Delayed exposure of rice to light partially phenocopies C4 bundle sheath characteristics by reducing cell length

C4 photosynthesis has arisen in over sixty lineages of angiosperms. Despite significant variation in characteristics of the C4 leaf, in all cases a compartment rich in chloroplasts allows high concentrations of CO2 to be supplied to RuBisCO. In most C4 lineages this is associated with increased chloroplast content of bundle sheath cells. As characteristics of C4 leaves are derived from processes operating in the ancestral C3 state, we studied greening of the bundle sheath in C3 rice. Total visible chloroplast area in bundle sheath and mesophyll cells was similar but the larger volume of bundle sheath cells reduced the proportion of cell occupied by chloroplast, and the longest bundle sheath cells had the lowest chloroplast content. Application of exogenous cytokinin and gibberellin increased chloroplast content of the longest bundle sheath cells but did not change cell shape. In contrast, delayed exposure to light reduced the number of very long bundle sheath cells but increased leaf length indicating delayed exit from the cell cycle. We conclude that in the C3 state, the plant hormones cytokinin and gibberellin are regulators of the bundle sheath cell-chloroplast relationship and that final bundle sheath length is affected by light-mediated control of exit from the cell cycle. HighlightExtended darkness can reduce maximum bundle sheath cell length in rice and hormones condition the set-point between cell size and chloroplast content.