Endo-(1,4)-β-glucanase gene families in the grasses: temporal and spatial co-transcription of orthologous genes.

Margaret Buchanan,Antoni J Rafalski,Scott V Tingey,Kanwarpal S Dhugga,Geoffrey B Fincher,Rachel A Burton,Neil J Shirley

doi:10.1186/1471-2229-12-235

Abstract

BackgroundEndo-(1,4)-β-glucanase (cellulase) glycosyl hydrolase GH9 enzymes have been implicated in several aspects of cell wall metabolism in higher plants, including cellulose biosynthesis and degradation, modification of other wall polysaccharides that contain contiguous (1,4)-β-glucosyl residues, and wall loosening during cell elongation.ResultsThe endo-(1,4)-β-glucanase gene families from barley (Hordeum vulgare), maize (Zea mays), sorghum (Sorghum bicolor), rice (Oryza sativa) and Brachypodium (Brachypodium distachyon) range in size from 23 to 29 members. Phylogenetic analyses show variations in clade structure between the grasses and Arabidopsis, and indicate differential gene loss and gain during evolution. Map positions and comparative studies of gene structures allow orthologous genes in the five species to be identified and synteny between the grasses is found to be high. It is also possible to differentiate between homoeologues resulting from ancient polyploidizations of the maize genome. Transcript analyses using microarray, massively parallel signature sequencing and quantitative PCR data for barley, rice and maize indicate that certain members of the endo-(1,4)-β-glucanase gene family are transcribed across a wide range of tissues, while others are specifically transcribed in particular tissues. There are strong correlations between transcript levels of several members of the endo-(1,4)-β-glucanase family and the data suggest that evolutionary conservation of transcription exists between orthologues across the grass family. There are also strong correlations between certain members of the endo-(1,4)-β-glucanase family and other genes known to be involved in cell wall loosening and cell expansion, such as expansins and xyloglucan endotransglycosylases.ConclusionsThe identification of these groups of genes will now allow us to test hypotheses regarding their functions and joint participation in wall synthesis, re-modelling and degradation, together with their potential role in lignocellulose conversion during biofuel production from grasses and cereal crop residues.

Highlights

Endo-(1,4)-β-glucanase glycosyl hydrolase GH9 enzymes have been implicated in several aspects of cell wall metabolism in higher plants, including cellulose biosynthesis and degradation, modification of other wall polysaccharides that contain contiguous (1,4)-β-glucosyl residues, and wall loosening during cell elongation
Endo-(1,4)-β-glucanases clearly function in cell wall degradation, but there is a good deal of evidence that points to an additional and important role for these enzymes in cellulose synthesis during cell growth
Evidence for homoeologues from an ancient allotetraploidy event or for segmental duplications was found in five pairs of maize genes, where the following genes had amino acid sequence identities of more than 90% and were found at two or more map locations: ZmCEL7 and ZmCEL7B, ZmCEL8 and ZmCEL29, ZmCEL14 and ZmCEL30, ZmCEL25 and ZmCEL26 and ZmCEL12, the latter being found at three different locations on the genome. These gene numbers in the grasses are similar to Arabidopsis, where 25 putative endo-(1,4)-β-glucanase genes have been identified; the majority of genes in each case fall into the GH9B group (Table 1)

Summary

Introduction

Endo-(1,4)-β-glucanase (cellulase) glycosyl hydrolase GH9 enzymes have been implicated in several aspects of cell wall metabolism in higher plants, including cellulose biosynthesis and degradation, modification of other wall polysaccharides that contain contiguous (1,4)-β-glucosyl residues, and wall loosening during cell elongation. It has been suggested that the sub-group of endo-(1,4)-β-glucanases that carry transmembrane helices may be specific to cellulose synthesis rather than being involved in the hydrolysis of cellulose or non-cellulosic polysaccharides [20]. In this connection, the endo-(1,4)-β-glucanase family has been divided into three sub-families on the basis of variations in protein sequences [21]. Members of the GH9C group of endo(1,4)-β-glucanases from rice and tomato have been shown to have a broader substrate specificity, insofar as they can hydrolyse (1,4)-β-xylans and in some cases (1,4)-β-mannans [22,23]

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