Mechanical induction of lateral root initiation

Unlike mammals whose development is limited to a short temporal window, plants produce organs de novo throughout their lifetime in order to adapt their architecture to the prevailing environmental conditions. Development of the root system represents a morphogenetic program where the positioning of new lateral organs occurs through the periodic recruitment of pericycle cells to become founder cells of a new lateral root (LR) primordium. While the hormone auxin appears intimately involved in specifying LR formation, it remains unclear why some pericycle cells are specified to initiate a LR while others are not. In the following thesis, I show that mechanical forces can act as one of the triggers for founder cell formation and so entrain the pattern of LR production to the environment. I observed that transient physical bending of the root was capable of eliciting LR formation to the convex side of the curve. Such mechanical stimulation triggered a Ca2+ transient within the pericycle, which was associated with the recruitment of ordinary pericycle cells to a LR founder cell fate. The initial establishment of the mechanically induced LR primordium was independent of an auxin supply from the shoot and was not disrupted by mutants in a suite of auxin transporters and receptor/response elements.;Mechanical forces have long been proposed to act as plant morphogenetic factors, however the cellular elements that translate mechanical force to a developmental signaling cascade have remained obscure. My observations indicate that in the case of mechanical induction of LR formation, the program of organogenesis may be triggered by mechanically elicited Ca2+ changes that can even suppress the requirement for many auxin-related elements normally involved in founder cell recruitment. Thus, the mechano-sensitive Ca2+ signaling system responsible for eliciting LR founder cell fate provides a potentially widespread mechanism whereby external and endogenous mechanical forces could be translated into morphogenetic programs during plant growth and development.