Abstract: Drought severely impedes plant growth and production as a primary abiotic stress. GIGANTEA (GI) regulates flowering and responds to various stresses in model plants; however, its function remains poorly understood in non-model plants. In this study, a Citrus limon GI homologous (CiGI) was identified and two alternative splicing transcripts (CiGIα and CiGIβ) were found. CiGIα overexpressing tobacco exhibited early flowering and drought sensitivity, whereas the phenotype of CiGIβ-overexpressing plants was similar to that of wild-type (WT) plants. Overexpression of CiGIα in citrus increased drought sensitivity and upregulated citrus FLOWERING LOCUS T (CiFT) expression, and downregulation of CiGI enhanced drought tolerance. Further studies revealed that CiGIα, CiGIβ, and LATE ELONGATED HYPOCOTYL (CiLHY) form a complex that binds to the Nuclear Factor YA1 (CiNF-YA1) promoter and activates its expression. Subsequently, CiNF-YA1 activates the expression of NADP-DEPENDENT MALIC ENZYME 2 (CiNADP-ME2) by binding its promoter, leading to increased reactive oxygen species (ROS) accumulation, which enhances plant drought sensitivity. Exogenous ROS treatment induced citrus flowering and reduced drought tolerance. Furthermore, the CiGI–CiLHY complex also activates CiFT and may participate in the regulation of citrus flowering. These results reveal a novel mechanism by which CiGI regulates citrus flowering and drought tolerance.

Key words: citrus, drought, flowering, GIGANTEA, reactive oxygen species