Auxin modulates the enhanced development of root hairs in Arabidopsis thaliana (L.) Heynh. under elevated CO2

Root hairs may play a critical role in nutrient acquisition of plants grown under elevated CO2. This study investigated how elevated CO2 enhanced the development of root hairs in Arabidopsis thaliana (L.) Heynh. The plants under elevated CO2 (800 mu L L-1) had denser and longer root hairs, and more H-positioned cells in root epidermis than those under ambient CO2 (350 mu L L-1). The elevated CO2 increased auxin production in roots. Under elevated CO2, application of either 1-naphthoxyacetic acid (1-NOA) or N-1-naphthylphthalamic acid (NPA) blocked the enhanced development of root hairs. The opposite was true when the plants under ambient CO2 were treated with 1-naphthylacetic acid (NAA), an auxin analogue. Furthermore, the elevated CO2 did not enhance the development of root hairs in auxin-response mutants, axr1-3, and auxin-transporter mutants, axr4-1, aux1-7 and pin1-1. Both elevated CO2 and NAA application increased expressions of caprice, triptychon and rho-related protein from plants 2, and decreased expressions of werewolf, GLABRA2, GLABRA3 and the transparent testa glabra 1, genes related to root-hair development, while 1-NOA and NPA application had an opposite effect. Our study suggests that elevated CO2 enhanced the development of root hairs in Arabidopsis via the well-characterized auxin signalling and transport that modulate the initiation of root hairs and the expression of its specific genes.