E69K mutation in β-tubulin 2 blocks cell wall integrity signaling during plant cell elongation.

The intact cell wall is the prerequisite for plant cell morphogenesis. Loss of function in FRA1/KINESIN-4A, which encodes a microtubule-based kinesin motor, causes dwarfed growth phenotypes with reduced cell wall mechanics. However, the underlying mechanisms remain elusive. Here, using genetic screening, we identify a suppressor of fra1 (sofa1) mutation that specifically suppresses the dwarf phenotype of the fra1 mutant. The sofa1 carries an E69K mutation in β-Tubulin 2 (TUB2), and the dominant suppressive effect of E69K mutation is conserved among β-tubulins. We further reveal that incorporation of TUB2E69K affects microtubule stability, yet fails to rescue the cell wall defects or lateral displacement of microtubules in fra1. Combining with transcriptomic analysis, we propose that the E69K mutation of TUB2 potentially restores the cell elongation by blocking the cell wall integrity (CWI) signaling. Our study sheds new light on the complex mechanism underlying the dwarfism of the fra1 mutant, and further proposes a potential model by which microtubules control plant cell elongation.