A grafting-rescued albino mutant reveals the essential function of CspTAC12 in cucumber chloroplast biogenesis

Chloroplast biogenesis requires precise coordination between the nuclear and plastid genomes, orchestrated by the plastid-encoded RNA polymerase (PEP) complex. In this study, we characterized a spontaneous albino lethal mutant, XNS106_1, in cucumber. The mutant exhibits complete chlorophyll deficiency and photosynthetic failure, leading to post-germinative lethality. Genetic analysis revealed that the phenotype is controlled by a single recessive locus, Csal, with significant segregation distortion. Utilizing a grafting technique to rescue the mutant for propagation, we fine-mapped Csal to a 75.1-Kb interval on chromosome 6. Within this region, a causative G-to-C single-nucleotide polymorphism was identified in the fifth exon of CsaV3_6G052070, which encodes the cucumber homolog of Plastid Transcriptionally Active 12 (CspTAC12), an essential nuclear-encoded component of the PEP complex. Subcellular localization confirmed that CspTAC12 is targeted to both chloroplasts and the nucleus. Integrated transcriptomic and physiological profiling demonstrated that the csptac12 mutation collapses PEP-dependent transcription, which in turn activates a comprehensive retrograde signaling network. This signaling cascade involves a burst of reactive oxygen species, altered salicylic acid and auxin levels, and extensive transcriptional reprogramming in the nucleus, including the suppression of photosynthesis-associated nuclear genes and the activation of defense-related pathways. Our work identifies CspTAC12 as a critical regulator of chloroplast development in cucumber and delineates the complex retrograde communication network triggered by its dysfunction.