Morpho-agronomic characterization and genetic variability assessment of a guar germplasm collection by a novel SSR panel

Guar (Cyamopsis tetragonoloba (L.) Taub.) is mainly cultivated as an industrial crop due to its high-quality galactomannan gum used as a thickener, flocculant, emulsion stabilizer and gelling agent in many industrial activities. In the present study, carried out in Southern Italy, a core collection of 46 guar genotypes, collected across the natural distribution area of the species, was characterized through a multidisciplinary approach, focusing on morphological (plant, leaf, stem and branch), productive (1000-seed weight and seeds per plant) and genetic traits. Plant height ranged from 37.7 to 97.3 cm. Branch number ranged from 0 to 12; branching genotypes showed a lower height of insertion of the first cluster. Seed weight showed an average value of 40.1 g per 1000 seeds with a narrow CV (10.1%) and nine genotypes yielded more than 45 g of seeds per plant. Positive correlation was observed between seeds and clusters per plant (0.77), seeds and pods per plant (0.94) and seeds per plant and seeds per pod (0.44). The high-throughput sequencing allowed us to isolate a set (157) of nuclear microsatellite (nSSR) markers from guar. Fourteen out of 48 analyzed nSSR markers showed polymorphisms and a sizeable discrimination power (PIC = 0.399; PI = 0.380) and gene diversity (0.276 ≤ He≤ 0.728) within the germplasm collection. The Discriminant Analysis of Principal Components (DAPC) highlighted three main clusters, with a private group in which three accessions from Delhi-India were included. Interestingly, the DAPC and morphological (PCA) analyses appeared to be in agreement, but with a partial discrimination across samples on the basis of geographic origin. However, for the first time in guar, a relationship between morpho-productive and genetic traits was underlined, showing significant p values (p < 0.05) in the Mantel test. Therefore, the novel set of molecular markers (nSSR) proposed here could be adopted as a useful tool to characterize the guar accessions for future breeding programs, looking for morpho-productive and productive valuable genotypes.