Christopher Kesten, Staffan Persson

　　University of Melbourne, School of Biosciences, Parkville, VIC 3010, Melbourne, Australia

　　Abiotic stress, such as salinity, drought, and cold, causes detrimental yield losses for all major plant crop species. Understanding mechanisms that improve plants' ability to produce biomass, which largely is constituted by the plant cell wall, is therefore of upmost importance for agricultural activities. Cellulose is a principal component of the cell wall and is synthesized by microtubule-guided cellulose synthase enzymes at the plasma membrane. We identified two components of the cellulose synthase complex, which we call companion of cellulose synthase (CC) proteins (Endler et al., 2015, Cell). The cytoplasmic tails of these membrane proteins could bind to microtubules and promote microtubule dynamics in vivo. This activity supported microtubule organization, cellulose synthase localization at the plasma membrane, and rendered seedlings less sensitive to stress. NMR analyses revealed four regions of the CC proteins that could interact with microtubules, which we confirmed using cross-linking experiments and mass spectrometry analyses. The latter analyses, furthermore, revealed what parts of the tubulin dimers the CC proteins could interact with. Electron microscopy analyses revealed that the CC proteins enhanced microtubule bundling and that the CC proteins are arranged in rows along adjacent microtubules, and that they were evenly spaced of approx. 12 nm. These data indicate that the CC proteins bind to microtubules along their seams following the three start helical confirmation of the microtubules, and that this binding supports bundling of microtubules. Our findings thus offer a mechanistic model for how the CC proteins sustain microtubule organization and cellulose synthase localization and thus aid plant biomass production during salt stress.