Interactions between auxin, microtubules and XTHs mediate green shade-induced petiole elogation in Arabidopsis

Plants are highly attuned to translating environmental changes to appropriate modifications in growth. Such phenotypic
plasticity is observed in dense vegetations, where shading by neighboring plants, triggers rapid unidirectional shoot growth
(shade avoidance), such as petiole elongation, which is partly under the control of auxin. This growth is fuelled by cellular
expansion requiring cell-wall modification by proteins such as xyloglucan endotransglucosylase/hydrolases (XTHs). Cortical
microtubules (cMTs) are highly dynamic cytoskeletal structures that are also implicated in growth regulation. The objective
of this study was to investigate the tripartite interaction between auxin, cMTs and XTHs in shade avoidance. Our results
indicate a role for cMTs to control rapid petiole elongation in Arabidopsis during shade avoidance. Genetic and
pharmacological perturbation of cMTs obliterated shade-induced growth and led to a reduction in XTH activity as well.
Furthermore, the cMT disruption repressed the shade-induced expression of a specific set of XTHs. These XTHs were also
regulated by the hormone auxin, an important regulator of plant developmental plasticity and also of several shade
avoidance responses. Accordingly, the effect of cMT disruption on the shade enhanced XTH expression could be rescued by
auxin application. Based on the results we hypothesize that cMTs can mediate petiole elongation during shade avoidance
by regulating the expression of cell wall modifying proteins via control of auxin distribution.