Proteolytic activity during senescence of plants

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.CONTENTSSummary 200 I. Introduction 200 II. Protein turnover 201 III. Proteases 201 IV. Plant proteases 202 V. Senescence 203 VI. Proteolytic activity during senescence 205 VII. Chloroplasts 207 VIII. Proteolytic activities associated with chloroplasts 209 IX. Proteolytic activities associated with stromal fractions 210 X. Thylakoid proteins 211 XI. Proteolytic activities associated with thylakoid fractions 214 XII. Targeting proteins for degradation 215References 223