Regulation of resource allocation during reproductive growth in Arabidopsis thaliana L. Heynh
The abi3-1 mutant causes moderate perturbation of seed metabolism relative to wild-type seeds, so offering a distinct and discrete treatment for use in experiments investigating regulation of allocation between sources and sinks. abi3-1 plants continue to initiate new flowers, and hence siliques, for longer than wild-type plants. Total rates of carbon assimilation in the short-term were the same in both genotypes during early reproductive growth. This rate fell to 50-70 % in wild-type plants during later reproductive growth, but did not change in the mutant, consistent with delayed senescence of cauline leaves in abi3-1 plants. It was found that restriction of carbon and/or nitrogen availability restricted growth in both genotypes, and abolished the mutant phenotype. Specific leaf areas increased under shading and decreased when nitrate was limiting. Reduction in nitrate limitation from 90 to 80 % was considerably less limiting for wild-type plant growth, but remained grossly limiting for abi3-1 plants. In previously non-acclimatised plants of both genotypes, no difference was found in assimilation and allocation of 14C-radiolabel supplied at 200 ppm CO 2. 800 ppm CO 2 similarly had no effect on the wild-type, but caused abolition or inversion of normal source-sink relationships in abi3-1 plants. A significantly large amount of radiolabel was initially incorporated into starch in all tissues in abi3-1 plants, and later moved into the water soluble fraction in each tissue, most likely as sucrose. It is proposed that resource allocation is regulated by competition for resources between sinks maintaining sucrose concentration gradients in the phloem, and that sucrose is both the transport and signalling molecule in the mechanism described. No difference in concentration of sucrose in tissues was found between genotypes, but it was found that mutant siliques imported [U-14C]glucose into siliques significantly more slowly from the phloem than wild-type siliques. In conclusion, abi3-1 seeds may be seen as having reduced capacity for growth which causes stimulation of floral meristem development by feedback of sucrose in the phloem. Silique initiation is thereby prolonged, creating a demand for assimilates that delays cauline leaf senescence.