Submitted to: Planta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/1/1996
Publication Date: N/A
Interpretive Summary: Efficient production of economic yield by crop plants is dependent on the maintenance of photosynthesis (Ps) during the time when yield is being produced. Unfortunately, crop canopies exhibit declining Ps and begin to die or senesce just as the yield production process is starting. A primary reason for the declining Ps is associated with the loss of a single leaf protein, called rubisco, the captures CO2 from the atmosphere during Ps but also provides an abundant nitrogen source used for yield production. Therefore, as yield is formed this leaf protein is degraded and the nitrogen from the protein is transported to the sites of yield production. In this paper we have demonstrated that the leaves of common bean plants begin to senesce just as they reach maximum size, in a manner analogous to the senescence of leaves of fruit-bearing plants. We show that the loss of rubisco is closely associated with the loss of the RNA that is needed to make the protein. Other, but not all, proteins important to Ps respond likewise indicating that the genetic regulation of the process is very precise. We also show that a newly discovered protein-degradation system does not seem to be involved in the loss of rubisco during senescence. The results suggest that loss of rubisco protein is a key factor in senescence. Once the regulation of the synthesis and degradation of rubisco are better detailed, it may be possible to manipulate the process and significantly improve the efficiency of yield production.
Technical Abstract: Our objective was to determine the coordination of transcript and/or protein abundances of stromal enzymes during leaf senescence. Leaves of Phaseolus vulgaris L. plants were sampled beginning at the time full expansion; at this same time half of the plants were switched to nutrient solution lacking N. RNA and protein abundances decreased after leaf expansion whereas chlorophyll remained constant; N stress enhanced the decline in these traits. Abundances of rubisco, rubisco activase and R5P kinase decreased after expansion in a coordinated manner for both treatments. In contrast, ADP glc pyrophosphorylase abundance was relatively constant during natural senescence but did decline similar to the other enzymes under N stress. Northern analyses indicated that transcript abundances for all enzymes declined markedly on a fresh weight basis just after full leaf expansion. This rapid decline was particularly strong for the rubisco small subunit (rbcS) transcript. Transcripts of the Clp protease subunits clpC and clpP declined in abundance just after full leaf expansion, similar to the other mRNA species. The results indicated that senescence was initiated near the time of full leaf expansion, was accelerated by N stress, and was characterized by large declines in transcripts of stromal enzymes. Transcript for the Clp subunits were constitutive throughout development suggesting that ClpC and ClpP do not function as a senescence-specific proteolytic system in Phaseolus.