Arabidopsis chloroplast J protein DJC75/CRRJ mediates nitrate-promoted seed germination in the dark

Abstract

Background and Aims Nitrate can stimulate seed germination of many plant species in the absence of light; however, the molecular mechanism of nitrate-promoted seed germination in the dark remains largely unclear and no component of this pathway has been identified yet. Here, we show that a plastid J-domain protein, DJC75/CRRJ, in arabidopsis (Arabidopsis thaliana) is important for nitrate-promoted seed germination in the dark. Methods The expression of DJC75 during imbibition in the dark was investigated. The seed germination rate of mutants defective in DJC75 was determined in the presence of nitrate when light cues for seed germination were eliminated by the treatment of imbibed seeds with a pulse of far-red light to inactivate phytochrome B (phyB), or by assaying germination in the dark with seeds harbouring the phyB mutation. The germination rates of mutants defective in CRRL, a J-like protein related to DJC75, and in two chloroplast Hsp70s were also measured in the presence of nitrate in darkness. Key Results DJC75 was 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 gibberellin biosynthetic gene GA3ox1 expression by nitrate in imbibed phyB mutant seeds was diminished when DJC75 was knocked out. Conclusions Our data suggest that plastid J-domain protein DJC75 regulates nitrate-promoted seed germination in the dark by upregulation of expression of the gibberellin 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.

Research Progress in J-Proteins in the Chloroplast

“…This suggests that there may be an evolutionary relationship between DJA5/6 and CDJ3–5. Recently, it was found that DJC75 and DNAJD15 cooperated with each other and participated in nitrate-promoted seed germination in the dark [ 26 , 27 ]. These findings provide a clear example that J-like protein can cooperate with J-proteins to synergistically regulate metabolic activity.…”

“…Recently, it was found that DJC75 and DNAJD15 cooperate with each other and participate in nitrate-promoted seed germination in the dark [ 26 , 27 ]. This also suggests that the two classes of proteins can act synergistically to regulate life activities.…”

“…djc75 , dnajd15 , cpHSP70-1 or cpHSP70-2 mutation heavily abolished the nitrate-stimulated germination of phyB mutant seeds, whose germination rate increased significantly under the culture condition containing both micronutrients and nitrate [ 28 , 29 ]. Therefore, it is suggested that DJC75 may activate cpHSP70s ATPase through its HPD (histidine-proline-aspartic acid) motif, so as to recruit cpHSP70s to participate in nitrate-stimulated seed germination in the dark [ 27 ]. So far, it is unclear whether NDH is involved in nitrate-stimulated seed germination in the dark.…”

“…If not, DJC75 and DNAJD15 may play other functions that are independent of NDH. Furthermore, it was found that DJC75 regulates the transcription of GA30ox1 , a key GA biosynthetic gene, that may contribute to the regulation of nitrate-promoted seed germination in the dark, although the mechanism needs further investigation [ 27 ].…”