Root hairs are tubular outgrowths of epidermal cells that extend the absorbing surface of roots into the surrounding soil. They play essential functions in nutrient acquisition and are particularly important for the uptake of nutrients. Root hairs emerge from the outer surface of cells in the root epidermis which contains hairless epidermal cells (Dolan, 2017). The spatial pattern of root hair cells and hairless epidermal cells varies in different vascular plants. In rice, root hair cells (50%) and hairless epidermal cells (50%) alternate along each longitudinal file that makes up the epidermis (Kim and Dolan, 2011). This pattern contrasts with the A. thaliana root hair cell pattern. RSL class I genes, RSL class II genes, and RHL1 are positive regulators of root hair development in rice (Ding et al., 2009; Kim et al., 2016; 2017). Three genes: CSLD1, EXPA17, and XXT1 involved in the synthesis of cell wall polysaccharides and their modification are required for root hair elongation (Kim et al., 2007; ZhiMing et al., 2011; Wang et al., 2014). ABA-mediated auxin accumulation in root tips also promotes root hair elongation (Wang et al., 2017).
