The role of pectin metabolism, cell wall pH, mechano- and cell wall integrity sensing in growth control
The cell walls of growing Arabidopsis organs consist for 20% of homogalacturonan (HG). HG is synthesized in a highly methylesterified form and selectively de-methylesterified in the cellwall by Pectin MethylEsterases (PMEs). In vitro studies have shown that pectin de-methylesterification promotes cell wall acidification and can, depending on the conditions, trigger the formation of more or less elastic pectin gels. We previously showed that in vivo pectin de-methylesterification triggers an increase in wall elasticity and growth of young cells. We also showed that the inhibition of cellulose synthesis prevents wall loosening and that mutations in the receptor kinase THESEUS1 reduce the growth-inhibiting effect of cellulose synthesis inhibition. THE1 is a member of the CrRLKL1 family to which also belongs FERONIA (FER), a receptor for the Rapid Alkalinisation Factor (RALF), which is required for mechanosensing. Here we show that THE1 is a receptor for another RALF-like (RALFL) peptide, which negatively regulates FER activity. We propose a conceptual framework for the regulation of growth that integrates strain sensing, cell wall pH control, PME activity and pH-dependent wall loosening and which accounts for the maintenance of cell wall integrity during cell expansion.