Manipulating cell cycle of rice bundle sheath cells reduces their chloroplast occupancy and inhibits leaf growth

Loss of OsCNR1 in rice increases chloroplast number per bundle sheath cell, but bundle sheath chloroplast occupancy is not increased by OsdnDAR2 or OsrGRF10 overexpression.

• Why it matters: Engineering efficient C4 photosynthesis into C3 crops requires coordinating bundle sheath cell number and chloroplast biogenesis, yet the cell-cycle link is not well defined.
• What they did: Rice knockouts for OsCNR1 and bundle sheath-specific overexpression of OsrGRF10 and OsdnDAR2 were generated, and leaf growth plus transcriptome changes were analyzed alongside chloroplast number, size, and occupancy.
• The result: OsCNR1 loss shifts bundle sheath development toward earlier differentiation while increasing chloroplast number per bundle sheath cell; OsdnDAR2 and OsrGRF10 reduce chloroplast size and do not raise total chloroplast occupancy, implying C4 bundle sheath traits cannot be fully recapitulated.