Guar: Agronomy, Production, Industrial Use, and Nutrition

Guar, [Cyamopsis tetragonoloba (L.) Taub] is an annual summer legume that is widely grown in arid and semi-arid regions. It is a diploid legume (2n = 2x = 14) with a genome size of approximately 560 Mb. Guar can fix atmospheric nitrogen via a symbiotic process with rhizobial bacteria, thus enhancing soil fertility and decreasing the need for the use of expensive nitrogen (N) fertilizers. Guar is a drought- and heat-tolerant crop and can be grown using minimal agricultural inputs. Guar can be used as vegetable, feed/fodder, or green manure. One of the primary driving factors for guar cultivation is galactomannan gum, known as guar gum, found in the seed endosperm. Guar gum has various industrial uses that range from cosmetic applications, food ingredients, to the oil industries. India is the lead guar production and accounts for more than 80% of the world production. The U.S. is the world largest guar market. This review discusses the status of genomic resources in guar, the progress made for germplasm evaluation of various agronomic traits, and the genetic engineering efforts in guar.

Genetic and genomic resources in guar: a review

Legume crops, belonging to the Fabaceae family, are of immense importance for sustaining global food security. Many legumes are profitable crops for smallholder farmers due to their unique ability to fix atmospheric nitrogen and their intrinsic ability to thrive on marginal land with minimum inputs and low cultivation costs. Recent progress in genomics shows promise for future genetic gains in major grain legumes. Still it remains limited in minor legumes/underutilized legumes, including adzuki bean, cluster bean, horse gram, lathyrus, red clover, urd bean, and winged bean. In the last decade, unprecedented progress in completing genome assemblies of various legume crops and resequencing efforts of large germplasm collections has helped to identify the underlying gene(s) for various traits of breeding importance for enhancing genetic gain and contributing to developing climate-resilient cultivars. This review discusses the progress of genomic resource development, including genome-wide molecular markers, key breakthroughs in genome sequencing, genetic linkage maps, and trait mapping for facilitating yield improvement in underutilized legumes. We focus on 1) the progress in genomic-assisted breeding, 2) the role of whole-genome resequencing, pangenomes for underpinning the novel genomic variants underlying trait gene(s), 3) how adaptive traits of wild underutilized legumes could be harnessed to develop climate-resilient cultivars, 4) the progress and status of functional genomics resources, deciphering the underlying trait candidate genes with putative function in underutilized legumes 5) and prospects of novel breeding technologies, such as speed breeding, genomic selection, and genome editing. We conclude the review by discussing the scope for genomic resources developed in underutilized legumes to enhance their production and play a critical role in achieving the “zero hunger” sustainable development goal by 2030 set by the United Nations.

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Progress of Genomics-Driven Approaches for Sustaining Underutilized Legume Crops in the Post-Genomic Era

Cluster bean popularly known as “guar” is a drought-tolerant, annual legume that has recently emerged as an economically important crop, owing to its high protein and gum content. The guar gum has wide range of applications in food, pharma, and mining industries. India is the leading exporter of various cluster bean-based products all across the globe. Non-coding RNAs (miRNAs) are involved in regulating the expression of the target genes leading to variations in the associated pathways or final protein concentrations. The understanding of miRNAs and their associated targets in cluster bean is yet to be used to its full potential. In the present study, cluster bean EST (Expressed Sequence Tags) database was exploited to identify the miRNA and their predicted targets associated with metabolic and biological processes especially response to diverse biotic and abiotic stimuli using in silico approach. Computational analysis based on cluster bean ESTs led to the identification of 57 miRNAs along with their targets. To the best of our knowledge, this is the first report on identification of miRNAs and their targets using ESTs in cluster bean. The miRNA related to gum metabolism was also identified. Most abundant miRNA families predicted in our study were miR156, miR172, and miR2606. The length of most of the mature miRNAs was found to be 21nt long and the range of minimal folding energy (MFE) was 5.8–177.3 (−kcal/mol) with an average value of 25.4 (−kcal/mol). The identification of cluster bean miRNAs and their targets is predicted to hasten the miRNA discovery, resulting in better knowledge of the role of miRNAs in cluster bean development, physiology, and stress responses.

In silico Identification of miRNAs and Their Targets in Cluster Bean for Their Role in Development and Physiological Responses

In this post-genomic era, we now have easy access to the genetic information of entire living organisms, and that information has been essential for biological research. The prosperity of genomics resulted from the progress of DNA sequence technologies, the development of computational analysis environments, and the establishment of biological resources. Plant genomics is one of the research fields that has strongly benefited from these technical advances. This chapter presents the evolution and transition of DNA sequence technologies and gives concrete examples of the proliferation in plant genomic research. 

Plant Genomics

Clusterbean (Cyamopsis tetragonoloba (L.) Taub.), also known as Guar is a widely cultivated dryland legume of Western India and parts of Africa. Apart from being a vegetable crop, it is also an abundant source of a natural hetero-polysaccharide called guar gum or galactomannan which is widely used in cosmetics, pharmaceuticals, food processing, shale gas drilling, etc. Here, for the first time we are reporting a chromosome-scale reference genome assembly of clusterbean, from a high galactomannan containing popular guar cultivar, RGC-936, by combining sequenced reads from Illumina, 10x Chromium and Oxford Nanopore technologies. The initial assembly of 1580 scaffolds with an N50 value of 7.12 Mbp was generated. Then, the final genome assembly was obtained by anchoring these scaffolds to a high density SNP map. Finally, a genome assembly of 550.31 Mbp was obtained in 7 pseudomolecules corresponding to 7 chromosomes with a very high N50 of 78.27 Mbp. We finally predicted 34,680 protein-coding genes in the guar genome. The high-quality chromosome-scale cluster bean genome assembly will facilitate understanding of the molecular basis of galactomannan biosynthesis and aid in genomics-assisted breeding of superior cultivars.

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Anitha Mithra Sv

Papers

Chromosome scale reference genome of Cluster bean (Cyamopsis tetragonoloba (L.) Taub.)