Functional Characterization of the GATA Transcription Factors GNC and CGA1 Reveals Their Key Role in Chloroplast Development, Growth, and Division in Arabidopsis

Chloroplasts develop from proplastids in a process that requires the interplay of nuclear and chloroplast genomes, but key steps in this developmental process have yet to be elucidated. Here, we show that the nucleus-localized transcription factors GATA NITRATE-INDUCIBLE CARBON-METABOLISM-INVOLVED (GNC) and CYTOKININ-RESPONSIVE GATA1 (CGA1) regulate chloroplast development, growth, and division in Arabidopsis (Arabidopsis thaliana). GNC and CGA1 are highly expressed in green tissues, and the phytohormone cytokinin regulates their expression. A gnc cga1 mutant exhibits a reduction in overall chlorophyll levels as well as in chloroplast size in the hypocotyl. Ectopic overexpression of either GNC or CGA1 promotes chloroplast biogenesis in hypocotyl cortex and root pericycle cells, based on increases in the number and size of the chloroplasts, and also results in expanded zones of chloroplast production into the epidermis of hypocotyls and cotyledons and into the cortex of roots. Ectopic overexpression also promotes the development of etioplasts from proplastids in dark-grown seedlings, subsequently enhancing the deetiolation process. Inducible expression of GNC demonstrates that GNC-mediated chloroplast biogenesis can be regulated postembryonically, notably so for chloroplast production in cotyledon epidermal cells. Analysis of the gnc cga1 loss-of-function and overexpression lines supports a role for these transcription factors in regulating the effects of cytokinin on chloroplast division. These data support a model in which GNC and CGA1 serve as two of the master transcriptional regulators of chloroplast biogenesis, acting downstream of cytokinin and mediating the development of chloroplasts from proplastids and enhancing chloroplast growth and division in specific tissues

Cytokinin Stimulates Chloroplast Transcription in Detached Barley Leaves1[OA]

Chloroplasts are among the main targets of cytokinin action in the plant cell. We report here on the activation of transcription by cytokinin as detected by run-on assays with chloroplasts isolated from apical parts of first leaves detached from 9-d-old barley (Hordeum vulgare) seedlings and incubated for 3 h on a 2.2 × 10−5 m solution of benzyladenine (BA). Northern-blot analysis also detected a BA-induced increase in the accumulation of chloroplast mRNAs. A prerequisite for BA activation of chloroplast transcription was preincubation of leaves for 24 h on water in the light, resulting in a decreased chloroplast transcription and a drastic accumulation of abscisic acid. Cytokinin enhanced the transcription of several chloroplast genes above the initial level measured before BA treatment, and in the case of rrn16 and petD even before preincubation. Cytokinin effects on basal (youngest), middle, and apical (oldest) segments of primary leaves detached from plants of different ages revealed an age dependence of chloroplast gene response to BA. BA-induced stimulation of transcription of rrn16, rrn23, rps4, rps16, rbcL, atpB, and ndhC required light during the period of preincubation and was further enhanced by light during the incubation on BA, whereas activation of transcription of trnEY, rps14, rpl16, matK, petD, and petLG depended on light during both periods. Our data reveal positive and differential effects of cytokinin on the transcription of chloroplast genes that were dependent on light and on the age (developmental stage) of cells and leaves