Collet Hairs Are Critical for Anchoring and Survival of Arabidopsis thaliana Seedlings During Early Development.

Rapid and successful establishment of seedlings, beginning with the penetration of the emerging radicle into the soil, is essential for survival. In roots, root hairs increase the surface area for the absorption of water and solutes while providing the mechanical anchorage necessary for deeper soil penetration during elongation. However, the role of collet hairs, which are morphologically similar to root hairs but synchronously develop at the hypocotyl-root junction during germination, has not been clearly distinguished from that of true root hairs. Furthermore, their physiological significance under soil conditions remains to be validated. During an investigation of the genetic interaction between two genes involved in both cysteine metabolism and root hair development, an independent mutation causing root hair defects was identified. In addition to exhibiting extremely short or collapsed root hairs compared to the wild-type plants, the mutant lacked nearly all collet hairs at the hypocotyl-root junction and frequently displayed an 'exposed-root' phenotype. We mapped this phenotype to a 5-bp deletion in Shaven 3 (SHV3; AT4G26690). Analysis of the collet hair-defective shv3 mutant revealed that collet hairs provide the primary anchorage required for successful radicle penetration and subsequent seedling survival, a process later reinforced by the development of root hairs.