CML24 is Involved in Root Mechanoresponses and Cortical Microtubule Orientation in Arabidopsis

Mechanostimuli can influence plant root system architecture by causing alterations in the root tip growth direction and triggering lateral root initiation. However, how a plant root senses and translates mechanostimulation into appropriate growth and/or developmental responses remains largely unclear. The fast expression induction and transcript turnover of the Arabidopsis TCH genes by touch stimulation suggest that the TCH genes may function in mechano-related events. However, the physiological functions of the TCH genes in Arabidopsis mechanoresponses remain undetermined. Here we screened a suite of tch mutants by characterizing their root growth behaviors on hard-agar surfaces. Two calmodulin-like 24 (CML24 or TCH2) mutants, cml24-2 and cml24-4, exhibited reduced root length, biased skewing, and altered epidermal cell file rotation (CFR) phenotypes compared with wild type (Col-0). The mutant phenotypes were dependent on hard-agar surface contact and disappeared when seedlings were grown in liquid medium. Abnormal glass barrier responses of cml24 mutants further indicate touch response defects. Pharmacological tests revealed differential sensitivity of cml24 mutants to microtubule-targeted agents. Nonadditive effects of mutations in CML24 and transgenic expression of a functional microtubule label, MBD-GFP, on root skewing and CFR phenotypes suggest a potential microtubule-related role of CML24. In vivo visualization of microtubule structures with the MBD-GFP reporter revealed altered cortical microtubule orientation in the epidermal cells in cml24-4. Our observations indicate that CML24 has a role in Arabidopsis root mechanoresponses, possibly through the regulation of cortical microtubule orientation.