Genotype By Environment Interaction in Shoot Branching

Plant development is highly plastic, allowing plants to adapt to constant changes in environmental conditions. An excellent example of developmental plasticity is shoot branching. The final architecture of the shoot system is determined by the integration of environmental cues such as light and nutrients with endogenous cues. In this thesis the effect of Nitrogen (N) availability on Arabidopsis shoot branching was used as a model to investigate plant developmental plasticity. In particular, natural variation in shoot branching response to N supply was investigated using a set of multi parent advanced generation inter cross (MAGIC) lines (Kover et al., 2009). Correlations between traits in a selected group of MAGIC lines revealed several interesting correlations, characterising two strategies for N response. One strategy involved flowering early, maintaining branch numbers of low N, and minimal shift in resource allocation to roots. This was associated with good seed yield and yield retention on low N. An alternative strategy involves late flowering, high branching on high N but low branching on low N, (i.e. high branching plasticity), and a substantial increase in root fraction on Low N. This was associated with high seed yields on high N, but poor yield retention on low N. The molecular basis for these different strategies are currently unknown, but it seems likely that plant hormones are involved. Analysis of bud activation on isolated nodal stem segments provided strong evidence that the regulation of branching by N availability requires strigolactone (SL), and that strigolactone acts by increasing the competition between buds. There was some evidence of strigolatone resistance in a low plasticity MAGIC line. Shoot system architecture is a key factor underlying crop yield, and yield stability under low N input is an agricultural priority. Therefore, in parallel the branching responses of a set of Brassica rapa lines to N limitation were determined. Results highlight many conserved features between Arabidopsis and Brassica, as well as some differences. These comparisons should aid breeding for shoot system architectures that can deliver improved yield under low N.