Rab-H1b is essential for trafficking of cellulose synthase and for hypocotyl growth in Arabidopsis thaliana.

Cell-wall deposition of cellulose microfibrils is essential for plant growth and development. In plant cells, cellulose synthesis is accomplished by cellulose synthase complexes located in the plasma membrane. Trafficking of the complex between endomembrane compartments and the plasma membrane is vital for cellulose biosynthesis; however, the mechanism for this process is not well understood. We here report that, in Arabidopsis thaliana, Rab-H1b, a Golgi-localized small GTPase, participates in the trafficking of CELLULOSE SYNTHASE 6 (CESA6) to the plasma membrane. Loss of Rab-H1b function resulted in altered distribution and motility of CESA6 in the plasma membrane and reduced cellulose content. Seedlings with this defect exhibited short, fragile etiolated hypocotyls. Exocytosis of CESA6 was impaired in rab-h1b cells, and endocytosis in mutant cells was significantly reduced as well. We further observed accumulation of vesicles around an abnormal Golgi apparatus having an increased number of cisternae in rab-h1b cells, suggesting a defect in cisternal homeostasis caused by Rab-H1b loss function. Our findings link Rab GTPases to cellulose biosynthesis, during hypocotyl growth, and suggest Rab-H1b is crucial for modulating the trafficking of cellulose synthase complexes between endomembrane compartments and the plasma membrane and for maintaining Golgi organization and morphology.