Strigolactones are involved in phosphate- and nitrate-deficiency-induced root development and auxin transport in rice

Abstract

Strigolactones (SLs) or their derivatives have recently been defined as novel phytohormones that regulate root development. However, it remains unclear whether SLs mediate root growth in response to phosphorus (P) and nitrogen (N) deficiency. In this study, the responses of root development in rice (Oryza sativa L.) to different levels of phosphate and nitrate supply were investigated using wild type (WT) and mutants defective in SL synthesis (d10 and d27) or insensitive to SL (d3). Reduced concentration of either phosphate or nitrate led to increased seminal root length and decreased lateral root density in WT. Limitation of either P or N stimulated SL production and enhanced expression of D10, D17, and D27 and suppressed expression of D3 and D14 in WT roots. Mutation of D10, D27, or D3 caused loss of sensitivity of root response to P and N deficiency. Application of the SL analogue GR24 restored seminal root length and lateral root density in WT and d10 and d27 mutants but not in the d3 mutant, suggesting that SLs were induced by nutrient-limiting conditions and led to changes in rice root growth via D3. Moreover, P or N deficiency or GR24 application reduced the transport of radiolabelled indole-3-acetic acid and the activity of DR5::GUS auxin reporter in WT and d10 and d27 mutants. These findings highlight the role of SLs in regulating rice root development under phosphate and nitrate limitation. The mechanisms underlying this regulatory role involve D3 and modulation of auxin transport from shoots to roots.

The role of strigolactones in root development

“…SLs promote the elongation of seminal/primary roots and adventitious roots (ARs) [1][2][3][4] and they repress lateral root (LR) formation. [1][2][3]5 In Arabidopsis, the length of the primary root in SL mutants was shown to be shorter than that in wild type plants due to a reduction in meristem cell number.…”

“…2 Interestingly, in rice, SLs also positively control the elongation of ARs. [3][4] In the case of LR formation, LR density was shown to be increased in SL-deficient and -insensitive mutants of tomato, 2,6,7 while GR24 application decreased the LR density via the signaling gene MAX2, which suppresses LR outgrowth in Arabidopsis. 2 SLs positively or negatively regulate AR formation depending on the plant species and experimental conditions.…”

“…22 Interestingly, several lines of evidence suggest that SLs are involved in rice root growth in response to N and P deficiencies. 3,4 The application of GR24 to wild-type plants and SL synthetic mutants, but not signaling mutants (d3), under nutrientsufficient conditions led to the complete recovery of seminal root length and LR density compared to wild-type plants under Nand P-deficient conditions. This confirmed that elevated SL levels under nutrient-limiting conditions can lead to the D3-dependent induction of seminal root elongation and a reduction in LR density.…”

“…This confirmed that elevated SL levels under nutrient-limiting conditions can lead to the D3-dependent induction of seminal root elongation and a reduction in LR density. 4 Hormonal crosstalk between SLs and auxin during root development…”

“…In addition to SLs, auxins play a key role in establishing patterns of root morphology and are regulated by the levels of N and P. 4,[23][24][25] In terms of seminal root and AR elongation and LR formation, based on an analysis of SL and auxin signaling mutants, it was found that auxin signaling acts downstream of SLs. 4,7,9,22 SLs may affect root growth via changes in auxin polar transport from shoot to root and/or auxin efflux in roots.…”