Abstract
Root hairs are tubular extensions of the epidermis. Root hairs of the monogenic recessive maize mutant roothairless 6 (rth6) are arrested after bulge formation during the transition to tip growth and display a rough cell surface. BSR-Seq in combination with Seq-walking and subsequent analyses of four independently generated mutant alleles established that rth6 encodes CSLD5 a plasma membrane localized 129 kD D-type cellulose synthase with eight transmembrane domains. Cellulose synthases are required for the biosynthesis of cellulose, the most abundant biopolymer of plant cell walls. Phylogenetic analyses revealed that RTH6 is part of a monocot specific clade of D-type cellulose synthases. D-type cellulose synthases are highly conserved in the plant kingdom with five gene family members in maize and homologs even among early land plants such as the moss Physcomitrella patens or the clubmoss Selaginella moellendorffii. Expression profiling demonstrated that rth6 transcripts are highly enriched in root hairs as compared to all other root tissues. Moreover, in addition to the strong knock down of rth6 expression in young primary roots of the mutant rth6, the gene is also significantly down-regulated in rth3 and rth5 mutants, while it is up-regulated in rth2 mutants, suggesting that these genes interact in cell wall biosynthesis.
Highlights
The epidermis of plant roots comprises of two types of cells: trichoblasts and atrichoblasts
A phylogenetic study on gene families related to cell wall formation revealed that mutant alleles of ZmCslD5 exhibit a root hair defect[14]
In the present study we report the cloning and functional characterization of the rth[6] gene of maize which encodes CSLD5, a monocot and root hair-specific D-type cellulose synthase required for the transition from bulge formation to tip growth in root hair formation
Summary
The epidermis of plant roots comprises of two types of cells: trichoblasts and atrichoblasts. Maize epidermis cells can differentiate into trichoblasts irrespective of their position[8,9]. Roothairless[5] (rth5) is the only gene known to date that controls root hair density and the differentiation of epidermis cells into trichoblasts[10]. The roothairless[5] (rth5) gene was shown to give rise to a monocot-specific NADPH oxidase, which is involved in the tip growth of root hairs[10]. Species of the monophyletic clade commelinids which includes the poaceae family, which comprise cereals such as maize, rice and wheat develop type II cell walls[16]. The synthesis of cell wall components in the α-layer of root hairs is instrumental for their elongation as illustrated by mutants defective in this process[13,20,21,22]
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