The root epidermis is patterned into trichoblasts, which form root hairs, and atrichoblasts, which do not. This patterning depends on a complex genetic network that converges on GLABRA2, which suppresses root hair development in atrichoblasts, but which is itself repressed in trichoblasts, leading to the activation of genes such as ROOT HAIR DEFECTIVE 6 (RHD6) involved in root hair differentiation. This genetic network mainly determines root hair density. For example, deletion of WEREWOLF, a component of this genetic network, transforms the majority of epidermal cells into trichoblasts bearing root hairs. However, root hair elongation is another crucial process that is important in increasing root surface and therefore in increasing the potential for nutrient uptake. As it was shown that phosphate starvation triggers root hair elongation through an auxin signal, the possibility that low nitrogen availability could trigger a similar response to increase nitrogen uptake efficiency was investigated.
The researchers found that low nitrogen indeed increases root hair length and that this depends on auxin biosynthesis and transport. Auxin accumulated initially in the root apex under low nitrogen, but later shifted towards the root hair differentiation zone, suggesting that auxin is first synthesized in the root apex and then transported to epidermal cells that differentiate into root hairs to boost elongation. Indeed, auxin biosynthesis and transport mutants have shorter root hairs under low nitrogen than the control. While this response is similar to the situation under low phosphate, the two responses diverge at the signal decoding step: under low phosphate, AUXIN RESPONSE FACTOR 19 (ARF19) conveys the signal to RHD6-LIKE 2 and 4 (RSL2 and RSL4) to trigger root hair elongation, under low nitrogen this task is mediated redundantly by ARF6 and ARF8 via an RHD6-LOTUS JAPONICA ROOT HAIRLESS-LIKE 3 (LRL3) module. It would be interesting to investigate which additional signals confer specificity to the two responses on the level of different ARFs for either phosphate or nitrogen limitation.
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