SPL14/17 act downstream of strigolactone signalling to modulate rice root elongation in response to nitrate supply.

Nitrogen (N) is an essential major nutrient for food crops. Although ammonium (NH ) is the primary N source of rice (Oryza sativa), nitrate (NO ) can also be absorbed and utilized. Rice responds to NO application by altering its root morphology, such as root elongation. Strigolactones (SLs) are important modulators of root length. However, the roles of SLs and their downstream genes in NO -induced root elongation remain unclear. Here, the levels of total N and SL (4-deoxyorobanchol) and the responses of seminal root (SR) lengths to NH and NO were investigated in rice plants. NO promoted SR elongation, possibly due to short-term signal perception and long-term nutrient function. Compared with NH conditions, higher SL signalling/levels and less D53 protein were recorded in roots of NO -treated rice plants. In contrast to wild-type plants, SR lengths of d mutants were less responsive to NO conditions, and application of rac-GR24 (SL analogue) restored SR length in d10 (SL biosynthesis mutant) but not in d3, d14, and d53 (SL-responsive mutants), suggesting that higher SL signalling/levels participate in NO -induced root elongation. D53 interacted with SPL17 and inhibited SPL17-mediated transactivation from the PIN1b promoter. Mutation of SPL14/17 and PIN1b caused insensitivity of the root elongation response to NO and rac-GR24 applications. Therefore, we conclude that perception of SLs by D14 leads to degradation of D53 via the proteasome system, which releases the suppression of SPL14/17-modulated transcription of PIN1b, resulting in root elongation under NO supply.