Low phosphorus promotes NSP1-NSP2 heterodimerization to enhance strigolactone biosynthesis and regulate shoot and root architecture in rice

Phosphorus is an essential macronutrient for plant development and metabolism, and plants have evolved ingenious mechanisms to overcome phosphate (Pi) starvation. However, the molecular mechanisms underlying the regulation of shoot and root architecture by low phosphorus conditions and the coordinated utilization of Pi and nitrogen remain largely unclear. Here, we show that Nodulation Signaling Pathway 1 (NSP1) and NSP2 regulate rice tiller number by promoting the biosynthesis of strigolactones (SLs), a class of phytohormones with fundamental effects on plant architecture and environmental responses. We found that NSP1 and NSP2 are induced by Oryza sativa PHOSPHATE STARVATION RESPONSE2 (OsPHR2) in response to low-Pi stress and form a complex to directly bind the promoters of SL biosynthesis genes, thus markedly increasing SL biosynthesis in rice. Interestingly, the NSP1/2-SL signaling module represses the expression of CROWN ROOTLESS 1 (CRL1), a newly identified early SL-responsive gene in roots, to restrain lateral root density under Pi deficiency. We also demonstrated that GR244DO treatment under normal conditions inhibits the expression of OsNRTs and OsAMTs to suppress nitrogen absorption but enhances the expression of OsPTs to promote Pi absorption, thus facilitating the balance between nitrogen and phosphorus uptake in rice. Importantly, we found that NSP1p:NSP1 and NSP2p:NSP2 transgenic plants show improved agronomic traits and grain yield under low- and medium-phosphorus conditions. Taken together, these results revealed a novel regulatory mechanism of SL biosynthesis and signaling in response to Pi starvation, providing genetic resources for improving plant architecture and nutrient-use efficiency in low-Pi environments.

WOX11 and CRL1 act synergistically to promote crown root development by maintaining cytokinin homeostasis in rice

Crown root (CR) morphogenesis is critical for normal growth and nutrition absorption in cereals. In rice, WUSCHEL-RELATED HOMEOBOX11 (WOX11) and CROWN ROOTLESS1 (CRL1) play vital roles in controlling CR development. Despite their importance, whether and how the two regulators coordinate CR formation remains unclear. Electrophoretic mobility shift assays, transient expression, and chromatin immunoprecipitation qPCR suggested that WOX11 and CRL1 directly bind to OsCKX4 to regulate its expression during CR development. CRL1 enhances OsCKX4 activation through direct interaction with WOX11 at root emergence and elongation stages. Genetic dissection showed that the wox11/crl1 double mutant exhibits a more severe root phenotype. OsCKX4 knockout plants generated by CRISPR/Cas9 exhibited fewer CRs and higher cytokinin levels in the root meristem. Increased expression of OsCKX4 could partially complement the CR phenotypes of both crl1 and wox11 mutants. Furthermore, cytokinin can promote WOX11 protein accumulation in the root meristem. Together, these findings show that cytokinin accumulation is tightly regulated by the WOX11-CRL1 complex during CR elongation by counteracting the negative regulatory effects of cytokinin on root development. Importantly, these results reveal an intrinsic link between WOX11 protein accumulation and cytokinin to maintain CR growth.

CROWN ROOTLESS1 binds DNA with a relaxed specificity and activates OsROP and OsbHLH044 genes involved in crown root formation in rice

In cereals, the root system is mainly composed of post-embryonic shoot-borne roots, named crown roots. The CROWN ROOTLESS1 (CRL1) transcription factor, belonging to the ASYMMETRIC LEAVES2-LIKE/LATERAL ORGAN BOUNDARIES DOMAIN (ASL/LBD) family, is a key regulator of crown root initiation in rice (Oryza sativa). Here, we show that CRL1 can bind, both in vitro and in vivo, not only the LBD-box, a DNA sequence recognized by several ASL/LBD transcription factors, but also another not previously identified DNA motif that was named CRL1-box. Using rice protoplast transient transactivation assays and a set of previously identified CRL1-regulated genes, we confirm that CRL1 transactivates these genes if they possess at least a CRL1-box or an LBD-box in their promoters. In planta, ChIP-qPCR experiments targeting two of these genes that include both a CRL1- and an LBD-box in their promoter show that CRL1 binds preferentially to the LBD-box in these promoter contexts. CRISPR/Cas9-targeted mutation of these two CRL1-regulated genes, which encode a plant Rho GTPase (OsROP) and a basic helix-loop-helix transcription factor (OsbHLH044), show that both promote crown root development. Finally, we show that OsbHLH044 represses a regulatory module, uncovering how CRL1 regulates specific processes during crown root formation.