CYP75B4-Mediated Tricin and Lignin Accumulation Improve Salt Tolerance in Rice

Salt stress limits plant growth and agricultural productivity and plants have evolved suitable mechanisms to adapt to salinity environments. It is important to characterize genes involved in plant salt tolerance, which will advance our understanding of mechanisms mediating salt tolerance and help researchers design ways to improve crop performances under high salinity environments. Here, we reported a CYP450 family member, CYP75B4, improves salt tolerance of rice seedlings by inducing flavonoid tricin and cell wall lignin accumulation. The CYP75B4 is highly expressed in tissues rich in cell walls and induced by salt treatment. Subcellular localization analysis revealed that CYP75B4 is localized in the endoplasmic reticulum (ER). The CYP75B4 overexpressing (CYP75B4-OE) lines showed significant enhancement in stem mechanical strength, whereas the cyp75b4 null mutants displayed weaker stems, as compared to the wild-type. Notably, the cyp75b4 and CYP75B4-OE lines showed decreased and improved, respectively, salt tolerance performances in terms of survival rate, ROS accumulation, and Na⁺/ K⁺ homeostasis. Additionally, the cyp75b4 mutants had a decreased tricin level, whereas CYP75B4-OE lines showed an increased tricin content, under both control or salinity conditions. Furthermore, treating the cyp75b4 mutants with tricin partly resorted salt stress tolerance to the wild-type levels, indicating a role of CYP75B4-mediated tricin production in rice response to salinity. Consistently, another tricin-deficient mutant cyp93g1 also displayed salt sensitivity and the tricin application could partly restore its salt-sensitive phenotypes. Moreover, the CYP75B4 significantly promotes lignin deposition in cell walls of mature stems and seedlings under salinity conditions, which probably contributes to the enhanced stem mechanical strength and improved salt tolerance in CYP75B4-OE plants. Our findings reveal a novel function of CYP75B4 in rice salt tolerance and lodging resistance by inducing tricin accumulation and lignin deposition in cell walls.