Proteolytic activity during senescence of plants

Abstract

summary Although information has rapidly developed concerning the intracellular localization of plant proteins, relatively few reports concern the intracellular location of endo‐ and exo‐proteolytic activities. Relatively few proteases have been purified, characterized, and associated with a specific cellular location. With the exception of the processing proteases involved in transport of proteins across membranes, little progress has yet been made concerning determination of in vivo products of specific proteases. Information on the turnover of individual proteins and the assessment of rate‐limiting steps in pathways as proteins are turned over is steadily appearing. Since chloroplasts are the major site of both protein synthesis and, during senescence, degradation, it was important to show unambiguously that chloroplasts can degrade their own constituents. Another important contribution was to obtain evidence that the chloroplasts contain proteases capable of degrading their constituents. This work has been more tenuous because of the low activities found and the possibility of contamination by vacuolar enzymes during the isolation of organelles. The possible targeting of cytoplasmic proteins for degradation by facilitating their transport into vacuoles is a field which hopefully will develop more rapidly in the future. Information on targeting of proteins for degradation via the ubiquitin (Ub) degradation pathway is developing rapidly. Future research must determine how much unity exists across the different eukaryotic systems. At present, it has important implications for protein turnover in plants, since apparently Ub is involved in the degradation of phytochrome. Little information has been developed regarding what triggers increased proteolysis with the onset of senescence, although it appears to involve protein synthesis. Thus far, the evidence indicates that the complement of proteases prior to senescence is sufficient to carry out the observed protein degradation. This field of study has great practical implications, e.g. maintaining photosynthesis during seed‐fill in order to obtain greater crop yields. The current use of ‘stay green’ variants in the populations of several crop plants to produce increased yields shows the potential for future development. The near future should see exciting discoveries in these areas of research that will have far reaching effects on the construction of transgenic plants for future research accomplishments and agricultural use.

A proteomic approach in analyzing heat‐responsive proteins in rice leaves

“…In plants, protein degradation was linked to different development stages, such as germination, differentiation and morphogenesis, senescence, and programmed cell death [60,61]. Proteolysis is also associated with oxidative stress that is promoted by ROS [62].…”

Regulation of Photosynthesis during Leaf Development in RbcS Antisense DNA Mutants of Tobacco

“…Vol. 107,1995 The changes in photosynthesis capacity that characterize leaf ontogeny appear to be part of a developmental program that specifies alterations in the expression of nuclear and chloroplast genes for photosynthetic proteins (reviewed by Gepstein, 1988;Huffaker, 1990). One issue that is poorly understood is whether these alterations are due, in part, to feedback regulation by the concentrations of various photosynthetic proteins.…”

“…We have also found this to be true in soybean (Jiang et al, 1993), and similar conclusions have been drawn in wheat and Pkaseolus (Brady, 1988;Bate et al, 1991). This is not a general phenomenon, however, since in some species posttranscriptional controls regulate Rubisco accumulation during at least part of the leaf developmental process (reviewed by Brady, 1988;Mullet, 1988;Huffaker, 1990). The signals that integrate RbcS transcription in the nuclear-cytoplasmic compartment with rbcL transcription in the plastid are poorly understood but involve both endogenous (e.g.…”

Proteolytic activities in Phaseolus vulgaris cotyledons under copper stress

“…Exopeptidases can be classified according to the substrate specificity as aminopeptidases or carboxypeptidases (Dalling 1986;Huffaker 1990). Endopeptidases are classified according to their catalytic mechanism, which implies specificity in the enzyme active sites.…”