Comparative genomic analysis of C4 photosynthetic pathway evolution in grasses.
BACKGROUND
Sorghum is the first C4 plant and the second grass with a full genome sequence available. This makes it possible to perform a whole-genome-level exploration of C4 pathway evolution by comparing key photosynthetic enzyme genes in sorghum, maize (C4) and rice (C3), and to investigate a long-standing hypothesis that a reservoir of duplicated genes is a prerequisite for the evolution of C4 photosynthesis from a C3 progenitor.


RESULTS
We show that both whole-genome and individual gene duplication have contributed to the evolution of C4 photosynthesis. The C4 gene isoforms show differential duplicability, with some C4 genes being recruited from whole genome duplication duplicates by multiple modes of functional innovation. The sorghum and maize carbonic anhydrase genes display a novel mode of new gene formation, with recursive tandem duplication and gene fusion accompanied by adaptive evolution to produce C4 genes with one to three functional units. Other C4 enzymes in sorghum and maize also show evidence of adaptive evolution, though differing in level and mode. Intriguingly, a phosphoenolpyruvate carboxylase gene in the C3 plant rice has also been evolving rapidly and shows evidence of adaptive evolution, although lacking key mutations that are characteristic of C4 metabolism. We also found evidence that both gene redundancy and alternative splicing may have sheltered the evolution of new function.


CONCLUSIONS
Gene duplication followed by functional innovation is common to evolution of most but not all C4 genes. The apparently long time-lag between the availability of duplicates for recruitment into C4 and the appearance of C4 grasses, together with the heterogeneity of origins of C4 genes, suggests that there may have been a long transition process before the establishment of C4 photosynthesis.
Wang
Xiyin
X
Plant Genome Mapping Laboratory, University of Georgia, Athens, GA 30602, USA. wang.xiyin@gmail.com
Gowik
Udo
U
Tang
Haibao
H
Bowers
John E
JE
Westhoff
Peter
P
Paterson
Andrew H
AH
D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D017360:Arabidopsis / Q000235:genetics; D002244:Carbon / Q000378:metabolism*; D002256:Carbonic Anhydrases / Q000235:genetics; D019143:Evolution, Molecular; D020440:Gene Duplication; D018745:Genome, Plant / Q000235:genetics*; D007527:Isoenzymes / Q000235:genetics; D008291:Malate Dehydrogenase / Q000378:metabolism; D012275:Oryza / Q000235:genetics; D010730:Phosphoenolpyruvate Carboxylase / Q000235:genetics; D010788:Photosynthesis / Q000235:genetics*; D010802:Phylogeny; D006109:Poaceae / Q000201:enzymology / Q000235:genetics*; D011772:Pyruvate, Orthophosphate Dikinase / Q000235:genetics; D045868:Sorghum / Q000201:enzymology / Q000235:genetics; D003313:Zea mays / Q000201:enzymology / Q000235:genetics
D007527:Isoenzymes; D002244:Carbon; D008291:Malate Dehydrogenase; C031917:malate dehydrogenase (oxaloacetate-decarboxylating) (NADP+); D011772:Pyruvate, Orthophosphate Dikinase; D010730:Phosphoenolpyruvate Carboxylase; D002256:Carbonic Anhydrases