Natural Variations of <i>ZmRLR1</i> Mediate the Root Lodging Resistance of Maize by Regulating Root Ascorbate and Auxin Homeostasis

Root lodging is a major problem limiting crop yield and seed quality. Here, we showed that Root Lodging Resistance 1 (ZmRLR1) is specifically expressed in maize roots. The overexpression of ZmRLR1 increased the rhizosphere soil weight, stem pulling force of the internode 50 cm from the ground, total root length, and root volume. In contrast, the rhizosphere soil weight, stem pulling force of the internode 50 cm from the ground, total root length, and root volume were reduced in the Zmrlr1_ems and Zmrlr1 mutants. Natural variations in the promoter of ZmRLR1 are significantly associated with the root lodging resistance of maize. ZmRLR1 positively regulates the ascorbate (AsA) content in maize roots. We further demonstrated that ZmRLR1 improved the interaction between the ZmAP2 σ subunit and ZmCHC2. The internalization of the endocytic tracer FM4-64 is substantially reduced in the Zmrlr1_ems and Zmrlr1 mutants, while auxin distribution and ZmPIN1a-YFP localization are altered in Zmrlr1_ems. The defect in the total root length of Zmrlr1_ems is rescued by the exogenous application of 10 µmol L^-1 AsA plus 0.01 µmol L^-1 1-naphthaleneacetic acid. Taken together, these results suggest that ZmRLR1 affects the root lodging resistance of maize by regulating root AsA and auxin homeostasis.