Bundle sheath suberisation is required for C4 photosynthesis in a Setaria viridis mutant

Abstract

C4 photosynthesis provides an effective solution for overcoming the catalytic inefficiency of Rubisco. The pathway is characterised by a biochemical CO2 concentrating mechanism that operates across mesophyll and bundle sheath (BS) cells and relies on a gas tight BS compartment. A screen of a mutant population of Setaria viridis, an NADP-malic enzyme type C4 monocot, generated using N-nitroso-N-methylurea identified a mutant with an amino acid change in the gene coding region of the ABCG transporter, a step in the suberin synthesis pathway. Here, Nile red staining, TEM, and GC/MS confirmed the alteration in suberin deposition in the BS cell wall of the mutant. We show that this has disrupted the suberin lamellae of BS cell wall and increased BS conductance to CO2 diffusion more than two-fold in the mutant. Consequently, BS CO2 partial pressure is reduced and CO2 assimilation was impaired in the mutant. Our findings provide experimental evidence that a functional suberin lamellae is an essential anatomical feature for efficient C4 photosynthesis in NADP-ME plants like S. viridis and have implications for engineering strategies to ensure future food security.

Accelerated remodeling of the mesophyll-bundle sheath interface in the maize C4 cycle mutant leaves

“…Abnormal suberin layers in the BS cell wall of mutant Setaria viridis leaves caused loss of CO2 from BS cells, decreasing their C4 photosynthetic efficiency 39 . The concentration of CO2 to BS chloroplasts is expected to be affected in ppdk-1 or dct2 leaf cells as C4 cycle is aberrant in them.…”

“…Because sucrose in the vascular parenchyma enters the companion cell after being exported to the apoplast, the hydrophobic stratum of the vascular bundle may inhibit the loss of sucrose 37 , 38 . It was recently shown that the suberin layer in the Setaria BS cell wall is critical for keeping CO 2 from leaking away from BS 39 .…”

Elucidating the role of SWEET13 in phloem loading of the C4 grass Setaria viridis

“…In some instances, localization occurred predominantly on the side of the bundle sheath cell in direct contact with the phloem (Figures 4c and 5a,c,e). Observations in Setaria have shown that the part of the bundle sheath cell in direct contact with the mesophyll cell is highly suberized, which would rule out membrane transport of sugars across this interface (Danila et al., 2021). This is consistent with the observations here that SvSWEET13s is localized only to the phloem side of the bundle sheath cells (Figure 5).…”

Evolution of a biochemical model of steady‐state photosynthesis

“…Like g m , g bs varies with leaf age or N content (Yin et al, 2011), temperature (Alonso-Cantabrana et al, 2018Kiirats, Lea, Franceschi, & Edwards, 2002;Yin, van der Putten, Driever, & Struik, 2016), and growth light conditions (Bellasio & Griffiths, 2014;Kromdijk, Griffiths, & Schepers, 2010;Ubierna, Sun, Kramer, & Cousins, 2013). Danila et al (2021) showed that suberization of the BS lamellae is required for a low g bs to minimize leakage.…”

“…Like g m , g bs varies with leaf age or N content (Yin et al, 2011), temperature (Alonso‐Cantabrana et al, 2018; Kiirats, Lea, Franceschi, & Edwards, 2002; Yin, van der Putten, Driever, & Struik, 2016), and growth light conditions (Bellasio & Griffiths, 2014; Kromdijk, Griffiths, & Schepers, 2010; Ubierna, Sun, Kramer, & Cousins, 2013). Danila et al (2021) showed that suberization of the BS lamellae is required for a low g bs to minimize leakage. As g bs is a lumped model parameter, its value may also depend on other anatomical characteristics (like BS cell wall thickness, plasmodesmata density, bundle sheath surface area‐to‐leaf area ratio, intervein spacing, sheath layers) as well as biochemical characteristics (like the location of decarboxylation).…”