Simultaneous but spatially different regulation of non-photosynthetic callus formation and photosynthetic root development after shoot removal.

Plants coordinate plastid differentiation and cellular differentiation during tissue development. Recently, we reported a mechanism of chloroplast development and photosynthetic improvement in Arabidopsis roots after shoot removal. Shoot removal induces the expression of WOUND INDUCED DEDIFFERENTIATION (WIND) transcription factors at the wound site, which activates type-B ARABIDOPSIS RESPONSE REGULATOR (ARR)-mediated cytokinin signaling in roots. The type-B ARR signaling upregulates chloroplast-related transcription factors along with photosynthesis-associated genes, thereby inducing chloroplast development in detached roots. However, at the wound site, WINDs promote the development of non-green callus cells in a type-B ARR-dependent manner. Thus, in shoot-removed roots, WIND-mediated cytokinin signaling has spatially distinct roles: it represses chloroplast development at the wound site while activating the development in adjacent regions. Because WINDs are strong inducers of cell dedifferentiation, spatial differences in their expression levels between the wound site and other areas may determine the fate of chloroplast development.