Seed germination is a critical step in the life cycle of flowering plants. Since seeds contain limited reserves, seed germination has to be tightly regulated to ensure that germination takes place in a suitable environment where seedlings can reach the sunlight and carry out photosynthesis before seed reserves are exhausted. However, seeds may not be able to germinate even under favorable conditions, which is called seed dormancy. Huai-Syuan Ciou and colleagues tested to see if a plastid J-domain protein DJC75/CRRJ in Arabidopsis (Arabidopsis thaliana) is important for nitrate-promoted seed germination in the dark.
The ability of nitrate to stimulate seed germination has been known for more than a century. Nitrate has been shown to relieve seed dormancy and to stimulate germination of a variety of plants in the light or in darkness. Under illumination, nitrate can reduce the light requirement for dormancy release of dormant Arabidopsis seeds, but nitrate itself failed to release seed dormancy in the absence of light.
DJC75 is expressed during seed imbibition in the absence of light. Mutants defective in DJC75 showed seed germination defects in the presence of nitrate when light cues for seed germination were eliminated. Mutants defective in CRRL, and in two chloroplast Hsp70s also exhibited similar seed germination defects. Upregulation of GA biosynthetic gene GA3ox1 expression by nitrate in imbibed phyB mutant seeds was diminished when DJC75 was knocked-out.
The authors’ data suggest that plastid J-domain protein DJC75 regulates nitratepromoted seed germination in the dark by upregulation of expression of GA , biosynthetic gene GA3ox1 through an unknown mechanism and that DJC75 may work in concert with chloroplast Hsp70s to regulate nitrate-promoted seed germination. DJC75 is the first pathway component identified for nitrate-promoted seed germination in the dark.
“No matter what photoperiod plants grow in, DJC75-mediated nitrate promoted seed germination in the dark may act as a survival mechanism to stimulate the germination of more seeds,” say Ciou and colleagues.