Genes tagging and molecular diversity of red rot susceptible/tolerant sugarcane hybrids using c-DNA and unigene derived markers

Red rot of sugarcane (Colletotrichum falcatumWent)

Red rot of sugarcane was recorded more than 100 years before in Java, India, Argentina, USA and other countries, and it is one of the most devastating diseases of sugarcane. Since the cultivated sugarcane (Saccharum officinarum) has failed across the countries, systematic inter-specific hybridization betweenS. officinarumand the wild speciesS. spontaneumreferred as ‘nobilization’ was done to develop resistant varieties and the disease was managed in most of the countries. However, in the countries especially in Asia, varietal breakdown to red rot caused severe epiphytotics, by which the resistant varieties failed in the field at regular intervals. New pathogenic strains ofColletotrichum falcatumwith higher virulence were found responsible for varietal breakdown in sugarcane. Extensive cultivation of a single variety over large areas led to extensive crop damages due to ‘vertifolia’ effect in different decades in India. The current epiphytotic on the ruling variety Co 0238 has caused loss of more than one billion US dollars in the current season in the country. Detailed studies were done on pathogenic variation, epidemiology, screening methods, disease resistance mechanism, identifying effectors, pathogenicity determinants, antifungal genes and transgenics. Recently, complete genome and transcriptomes ofC. falcatumwere sequenced and pathogenicity hot spots and candidate secreted effector proteins were identified and this will further help to identify the candidate genes for further genetic manipulation. In spite of poor understanding on inheritance of resistance toC. falcatumin sugarcane, new varieties with red rot resistance were developed and deployed after each of the epiphytotic to save the crop. Further, other management practices including bioagents, chemicals and inducers were attempted and improved efficacy by mechanized sett treatment showed promising results to manage the disease under field conditions.

Assessing genetic diversity and population structure of sugarcane cultivars, progenitor species and genera using microsatellite (SSR) markers

Sugarcane is one among the most important commercial crops used to produce sugar, ethanol, and other byproducts, which significantly contributes in the GDP of India and many other countries around the world. Genetic diversity is a platform for any breeding program of a plant species. Estimation of the genetic variability and population structure play a vital role for conservation planning and management of plant genetic resources. Genetic variability serves as a source of noble alleles responsible for key agronomic and quality traits, which ultimately form basis for identification and selection of promising parents for breeding programs. In the present study genetic diversity and population structure of 139 accessions of the genus Saccharum, allied genera of family Poaceae and cultivars were assessed using informative microsatellite (SSR) markers. A sum of 427 alleles was produced using 61 polymorphic primers and number of alleles generated was ranged from 2 to 13 with an average of 7 alleles per locus. PIC values were ranged from 0.35 to 0.90, with a mean value of 0.66 for all the markers evaluated. Cluster analysis based on UPGMA method revealed three major clusters which were further subdivided into nine subclusters. Population structure analysis also established three subpopulations of used accession set, however there were no correlation of sub-groupings with that of place of origin. AMOVA analysis also confirmed that 83% and 17% of total variations were attributed to the within- and between-populations, correspondingly, demonstrating greater exchange of gene pool across places of origin. The principal component analysis (PCA) demonstrated the distribution of accessions in the scatter-plot was substantially dispersed, revealing rich genetic diversity among accessions of different species. The findings from this study will be useful in breeding programs for introgression of noble alleles into modern cultivars by exploiting natural genetic variation existing in sugarcane genetic resources.

Single nucleotide polymorphism in sugar pathway and disease resistance genes in sugarcane

Single nucleotide polymorphism in sugar pathway and disease resistance genes showing genetic association with sugar content and red rot resistance would be useful in marker-assisted genetic improvement of sugarcane. Validation and genotyping of potential sequence variants in candidate genes are necessary to understand their functional significance and trait association potential. We discovered, characterized, validated and genotyped SNPs and InDels in sugar pathway and disease resistance genes of Saccharum complex and sugarcane varieties using amplicon sequencing and CAPS assays. The SNPs were abundant in the non-coding 3'UTRs than 5'UTRs and coding sequences depicting a strong bias toward C to T transition substitutions than transversions. Sequencing of cloned amplicons validated 61.6 and 45.2 % SNPs detected in silico in 21 sugar pathway and 16 disease resistance genes, respectively. Sixteen SNPs in four sugar pathway genes and 10 SNPs in nine disease resistance genes were validated through cost-effective CAPS assay. Functional and adaptive significance of SNP and protein haplotypes identified in sugar pathway and disease resistance genes was assessed by correlating their allelic variation with missense amino acid substitutions in the functional domains, alteration in protein structure models and possible modulation of catalytic enzyme activity in contrasting high and low sugar and moderately red rot resistant and highly susceptible sugarcane genotypes. A strong genetic association of five SNPs in the sugar pathway and disease resistance genes, and an InDel marker in the promoter sequence of sucrose synthase-2 gene, with sugar content and red rot resistance, was evident. The functionally relevant SNPs and InDels, detected and validated in sugar pathway and disease resistance genes, and genic CAPS markers designed, would be of immense use in marker-assisted genetic improvement of sugarcane for sugar content and disease resistance.