RCN1 helps to alleviate the damage of drought stress to rice young spike differentiation

Drought is a major constraint to rice (Oryza sativa L.) yield. The young spike differentiation stage is crucial for yield determination, exhibiting heightened sensitivity to soil moisture deficits. RICE CENTRORADIALIS 1 (RCN1), a TERMINAL FLOWER-like gene, is a key regulator of young spike differentiation and may mediate drought-induced delays in floral transition in rice. This study examines the effect of drought stress during the young spike differentiation stage on rice spike structure and investigates the role of RCN1 in this process. Drought stress markedly suppresses branch differentiation, especially secondary branches. Compared to the wild type (WT), the spike structure of rcn1 mutants was more severely compromised under drought conditions, the ABA-mediated protection of spike structure was also reduced in the rcn1 mutants. Key flowering regulators, Ehd1 and Hd1, were implicated in the regulation of young spike differentiation under drought stress, with the drought response of Ehd1 negatively modulated by RCN1. Furthermore, RCN1 influenced the expression of branch-promoting genes, including LAX1, RCN2 and RCN4 under drought. RCN1 displayed high sensitivity in rice roots, the increase rate of new roots in rcn1 mutants was lower than in WT under 10% PEG-6000 treatment. Under 15% PEG-6000 treatment, rcn1 mutants showed lower survival rate and ABA content compared to WT. RCN1 contributes to the maintenance of spike structure under drought stress by promoting second branch formation and affecting the drought response of some spike differentiation-related genes (Ehd1, LAX1, RCN2 and RCN4). Additionally, RCN1 enhances drought tolerance of rice by promoting abscisic acid (ABA) biosynthesis and facilitating new root germination, thereby indirectly reducing yield losses.