COORDINATION OF PROTEIN AND MESSENGER-RNA ABUNDANCES OF STROMAL ENZYMES AND MESSENGER-RNA ABUNDANCES OF THE CLP PROTEASE SUBUNITS DURING SENESCENCE OF PHASEOLUS-VULGARIS (L) LEAVES

Our objective was to determine the coordination of transcript and/or p rotein abundances of stromal enzymes during leaf senescence. First tri foliate leaves of Phaseolus vulgaris is L. plants were sampled beginni ng at the time of full leaf expansion; at this same time, half of the plants were switched to a nutrient solution lacking N. Total RNA and s oluble protein abundances decreased after full leaf expansion whereas chlorophyll abundance remained constant; N stress enhanced the decline in these traits. Abundances of ribulose-1,5-bisposphate carboxylase/o xygenase (Rubisco; EC 4.1.1.39), Rubisco activase and phosphoribulokin ase (Ru5P kinase; EC 2.7.1.19) decreased after full leaf expansion in a coordinated manner for both treatments. In contrast, adenosine dipho sphate glucose (ADPGlc) pyrophosphorylase (EC 2.7.7.27) abundance was relatively constant during natural senescence but did decline similar to the other enzymes under hr stress. Northern analyses indicated that transcript abundances for all enzymes declined markedly on a fresh-we ight basis just after full leaf expansion. This rapid decline was part icularly strong for the Rubisco small subunit (rbcS) transcript. The d ecline was enhanced by N stress for rbcS and Rubisco activase (rca), b ut not for Ru5P kinase (prk) and ADPGlc pyrophosphorylase (agp). Trans cripts of the Clp protease subunits clpC and clpP declined in abundanc e just after full leaf expansion, similar to the other mRNA species, W hen Northern blots were analysed using equal RNA loads, rbcS transcrip ts still declined markedly just after full leaf expansion whereas rca and clpC transcripts increased over time. The results indicated that s enescence was initiated near the time of full leaf expansion, was acce lerated by N stress, and was characterized by large decline in transcr ipts of stromal enzymes. The decreased mRNA abundances were in general associated with steadily declining stromal protein abundances, with A DPGlc pyrophosphorylase being the notable exception. Transcript analys es for the Clp subunits supported a recent report (Shanklin et al., 19 95, Plant Cell 7: 1713-1722) indicating that the Clp protease subunits were constitutive throughout development and suggested that ClpC and ClpP do not function as a senescence-specific proteolytic system in Ph aseolus.