The response of the root system architecture to nutrient deficiencies is critical for sustainable agriculture. Nitric oxide (NO) is considered a key regulator of root growth, although the mechanisms remain unknown. Phenotypic, cellular and genetic analyses were undertaken in rice to explore the role of NO in regulating root growth and strigolactone (SL) signalling under nitrogen-deficient and phosphate-deficient conditions (LN and LP). LN-induced and LP-induced seminal root elongation paralleled NO production in root tips. NO played an important role in a shared pathway of LN-induced and LP-induced root elongation via increased meristem activity. Interestingly, no responses of root elongation were observed in SL d mutants compared with wild-type plants, although similar NO accumulation was induced by sodium nitroprusside (SNP) application. Application of abamine (the SL inhibitor) reduced seminal root length and pCYCB1;1::GUS expression induced by SNP application in wild type; furthermore, comparison with wild type showed lower SL-signalling genes in nia2 mutants under control and LN treatments and similar under SNP application. Western blot analysis revealed that NO, similar to SL, triggered proteasome-mediated degradation of D53 protein levels. Therefore, we presented a novel signalling pathway in which NO-activated seminal root elongation under LN and LP conditions, with the involvement of SLs.

Aims Potassium deficiency (LK) is a common abiotic stress that affects plant root development. We aimed to identify the role of auxin in LK effects on root growth. Methods We investigated the effects of LK on the formation and elongation of first-and second-order lateral roots (LRs), auxin concentration, DR5::GUS expression, [ 3 H]IAA transport and the expressions of six PIN genes in roots of tobacco plants. We also examined the effects of exogenous auxin (NAA) and a transport inhibitor (NPA) on LR growth and DR5::GUS expression. Results Potassium deficiency reduced root growth, mostly by impairing the formation and elongation of first-order LRs. Indole acetic acid (IAA) concentration and DR5::GUS expression levels decreased in leaves and roots subjected to LK, indicating that LK affected auxin distribution. [ 3 H]IAA transport and the expression levels of PIN genes were reduced by LK, showing that auxin polar transport was inhibited by LK. Application of NAA to LK-treated plants increased first-order LR formation and elongation to levels similar to the control, an outcome that was consistent with the similarity in DR5::GUS expression levels between treatments. NPA acted in a converse manner. Conclusions Potassium deficiency inhibited LR formation and elongation in tobacco plants by shifting auxin distribution.

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