Abstract

In the leaves of C3 rice, mesophyll cells (MCs) have a significantly greater chloroplast compartment than leaf bundle sheath cells (BSCs) and are the primary location for photosynthesis. In contrast, plants that use the more efficient C4 photosynthesis pathway have large chloroplast compartments in both MCs and BSCs. Engineering of rice BS chloroplasts such that they are larger and more abundant has the potential to drive a move to a C4-like photosynthetic state and improve photosynthetic efficiency. Targeting plant hormones such as brassinosteroids is one potential way to alter chloroplast development. In the work presented here, treatment of rice with brassinazole (a BR biosynthesis inhibitor) resulted in smaller but more abundant chloroplast in the BSCs. Additionally, constitutive and BSC -specific overexpression of OsBZR1, the primary transcription factor downstream of BRs, resulted in an increased BSC chloroplast size of up to 28%. Both results indicate a novel role for BRs in modulating chloroplast development which could be harnessed for downstream engineering.