Fig. 9. Model for TTL function in cellulose biosynthesis under salt stress.

The CSCs produce cellulose at the plasma membrane tracking along cortical microtubules under control conditions in both WT (Top) and ttl mutants (Bottom). Two hours of exposure to elevated NaCl concentrations cause microtubule depolymerization and CSC displacement from the plasma membrane into SmaCCs (also called MASCs) in both genotypes. However, microtubule depolymerization is not as pronounced in ttl1ttl3 as compared to WT. After 28 hours of NaCl treatment, TTL proteins establish a salt-tolerant microtubule array and allow the repopulation of the CSC pool at the plasma membrane to restore cellulose synthesis in WT plants. However, in the absence of TTL proteins, microtubules disintegrate at later stages of treatment, while the CSCs do not repopulate the plasma membrane, leading to defective cellulose synthesis.