Root hairs, like trichomes, are epidermal cell extensions, but root hairs are unbranched. Once initiated, root hairs grow to about 100 ^m long by polarised tip growth, which requires vesicle trafficking of new cell membrane and cell wall components to the root hair tip, and involves both actin and microtubules (Bibikova et al. 1999; Baluska et al. 2000).
To date, about 40 genes required for root hair development have been identified by forward genetics. A complementary approach compared global gene expression in the root hair differentiation zone of wild-type roots with a root hair-defective mutant, rhd2, and defined the "root hair morphogenesis tran-scriptome" (Jones et al. 2006). Using these data, six new root hair mutants were isolated, including mrh2. Unlike wild-type root hairs, which are straight and unbranched, mrh2 mutant root hairs are wavy and branched, and constantly reorient their direction of growth (Jones et al. 2006). Thus, MRH2 is not required for growth itself, but is required to maintain a steady direction of tip growth and a single growing tip.
The mrh2 root hairs resemble those with microtubule dynamics disrupted by chemical or genetic means (Bibikova et al. 1999; Bao et al. 2001; Whitting-ton et al. 2001). MRH2 protein contains Arm repeats and a kinesin domain (Fig. 1). Kinesins are ATP-dependent microtubule motors that can generate force. The Arm repeats of MRH2 may bind to a specific cargo whose transport along microtubules is required for directed root hair tip growth (Fig. 2). MRH2 is one of three closely related kinesin-Arm proteins in Arabidopsis (Reddy and Day 2001). MRH2 and its relatives could have tissue-specific roles in plant development and morphogenesis, although functional redundancy is also possible.
Interestingly, a screen for mutants in another tip-growing cell type, pollen tubes, identified the seth4 mutant. SETH4 has six Arm repeats and may affect pollen tube growth or guidance (Lalanne et al. 2004). However, the mechanism of SETH4 function is not yet clear.
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