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 con-sisting of B-BOX DOMAIN PROTEIN 22(BBX22)and ELONGATED HYPOCOTYL 5(HY5)finely tunes plant height and internode elongation in citrus.Loss-of-function mutations of BBX22 in an early-flowering citrus(Citrus hindsii"SJG")promoted internode elongation and reduced pigment accumulation,whereas ectopic ex-pression of BBX22 in SJG,sweet orange(C.sinensis),pomelo(C.maxima)or heterologous expression of BBX22 in tomato(Solanum lyco-persicum)significantly decreased internode length.Furthermore,exogenous application of gibberellin A3(GA3)rescued the shortened in-ternode and dwarf phenotype caused by BBX22 overexpression.Additional experiments re-vealed that BBX22 played a dual role in regu-lation internode elongation and pigmentation in citrus.On the one hand,it directly bound to and activated the expression of HY5,GA metabo-lism gene(GA2 OXIDASE 8,GA2ox8),car-otenoid biosynthesis gene(PHYTOENE SYN-THASE 1,PSY1)and anthocyanin regulatory gene(Ruby1,a MYB DOMAIN PROTEIN).On the other hand,it acted as a cofactor of HY5,en-hancing the ability of HY5 to regulate target genes expression.Together,our results reveal the critical role of the transcriptional cascade consisting of BBX22 and HY5 in controlling in-ternode 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.
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