Environmental and hormonal control of shoot branching

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Plant architecture is highly dependent on the pattern of shoot branching. From an agronomical perspective, the degree of branching is important because it affects the biomass and number of flowers (hence seeds), and in cultivated species it ultimately affects crop yield. Branching is controlled by environmental (e.g. temperature, water availability, photoperiod) and endogenous (hormones and other growth regulators) factors. The plant hormone auxin, which has been studied for decades, is considered the master regulator of branching. Recently, novel growth regulators called strigolactones have been identified. Both auxin and strigolactones are negative signals, meaning that they prevent the growth of lateral buds located on the stem, hence reducing shoot branching. A third class of growth regulators, cytokinins, act as positive regulators and promote branching. It is the ratio between the various hormones that determines if branching will occur. Environmental conditions influence branching, likely because they affect the levels of the different hormones. Our proposed research program aims to determine the underlying molecular mechanisms by which environmental factors influence the hormone-dependent signal transduction pathways that regulate shoot branching in the model plant Arabidopsis thaliana. We will use wild-type and mutant plants that show different levels of branching to determine which of the proteins from three families known to be involved in auxin responses are associated specifically with shoot branching. We will identify and characterize other candidate proteins associated with branching, such as regulators of gene expression. The proposed research program will therefore allow us to identify novel branching-associated proteins and determine how the environmental cues interact with endogenous signals to influence this very important physiological process. The knowledge gained will contribute to our understanding of plant growth determinants and will lead to the development of novel tools to improve plant performance and yield.