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

Florence R Danila,Susanne Von Caemmerer,Robert T Furbank,Vivek Thakur,Soumi Bala,William Paul Quick,Robert A Coe,Kelvin Acebron,Jolly Chatterjee

doi:10.1038/s42003-021-01772-4

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.

Highlights

C4 photosynthesis provides an effective solution for overcoming the catalytic inefficiency of Rubisco
Peptide sequence analysis of Sevir.9G451500 showed that R552 is located a few amino acids upstream of transmembrane domain (TMD) in an amphipathic α-helix (Supplementary Fig. 4), which is previously reported to be a key component of the transmission interface essential for ATP-binding cassette subfamily G (ABCG) protein folding, ATP hydrolysis, and substrate binding and transport[29]
Expression analysis performed using existing maize[21], rice[21] and S. viridis[22] data showed that this particular ABCG transporter gene is highly expressed in the basal region of Zea mays and S. viridis leaves, but not in Oryza sativa leaf (Fig. 1c). This corroborates the genetic association of the ABCG transporter gene with C4 photosynthesis and leaf anatomy, most likely in bundle sheath (BS) wall suberisation as Sevir.9G451500’s direct orthologues in monocot and dicot C3 plants (Supplementary Fig. 5) have previously been reported to be involved in suberin transport[30,31,32,33]

Summary

Introduction

C4 photosynthesis provides an effective solution for overcoming the catalytic inefficiency of Rubisco. Nile red staining, TEM, and GC/MS confirmed the alteration in suberin deposition in the BS cell wall of 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. The pathway is characterised by a biochemical CO2 concentrating mechanism that involves coordinated functioning of mesophyll (M) and bundle sheath (BS) cells within a leaf[3]. CO2 is initially assimilated into C4 acids by phosphoenolpyruvate (PEP) carboxylase in the mesophyll cells These acids diffuse to and are decarboxylated in BS cells where CO2 is concentrated. It has been hypothesised that low conductance to CO2 diffusion across the M and BS interface is an essential feature of the C4 photosynthetic CO2 concentrating mechanism[5,6].

Methods

Results

Conclusion