Dual role of Pectin Methyl Esterase activity for plant cell growth regulation.

Acid-growth theory has been postulated in the 70s as a model to explain rapid elongation of plant stems in response to plant hormone auxin. More recently, it was demonstrated that hormones like auxin as well as brassinoesteroids (BR) induce cell elongation through the activation of proton ATPs pumps (H+-ATPs) thereby inducing apoplastic acidification. However, the impact of this acidification on the mechanical properties of the cell wall remained largely unexplored. Here we use Atomic Force Microscopy (AFM) to demonstrate that acidification of the apoplast is sufficient and necessary to induce cell elongation through cell wall relaxation. Moreover, we demonstrate that a tight control of Pectin Methyl Esterase (PME) activity is required for proper hypocotyl elongation, and that PME can induce both cell walls softening or stiffening in extracellular calcium dependent-manner. ### Competing Interest Statement The authors have declared no competing interest.