Identification of novel regulators of growth angle control in Arabidopsis lateral roots and shoots

The angle of growth of root and shoot branches is a key determinant of plant architecture. Arabidopsis lateral roots and shoots grow maintaining non-vertical gravitropic setpoint angles (GSAs) for a considerable part of the plant life (Digby and Firn, 1995). Non-vertical GSAs are the result of two counteracting auxin-dependent components: the gravitropic response, responsible for auxin flux to the lower side of the organ, and the antigravitropic offset (AGO), responsible for auxin flux to the upper side (Roychoudhry et al., 2013). In lateral roots, this auxin transport is mediated by PIN3 and PIN7, which localization to the upper and lower side of the columella cells is controlled by their phosphorylation status (Grones et al., 2018; Roychoudhry et al., 2019). PIN3 dephosphorylation by PP2A/RCN1 phosphatase determines the protein to localise to the lower side of the columella cells, mediating the gravitropic auxin flux (Roychoudhry et al., 2019). In lateral shoots, the molecular regulation of non-vertical GSAs has been suggested to be the same of lateral roots, with PIN3, PIN4 and PIN7 mediating auxin fluxes from the endodermis to the sides of the stem (Rakusová et al., 2011; Roychoudhry et al., 2013; Bennett et al., 2016). During my PhD I identified two novel regulators of lateral root GSAs working in antagonism with RCN1. Their proposed role in membrane trafficking suggests their possible regulation of PIN localization at the upper side of the columella cells. Furthermore, I described a new phase of lateral shoot development preceding the setting of the GSA, and identified three novel putative regulators of the stem ad-abaxial polarity during early development. Taken together my results have advanced our understanding of growth angle control in shoot and root branches, for growth angles set both dependently and independently of gravity.