Arabidopsis lateral shoots display two distinct phases of growth angle control
Shoot growth angle is a fundamental determinant of plant form. In their later development, lateral branches maintain gravitropic setpoint angles (GSAs) in which growth is set and maintained relative to gravity. The typically non-vertical GSAs are the product of an auxin-dependent antigravitropic offset that counteracts underlying gravitropic response in the branch (Roychoudhry et al., 2013). Here we describe an earlier phase of branch development in which the young lateral shoot grows rootward, independently of gravity, promoting a spreading growth habit. In normal development, this phase of growth is terminated with the onset of the GSA programme, with branches then growing upwards to assume their mature form. The biophysical basis of the early rootward phase of branch growth can be traced back to greater cell proliferation on the upper, adaxial side that upon expansion, drives asymmetric growth. Our data indicate that cytokinin is involved in this process and that the transcription factor TCP1 is an important regulator of lateral shoot adaxial identity and differential ad-abaxial cell proliferation.