The roles of LRD2, sugars, and the hormones ABA and auxin in regulating Arabidopsis lateral root formation

Culture systems have long been used to study root system development and the effects of environment on root system architecture. My investigations into the mechanisms controlling these processes allow me to present a comprehensive model for the regulation of lateral root formation in Arabidopsis seedlings grown in culture. I demonstrate direct contact between aerial tissues and sucrose in the growth media is necessary and sufficient to promote emergence of lateral root primordia from the parent root. Sucrose enters the aerial tissues and acts as a metabolite to coordinately increase shoot system development and lateral root formation. Mild osmotic stress, previously shown to switch root system development towards an unbranched architecture, is perceived by the root, which then sends an ABA-dependent signal to the aerial tissues. ABA causes a decrease in the permeability of aerial tissues and hence reduces uptake of sucrose from the culture media resulting in reduced lateral root formation. Consistent with these findings, osmotic repression of lateral root formation in culture can be overcome by mutations that cause the cuticle of a plant's aerial tissues to become more permeable. Indeed, I report here that the lateral root development2 (lrd2) mutant, carries a point mutation in Long Chain Acyl-CoA Synthetase 2 (LACS2), a gene essential for cutin biosynthesis. Together, my findings: (1) Strongly impact the interpretation of experiments that utilize Arabidopsis grown in culture to study root system architecture; (2) Identify sucrose as an unexpected regulator of lateral root formation; (3) Demonstrate important mechanisms by which roots communicate information to aerial tissues and receive information in turn; and (4) Provide new insights into the regulatory pathways that allow plants to be developmentally plastic while preserving the essential balance between above-ground and underground organs.