Bundle sheath suberisation is required for C4 photosynthesis in a Setaria viridis mutant (Nature Comms)

An efficient C₄ photosynthetic pathway relies on the coordination between the biochemical carbon concentration mechanism that secures high rates of C₄ acid flow into the bundle sheath (BS) and anatomical modifications that minimize CO₂ leakage out of the BS cells. Suberin deposition in BS cell walls has been believed to seal BS cells against leakiness of CO₂ in NADP-ME C₄ grasses; however, direct evidence proving suberin’s role was lacking. Through screening of the N-nitroso-N-methylurea induced Setaria viridis (a C₄ NADP-ME) mutants for impaired photosynthesis, Danila et al. isolated a mutant with an SNP in the coding region of an ABCG transporter gene (Sevir.9G451500) that was expressed exclusively in the developing leaf section of the C₄ NADP-ME species – maize and Setaria, but not the C₃ species rice. Cell wall imaging confirmed deposition defect and GC/MS analysis detected a 50% reduction of aliphatic suberin component in the bundle sheath cells of the mutants providing direct evidence in the role of Sevir.9G451500 in suberin synthesis. The mutants exhibited a higher BS conductance to CO₂ due to suberin defect, grew poorly in ambient CO₂, and showed reduced biomass production, CO₂ assimilation rate, and stomatal conductance in high CO₂. With elaborate physiological evaluation, Danila et al. linked the leaky BS to the cell wall suberin defect and verified that bundle sheath suberin lamella is an essential ingredient to assemble an efficient C₄ pathway in NADP-ME C₄ grasses. (Summary by Roxana Khoshravesh @RoxiKh) Comms.  Biol. 10.1038/s42003-021-01772-4