Salinity is a major hazard to crop plant growth and significantly reduces grain yield. When subjected to high salinity, plants maintain ion homeostasis through Na⁺ compartmentalization and exclusion, enabling them to mitigate salt stress. In this study, we revealed the role of rice (Oryza sativa) MYB9 (OsMYB9) in regulating salt stress tolerance. OsMYB9 was expressed in the vascular bundles of roots and leaves, particularly in the parenchyma cells. The null mutation of OsMYB9 resulted in increased sodium ion accumulation in shoot tissues under salt stress, leading to salt-sensitive phenotypes. Real-Time Quantitative Reverse Transcription PCR analysis indicated the OsMYB9 mutation led to decreased expression of several genes associated with vacuolar Na⁺/H⁺ antiporters (OsNHX1, OsNHX2, OsNHX3, OsNHX4, and OsNHX5), high-K⁺ affinity transporters (OsHKT1;5), and SALT OVERLY SENSITIVE proteins (OsSOS2 and OsSOS3). Among these, OsMYB9 upregulated the expression of OsNHX1 and OsNHX2 by directly binding to their promoter regions. Furthermore, GIGANTEA (OsGI) interacted with OsMYB9, suggesting that OsGI negatively acts upstream of OsMYB9 and regulates the expression levels of OsNHX1. Collectively, OsMYB9 alleviates the excess accumulation of Na⁺ ions in the xylem by retrieving Na⁺ ions from xylem parenchyma cells and compartmentalizing them into vacuoles. These regulatory mechanisms mediated by OsMYB9 are crucial for minimizing Na⁺ toxicity in photosynthetic tissues and enhancing salt stress tolerance in rice.

The transition from the vegetative to reproductive stage (flowering) is a critical stage in the life cycle of plants. Transition to flowering is controlled by a complex photoperiod-dependent network. Here, we report the functional analysis of NAC transcription factor ONAC005 as a positive regulator of flowering in rice. An onac005-1 T-DNA insertion mutant showed late flowering only under long-day (LD) conditions, indicating that ONAC005 is an LD-dependent flowering activator. Expression analysis of flowering time genes revealed that ONAC005 negatively regulates expression of OsLFL1, which is a LD-specific repressor of flowering in rice. Consequently, ONAC005 up-regulates expression of downstream genes Ehd1, Hd3a, and RFT1 under LD conditions. ONAC005 physically interacts with previously reported upstream regulators of OsLFL1, OsMADS50 and OsMADS56. Binding assays showed that ONAC005 binds to the promoter regions of OsLFL1. We further found that the ONAC005-OsMADS50-OsMADS56 complex weakly repressed OsLFL1 at the early vegetative stage and then gradually repressed it at the transcriptional level as the plant developed. Taken together, our results suggest that ONAC005 specifically affects flowering under LD-dependent conditions by interacting with an antagonistic protein complex.

Senescence-induced NAC (senNAC) TFs play a crucial role in senescence during the final stage of leaf development. In this study, we identified a rice senNAC, ONAC016, which functions as a positive regulator of leaf senescence. The expression of ONAC016 increased rapidly in rice leaves during the progression of dark-induced and natural senescence. The onac016-1 knockout mutant showed a delayed leaf yellowing phenotype, whereas the overexpression of ONAC016 accelerated leaf senescence. Notably, ONAC016 expression was upregulated by abscisic acid (ABA), and thus detached leaves of the onac016-1 mutant remained green much longer under ABA treatment. Quantitative RT-PCR analysis showed that ONAC016 upregulates the genes associated with chlorophyll degradation, senescence, and ABA signaling. Yeast one-hybrid and dual-luciferase assays revealed that ONAC016 binds directly to the promoter regions of OsNAP, a key gene involved in chlorophyll degradation and ABA-induced senescence. Taken together, these results suggest that ONAC016 plays an important role in promoting leaf senescence through the ABA signaling pathway involving OsNAP.