Defects in CSI1/POM2 impair microtubule-based guidance of cellulose synthase during secondary wall synthesis and reveal autonomous cell wall patterning

R Schneider and S Persson

School of Biosciences, University of Melbourne, Australia

The transition from primary and secondary cell wall synthesis represent a key developmental step in plant biology as it allows for efficient water transport throughout the plant body and supports plant stature. In addition, secondary walls are the prime contributors to the biomass of plants. Xylem cells are surrounded by evenly dispersed bands of secondary wall thickenings that are crucial for their functions. The patterning of these thickenings is determined by cortical microtubules. Cellulose is the major constituent of the cell wall thickenings and is synthesized by plasma membrane-based secondary wall cellulose synthases (CesAs) whose catalytic activity drives them forward through the membrane. Here, the direction of the CesAs is determined by the microtubules. However, the steering factors for this process remain elusive. We show that the protein Cellulose Synthase Interacting (CSI)1/POM2 is necessary for the alignment of the secondary wall CesAs and microtubules during the early stages of secondary wall formation. Consequently, impaired CSI1/POM2 function results in altered secondary wall patterns and in compromised cellulose microfibrils, both in Arabidopsis and rice. Surprisingly, even in the absence of CSI1/POM2 the CesAs can subsequently realign with the microtubules possibly by engaging with other secondary wall deposits. Hence, the initial stages of microtubule-based guidance of the secondary wall CesA depend on CSI1/POM2, while the later stages can occur in a microtubule autonomous manner. Our results reveal how secondary wall synthesis, a key developmental process in plant biology, is regulated and directed.