Abstract
Basic helix-loop-helix (bHLH) proteins constitute a large family of transcriptional regulators in plants. Although they have been shown to play important roles in a wide variety of developmental processes, relatively few have been functionally characterized. Here, we describe the map-based cloning of the Lotus japonicus ROOTHAIRLESS1 (LjRHL1) locus. Deleterious mutations in this locus prevent root hair development, which also aborts root hair-dependent colonization of the host root by nitrogen-fixing bacteria. We show that the LjRHL1 gene encodes a presumed bHLH transcription factor that functions in a nonredundant manner to control root hair development in L. japonicus. Homology search and cross-species complementation experiments defined three members of the Arabidopsis (Arabidopsis thaliana) bHLH protein family, At2g24260, At4g30980, and At5g58010, as functionally equivalent to LjRHL1. Curiously, At2g24260 and At4g30980 mRNA species accumulate independently from the known positive regulators of root hair cell fate, while all three genes act in a partially redundant manner to regulate root hair development in Arabidopsis.
Highlights
Basic helix-loop-helix proteins constitute a large family of transcriptional regulators in plants
Root hair mutants have been identified from genetic screens of which the primary goal has been the characterization of loci that support the development of root nodule symbiosis in legume plants (Kawaguchi et al, 2002; Karas et al, 2005; Murray et al, 2006) In many legumes, root hairs mediate the initial contact between the legume host and nitrogen-fixing soil bacteria, commonly known as Rhizobium
We investigated the molecular basis of root hair formation in L. japonicus by analyzing two mutant lines carrying deleterious mutations in the Lotus japonicus ROOTHAIRLESS1 (LjRHL1) locus
Summary
Basic helix-loop-helix (bHLH) proteins constitute a large family of transcriptional regulators in plants. Root hair mutants have been identified from genetic screens of which the primary goal has been the characterization of loci that support the development of root nodule symbiosis in legume plants (Kawaguchi et al, 2002; Karas et al, 2005; Murray et al, 2006) In many legumes, root hairs mediate the initial contact between the legume host and nitrogen-fixing soil bacteria, commonly known as Rhizobium They actively participate in the recognition of bacterially encoded lipochitin-oligosaccharide signaling molecules known as nodulation or Nod factors and, subsequently, in the colonization of the root by bacteria (Karas et al, 2005), processes that are unknown to Arabidopsis. This leads to the formation of new lateral organs, root nodules, which eventually host the symbiotic bacteria, providing the appropriate conditions for symbiotic nitrogen fixation to occur (Oldroyd and Downie, 2008)
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