been dissected.

THE ROLE OF FREE RADICALS IN SENESCENCE AND WOUNDING

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.

Metabolism of activated oxygen in detached wheat and rye leaves and its relevance to the initiation of senescence

The activities of several enzymes either generating or decomposing O 2 (-) or H2O2, were investigated during the course of senescence of detached wheat (Triticum aestivum L.) and rye (Secale cereale L.) leaves in light and in darkness. Most of the activities, although not in full synchrony, declined with the degradation of chlorophyll and protein. The decline was slower in light than in darkness (e.g. glycolate oxidase, EC 1.1.3.1; urate oxidase, EC 1.7.3.3.; catalase, EC 1.11.1.6) and was further retarded after application of kinetin. The activity of superoxide dismutase (EC 1.15.1.1) declined only very little or, in detached rye leaves, even remained unchanged. For lipoxygenase (EC 1.13.11.12) the decline was enhanced in light and not affected by kinetin. Total peroxidase (EC 1.11.1.7) activity strikingly increased after excision of the leaves. The increase was higher in the dark than in light and further enhanced by kinetin. Activity of L-amino-acid oxidase (EC 1.4.3.2) was not detected. The peroxide content of the detached leaves slowly increased during senescence, being higher in light than in darkness. The malondialdehyde content strongly increased in light, but not in darkness. Application of several chemicals known as scavengers for oxygen radicals (1,4-diazobicyclo(2,2,2)octane, α-tocopherol acetate, p-benzoquinone, D-penicillamine copper, 2-amino-2-(hydroxymethyl)-1,3-propanediol, formate) did not notably retard chlorophyll degradation in senescencing leaves. Thiourea and urate retarded chlorophyll breakdown in light, obviously because they were used as nitrogen sources. Chlorophyll breakdown was greatly accelerated by D2O, particularly in light, presumably by enhancing photooxidative damage. The results indicate that increased peroxide metabolism accompanies the senescence of detached leaves. They do not, however, support the free-radical theory that an accumulation of activated oxygen initiates leaf senescence.