Cucumber NUCLEAR FACTOR-YC2/-YC9 target translocon component CsTIC21 in chloroplast photomorphogenesis

Light signals promote photomorphogenesis and photosynthesis, allowing plants to establish photoautotrophic growth. Chloroplasts are organelles responsible for photosynthesis in which light energy is converted into chemical energy and stored as organic matter. However, how light regulates chloroplast photomorphogenesis remains unclear. Here, we isolated a cucumber (Cucumis sativus L.) mutant albino seedling (as) from an ethyl methane sulfonate mutagenesis library with an albino phenotype. Map-based cloning revealed that the mutation occurred in a component of cucumber translocon at the inner membrane of chloroplasts (CsTIC21). Subsequently, virus-induced gene silencing and CRISPR/Cas9 analyses confirmed the association between the mutant gene and the as phenotype. Loss-of-function of CsTIC21 induces malformation of chloroplast formation, leading to albinism and death in cucumber. Notably, CsTIC21 transcription was very low in etiolated seedlings grown in the dark and was upregulated by light, with expression patterns similar to those of Nuclear factor-YC (NF-YC) genes. Here, 7 cucumber NF-YC family genes (CsNF-YC) were identified, among which the expression of 4 genes (CsNF-YC1, -YC2, -YC9, and -YC13) responded to light. Gene silencing of all CsNF-YC genes in cucumber indicated that CsNF-YC2, -YC9, -YC11-1, and -YC11-2 induced distinct etiolated growth and decreased chlorophyll content. Interaction studies verified that CsNF-YC2 and CsNF-YC9 target the CsTIC21 promoter directly and promote gene transcription. These findings provide mechanistic insights on the role of the NF-YCs-TIC21 module in chloroplast photomorphogenesis promoted by light in cucumber.