AtOFPs regulate cell elongation by modulating microtubule orientation via direct interaction with TONNEAU2

As a group of plant-specific proteins, OVATE family protein (OFP) members have been shown to function as transcriptional repressors and involve in plant growth regulation in Arabidopsis and rice. It has also been shown that OFPs can interact with TONNEAU1 Recruiting Motif (TRM) proteins to regulate tomato fruit shape. In this study, we show that mutant plants with knock-down expression of OFP1, OFP2, OFP3, and OFF5 exhibit longer hypocotyls and cotyledons due to enhanced cell elongation. Overexpression of OFPs disturb the arrangement of cortical microtubule arrays in pavement cells and promote abnormal pavement cell expansion perpendicular to the direction of petiole growth, resulting in the kidney-shaped cotyledons in transgenic plants. OFP2 and OFP5 interact directly with the microtubule regulating protein TONNEAU2 (TON2), and genetic analysis suggests TON2 is required for the function of OFPs. We also show that altering the expression of OFPs affects light and BR regulated microtubule reorientation. BR treatment reduce the protein accumulation of OFP2, suggesting OFP2 mediates BR regulated microtubule reorientation. Taken together, our study provides evidences showing that OFP family proteins negatively regulate cell expansion by modulating microtubule reorganization, which requires the function of TON2.