Bbl1 Allele of the Receptor Kinase Subfamily VII PBL38 Is Required for Elongated Root Hairs in Arabidopsis

In a screen for mutations that alter cell morphology and adhesion as a marker for cell wall composition alterations, an Arabidopsis seedling with root hairs that fail to elongate was identified. Root hairs are rounded and bubble-like, and the mutation was named Bubbles or bbl1. No other visible whole plant phenotypes were observed. Using backcrosses with wild type, and genomic sequencing of pooled F2 individuals with the Bubbles phenotype, 14 candidates for the mutant allele were identified. T-DNA alleles of one of these candidates showed a similar but partial phenotype to bbl1, and indicate that bbl1 is an allele of AT2G39110 previously identified as PBL38 (AvrPphB SUSCEPTIBLE1-LIKE38), a member of receptor kinase subfamily VII. PBL38 has been associated with the response to pathogens. bbl1 is a point mutation that causes a Glutamic acid to Lysine change at position 140, and is complimented by a C-terminal GFP fusion to the coding region of AT2G39110, indicating that the BBL1 gene (PBL38) is involved in correct root hair formation. AT2G39110 is expressed only in roots early in seedling maturation, and the GFP fusion protein localizes to the cell surface, consistent with the prediction that the gene encodes a receptor-like protein kinase. In bbl1 root hairs, the actin cytoskeleton does not form, while in bbl1 roots and other tissues normal actin cytoskeleton is observed.