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

SUMMARY Nitrogen (N) is an essential major nutrient for food crops. Although ammonium (NH 4 + ) is the primary N source of rice ( Oryza sativa ), nitrate (NO 3 − ) can also be absorbed and utilized. Rice responds to NO 3 − 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 3 − ‐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 4 + and NO 3 − were investigated in rice plants. NO 3 − promoted SR elongation, possibly due to short‐term signal perception and long‐term nutrient function. Compared with NH 4 + conditions, higher SL signalling/levels and less D53 protein were recorded in roots of NO 3 − ‐treated rice plants. In contrast to wild‐type plants, SR lengths of d mutants were less responsive to NO 3 − 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 3 − ‐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 3 − 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 3 − supply.