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Originating in the subtropics, cucumbers require warm weather and plenty of light, low light inhibits the growth of cucumbers. For cucumbers to efficiently absorb light energy in low light, chlorophyll synthesis is necessary. Oxygenase for chlorophyllate is encoded by CsCAO. RNA interference (RNAi) was used to reduce CsCAO expression in cucumber, reduced chlorophyll b content and Fv/Fm (maximal photochemical efficiency of photosystem II) were found in RNAi- CsCAO lines. The Arabidopsis Atcao mutant lacks chlorophyll b and has a low Fv/Fm, but overexpressing CsCAO restored the mutant's phenotype. CsCAO gene expression was found to be upregulated in the presence of low light. The Y1H screening assay revealed that CsHsfA6b and CsBLH4 were potential CsCAO expression regulators. The results of the Y1H, LUC/REN, and EMSA assays confirmed that only CsBLH4 can bind to the CsCAO promoter. Furthermore, like Atcao , the Atblh4 mutant had low chlorophyll b content and Fv/Fm. CsCAO overexpression in the Atblh4 mutant increased the content of chlorophyll b and Fv/Fm, especially under low light conditions. CsBLH4 targeting CsCAO was found to positively regulate chlorophyll b synthesis and photosynthetic capacity in cucumber under low light conditions in our study. • Low light promoted the accumulation of chlorophyll b by up-regulating the transcription level of CsCAO. • It clarified the regulatory mechanism of BLH family member involved in chlorophyll b synthesis. • The synthesis of chlorophyll plays an important role in the response of cucumber to low light stress. [ABSTRACT FROM AUTHOR]