Source Strength Regulates an Early Phase Transition of Tobacco Shoot Morphogenesis

We have taken advantage of specific reductions in the ribulose-1,5-bisphosphate carboxylase/oxygenase concentration in rbcS antisense mutants of tobacco (Nicotiana tabacum L.) to assess the contribution of source strength (carbohydrate production) to the control of shoot development. Wild-type and antisense plants undergo distinct phases of shoot development that can be distinguished from one another on the basis of differences in stem elongation rates, internode distances, plastochron indices, leaf sizes, and leaf morphologies. An early phase of shoot morphogenesis is markedly prolonged in the antisense plants, and an increased number of leaves emerge during this phase in the mutants. This delay is specific, inasmuch as the duration and expression of traits characteristic of later phases of shoot development proceed normally. In addition to altered shoot developmental patterns, the antisense mutants have enhanced shoot/root ratios and markedly increased leaf longevities. It is likely that these are adaptations that enhance photosynthetic rates. Consistent with this proposal, the total leaf areas and dry weights of the mutant and wild type are similar at flowering. Collectively, our results indicate that source strength regulates the duration of an early phase of tobacco shoot development and the transition to a later phase. We suggest that this phase change may occur in response to the attainment of a threshold source strength, which is delayed in the mutant plants.