New evidence for grain specific C4 photosynthesis in wheat

Abstract

The C4 photosynthetic pathway evolved to allow efficient CO2 capture by plants where effective carbon supply may be limiting as in hot or dry environments, explaining the high growth rates of C4 plants such as maize. Important crops such as wheat and rice are C3 plants resulting in efforts to engineer them to use the C4 pathway. Here we show the presence of a C4 photosynthetic pathway in the developing wheat grain that is absent in the leaves. Genes specific for C4 photosynthesis were identified in the wheat genome and found to be preferentially expressed in the photosynthetic pericarp tissue (cross- and tube-cell layers) of the wheat caryopsis. The chloroplasts exhibit dimorphism that corresponds to chloroplasts of mesophyll- and bundle sheath-cells in leaves of classical C4 plants. Breeding to optimize the relative contributions of C3 and C4 photosynthesis may adapt wheat to climate change, contributing to wheat food security.

Figure 1. Grain specific expression of genes involved in NAD-ME type C₄ photosynthesis as RPKM values (mean of 31 genotypes) partitioned to the sub-genome level at 14 days-post-anthesis period.

(A) ppc (phosphoenolpyruvate carboxylase); (B) aat (aspartate aminotransferase); (C) mdh (malate dehydrogenase); (D) me2 (NAD-dependent malic enzyme); (E) gpt (alanine aminotransferase); (F) ppdk (orthophosphate, pyruvate dikinase). Abbreviations used in X-axis panel–ABD: three sub-genomes of hexaploid wheat; cp: chloroplast targeted; cy: cytosolic; L: chromosomal long arm; mt: mitochondria targeted; S: chromosomal short arm.

Figure 2. Comparison of expression levels (measured as RPKM) for genes involved in NAD-ME type C₄ photosynthesis in leaf and grain tissues of hexaploid wheat.

for X-axis, refer bottom most panel. (A) ppc (phosphoenolpyruvate carboxylase); (B) aat (aspartate aminotransferase); (C) mdh (malate dehydrogenase); (D) me2 (NAD-dependent malic enzyme); (E) gpt (alanine aminotransferase); (F) ppdk (orthophosphate, pyruvate dikinase). Abbreviations used in the X-axis panel and series’ legends – ABD: three sub-genomes of hexaploid wheat; cp: chloroplast targeted; cy: cytosolic; L: chromosomal long arm; mt: mitochondria targeted; S: chromosomal short arm; Zadok’s scales Z10: seedling stage; Z23: tillering stage; Z71: watery kernel stage; Z75: early-grain filling stage (app.14 days-post-anthesis); Z85: late-grain filling stage (app. 30 days-post-anthesis).

Figure 3. Variation in expression levels (measured as RPKM) for genes involved in NAD-ME type C₄ photosynthesis in 31 wheat genotypes at 14 days-post-anthesis. For X-axis, refer ‘panel F’.

(A) ppc (phosphoenolpyruvate carboxylase); (B) aat (aspartate aminotransferase); (C) mdh (malate dehydrogenase); (D) me2 (NAD-dependent malic enzyme); (E) gpt (alanine aminotransferase); (F) ppdk (orthophosphate, pyruvate dikinase). Abbreviations used in series’ legends – ABD: three sub-genomes of hexaploid wheat; cp: chloroplast targeted; cy: cytosolic; L: chromosomal long arm; mt: mitochondria targeted; S: chromosomal short arm.

Figure 4. Schema depicting a longitudinal cross section of developing wheat grains with one cross- and tube-cell in enlarged view illustrating the C₄-like type photosynthetic pathway through NAD-dependent malic enzyme (NAD-ME) subtype [adapted and modified from Sage and Monson15].

2-OG: 2-Oxyglutarate; Ala: Alanine; Asp: aspartate; Glu: glutamate; Mal: malate; OAA: oxaloacetate; PEP: phosphoenolpyruvate; Pyr: pyruvate; 1, Carbonic anhydrase (CA, EC 4.2.1.1); 2, PEP carboxylase (PPC, EC 4.1.1.31); 3, Aspartate aminotransferase (AAT, EC 2.6.1.1); 4, Malate dehydrogenase (MDH, EC 1.1.1.31); 5, NAD-dependent-malic enzyme (ME2, 1.1.1.39); 6, Alanine aminotransferase (GPT, EC 2.6.1.2); 7, Pyruvate orthophosphate dikinase (PPDK, EC 2.7.9.1).