Spatial and temporal regulation of chloroplast development in arabidopsis root

Abstract

In higher plants, heterotrophic organs develop non-green plastids whereas photosynthetic organs develop chloroplasts within the cells. In Arabidopsis, roots grow underground as heterotrophic organs with developing non-green plastids. Although the roots have a potential to develop chloroplasts, it may be usually repressed through the COP1/DET1-mediating degradation of HY5 even in the presence of light. On the other hand, our recent data demonstrate that the roots detached from the shoot develop chloroplasts through a modification of auxin/cytokinin signaling. To reveal the regulatory system of chloroplast biogenesis, in this study we investigated spatial and temporal patterns of chlorophyll accumulation, HY5 protein accumulation, and CHLH expression in Arabidopsis roots. While the patterns of chlorophyll accumulation and CHLH expression in roots were very similar, the distribution pattern of HY5 was largely different from those, suggesting that the accumulation of HY5 is not a rate-limiting factor for chlorophyll synthesis in roots. Meanwhile, overexpression of GOLDEN2-LIKE (GLK) 1 and GLK2 transcription factors resulted in an ectopic accumulation of chlorophyll in roots. Because GLKs are strong upregulators of genes associated with chlorophyll biosynthesis and light harvesting, the expression rate of GLK1 and GLK2 is likely one of limiting factors for chloroplast biogenesis in roots.