A transcriptional cascade involving BBX22 and HY5 finely regulates both plant height and fruit pigmentation in citrus.

ABSTRACT Dwarfing is a pivotal agronomic trait affecting both yield and quality. Citrus species exhibit substantial variation in plant height, among which internode length is a core element. However, the molecular mechanism governing internode elongation remains unclear. Here, we unveiled that the transcriptional cascade consisting of <italic>B‐BOX DOMAIN PROTEIN 22</italic> (<italic>BBX22</italic>) and <italic>ELONGATED HYPOCOTYL 5</italic> (<italic>HY5</italic>) finely tunes plant height and internode elongation in citrus. Loss‐of‐function mutations of <italic>BBX22</italic> in an early‐flowering citrus (<italic>Citrus hindsii</italic> “SJG”) promoted internode elongation and reduced pigment accumulation, whereas ectopic expression of <italic>BBX22</italic> in SJG, sweet orange (<italic>C. sinensis</italic>), pomelo (<italic>C. maxima</italic>) or heterologous expression of <italic>BBX22</italic> in tomato (<italic>Solanum lycopersicum</italic>) significantly decreased internode length. Furthermore, exogenous application of gibberellin A3 (GA3) rescued the shortened internode and dwarf phenotype caused by <italic>BBX22</italic> overexpression. Additional experiments revealed that BBX22 played a dual role in regulation internode elongation and pigmentation in citrus. On the one hand, it directly bound to and activated the expression of <italic>HY5</italic>, GA metabolism gene (<italic>GA2 OXIDASE 8</italic>, <italic>GA2ox8</italic>), carotenoid biosynthesis gene (<italic>PHYTOENE SYNTHASE 1</italic>, <italic>PSY1</italic>) and anthocyanin regulatory gene (<italic>Ruby1,</italic> a MYB DOMAIN PROTEIN). On the other hand, it acted as a cofactor of HY5, enhancing the ability of HY5 to regulate target genes expression. Together, our results reveal the critical role of the transcriptional cascade consisting of <italic>BBX22</italic> and <italic>HY5</italic> in controlling internode elongation and pigment accumulation in citrus. Unraveling the crosstalk regulatory mechanism between internode elongation and fruit pigmentation provides key genes for breeding of novel types with both dwarf and health‐beneficial fortification in citrus. [ABSTRACT FROM AUTHOR]