Carbohydrate regulation of leaf development: Prolongation of leaf senescence in Rubisco antisense mutants of tobacco

Previous investigations have shown that increased source strength as a resu lt of elevated CO2 can alter the timing of the phases of change that occur in photosynthetic rates during dicot leaf ontogeny [Miller et al. (1997) Pl ant Physiol 115: 1195-1200]. To evaluate the converse situation of decrease d source strength, we examined leaf development in rbcS antisense mutants o f tobacco. These mutants have depressed Rubisco levels and decreased rates of carbohydrate production. We found that antisense leaves are longer-lived than wild type leaves and that this appeared to be due to a prolongation o f the senescence phase of development, as monitored by photosynthetic rates , chlorophyll content, and the abundance and activity of Rubisco. Declines in these parameters during leaf ontogeny in both the wild type and mutant p lants were generally accompanied by coordinate reductions in the levels of rbcS mRNA and rbcL mRNA, as well as by reductions in chloroplast rRNA, chlo roplast DNA and total protein. We suggest that the prolongation of senescen ce in the antisense leaves is due to an impact of source strength on leaf d evelopmental programming that occurs, at least in part, at the level of tra nscript abundance of nuclear and chloroplast genes for chloroplast rRNAs an d proteins. We hypothesize that plants are capable of sensing a range of so urce strength conditions to initiate and modulate leaf developmental progra mming.