Search

Sugarcane is the world’s most widely grown sugar crop accounts for almost 75% of world sugar production. Diseases are the major productivity constraints for sugarcane that considerably reduce yield and sugar recovery. Identification of disease resistance is an important and sustainable approach for disease management. More recently, Brazilian researchers have characterized the whole genome sequences of a commercial sugarcane cv SP80-3280 and mapped 373,869 genes or ninety nine percent of the total genome. Use of next- generation sequencing (NGS) techniques have facilitated the availability of complete genomic information of various sugarcane pathogens, have led to the understanding of organization of the disease resistance gene clusters for unraveling the genetic mechanism involved in developing new resistance strategies to manage sugarcane diseases. Genome wide association study (GWAS) reveal polymorphisms related with composite characters in the whole genome. In the omics era, whole genome sequencing of host and pathogens combined with the availability of bioinformatics tools for their analyses led to the emergence of GWAS, a potent method for identifying genomics region covering resistance loci in various plants. This review summarizes the literature generated during last the 10 years on GWAS in sugarcane related to disease resistance and along with novel single nucleotide polymorphism and quantitative trait loci’s hotspots identified for economically important diseases of sugarcane.

Dragon fruit (Hylocereus spp.) is an important fruit crop for its high nutrition and profitability grown in India and many other countries. Dragon fruit stem samples exhibiting chlorosis and mottling symptoms, collected from Sangareddy district of Telangana, India revealed a flexuous rod-shaped virus with a length of ~ 600 nm in electron microscopic examination. The RT-PCR based detection of Cactus virus X using specific primers resulted in the production of ~ 900 bp amplicons covering the full length triple gene block protein 3 and coat protein regions. Further, cloning and sequence information of the ~ 900 bp amplicon revealed the presence of Cactus virus X (CVX) in dragon fruit. To our knowledge, this is the first record of CVX causing particular disease in dragon fruit from India.

Among 10 proteins encoded by Papaya ringspot virus (PRSV), only coat protein (CP) has been studied extensively from mainly the papaya-originated isolates i.e. PRSV-P. In this study, besides CP, the helper component proteinase (HC-pro) and nuclear inclusion protein-a (NIa-pro) coding regions from 19 PRSV-P and -W isolates originating from different geo-climatic locations and hosts were analyzed. Unlike CP, both HC-pro and NIa-pro did not reveal the variability in their length. HC-pro and NIa-pro sequences revealed less than 10% deduced amino acid (daa) diversity worldwide, the CP was found highly variable (up to 20% daa diversity). Highest variability in CP sequence was noticed among the cucurbit-originating PRSV-P Indian isolates, especially from Warm semi-arid climate, which was attributed to a large number of aa insertions/deletions. However, papaya-originating PRSV-P isolates from the Oceanic climate were less variable (up to 5% daa diversity). In NIa-pro, besides already known host/pathotype-specific switching of aa Lys/Asp27, switching of Lys27 → Arg27 and Asp27 → Glu27 was identified within P and W pathotypes. Additionally, a novel aa switching from Ser/Met134 → Thr134 between P and W pathotypes respectively was identified. The phylogeny of PRSV isolates was largely based on the geo-climatic locations and hosts. Through recombination detection analysis, three recombinant isolates were detected based on both NIa-pro and CP sequences, which possessed cucurbit-originated PRSV (either -P or -W) isolates as one of their parents (either major or minor.) Patterns of codon usage among the three protein coding sequences generated further revealed that the CP gene is under maximum selection pressure (with six sites) followed by NIa-pro gene (with one site). This investigation shows that the genetic recombination between the isolates originating from different hosts and geo-climatic locations as well as mutations are the significant mechanisms playing their role in generating diverse populations of PRSV.

Sugarcane mosaic virus (SCMV) one of the causative viruses of mosaic disease in sugarcane occurs in sugarcane growing countries worldwide. India is the second largest sugarcane producing country and genome of SCMV from India has not been characterized so far. Hence detailed studies were carried out to characterize the virus isolates based on its complete genome. Comparative genome analyses of five new isolates were performed with previously reported SCMV full genome sequences of isolates infecting sugarcane, maize, sorghum and Canna. Sequence identity matrix and phylogenetic analyses clearly represented that Indian isolates are closely related to sugarcane infecting isolates reported from Australia, Argentina, China and Iran and they diverged as a separate subgroup from other reported maize infecting isolates from Mexico, China, Ohio, Spain, Germany, Iran, Ethiopia, Kenya and Eucador. Selection pressure analysis clearly depicted the predomination of strong purifying selection throughout the viral genome, and strongest in CI and HC-Pro gene. Evidence for few positively selected sites was identified in all the cistrons except in 6 K1 and Nib rep. Among the genomic region, CI gene has exhibited comparatively more recombination hotspots followed by HC-Pro unlike other reported isolates. As the cultivation of sugarcane first originated in India, our results from the recombination events strongly suggest that Indian SCMV populations contribute for the emergence of upcoming new recombinant SCMV isolates not only within the sugarcane isolates but also with maize infecting isolates of SCMV in other countries irrespective of geographic origin and host type.

Loop-mediated isothermal amplification (LAMP) is a rapid, highly specific and reliable technique, which is widely incorporated in the field for diagnosis of pathogens at their early stage of infection. In present study, RT-LAMP assay was developed successfully to detect Sugarcane yellow leaf virus (SCYLV) causing yellow leaf disease in sugarcane and compared with conventional reverse transcription-polymerase chain reaction (RT-PCR) and quantitative real-time polymerase chain reaction (qRT-PCR) to assess its sensitivity. RT-LAMP was able to detect the SCYLV up to a limit of 10 pg (picogram), while RT-PCR assays were able to detect this virus up to 100 pg only. The sensitivity of the RT-LAMP method was tenfold higher than that of conventional RT-PCR assay. This study revealed high sensitivity of RT-LAMP in detecting SCYLV in Indian sugarcane cultivars.

Sugarcane, an important field crop, is cultivated under tropical and subtropical regions around the world. Fusarium sacchari causing wilt, is a stalk disease, inflicting severe damage to the crop in India and other countries. Similarly, pokkah boeng (PB) a foliar disease caused by different species of Fusarium also infects the crop throughout the world. In India, both the diseases occur in different states in various sugarcane varieties. Although both diseases occur independently in the field, we recorded that they occur together in a plant. Hence, a detailed investigation was conducted to characterize different Fusarium isolates from wilt- and PB-affected sugarcane varieties by sequencing TEF1-α gene. Gene sequencing of 48 isolates revealed that 44 were of F. sacchari and the remaining four belonged to F. proliferatum. Of the four F. proliferatum, three were associated with PB and one with wilt. Almost all the 41 wilt-associated isolates belonged to F. sacchari. Investigation carried out to identify Fusarium isolates from the plants exhibiting both the wilt and the PB in two varieties Co 0238 and MS 901 revealed that only F. sacchari caused wilt and PB symptoms in both. Further, several varieties showed progressive disease severity through different phases of PB and that resulted in wilt development. The results clearly established for the first time that the same fungal pathogen systematically infects sugarcane plant and exhibits both the diseases.

Sugarcane streak mosaic virus (SCSMV), a member of the genus Poacevirus, family Potyviridae, is an important viral pathogen affecting sugarcane cultivation in India. The complete nucleotide sequence of a SCSMV isolate from India (SCSMV-IND) was determined. The linear, assembled, single-stranded positive-sense RNA genome of SCSMV-IND was 9,786 nucleotides in length (excluding the poly (A) tail) and encoded a polyprotein of 3,131 amino acid residues. The genome of SCSMV-IND shared high degree of sequence identity with SCSMV-PAK (93 % at nucleotide and 97 % at amino acid), and shared only 81 % nucleotide and 94 % amino acid identities with all the four SCSMV isolates (SCSMV-JP1, -JP2, -ID, and -THA). Phylogenetic tree analysis of the complete genome sequences of SCSMV isolates revealed that they can be clustered into two groups. SCSMV-IND and -AP isolates showed 18 % (nucleotide) divergence within the highly conserved 3' partial genome, suggesting a high level of genetic diversity among the Indian SCSMV isolates.

Yellow leaf (YL) caused by Sugarcane yellow leaf virus (SCYLV) has become a serious constraint for sugarcane production in different countries. Worldwide seven genotypes have been reported, with five based on complete and two based on partial genome characterization. We have previously reported the occurrence of three different SCYLV genotypes in India based on their partial genome sequences. A further four SCYLV isolates from sugarcane from Coimbatore (in India) were characterized after complete genome sequencing (~ 5,875 nt). These isolates (SCYLV-IND) exhibited amino acid (aa) sequence differences of 29.2–31.8, 28.1–34.4 and 30.7–33.4 % with REU, HAW-PER and BRA in partial ORF0 sequences, respectively. Similarly IND isolates have 21.4–23.7, 22.5–25.0 and 21.4–23.9 % aa sequence differences with REU, HAW-PER and BRA, respectively in partial ORF1. However, the difference was found to be least in ORF5. The genotype reported from China, CHN1 shared a very close relationship with IND isolates with minimum differences of 4.3–5.3 %, 4.8–5.8 % and 2.5–3.0 % in ORF0, 1 and 5 in aa sequences, respectively and 4.4–5.3 % in complete nucleotide sequences. Phylogenetic analyses showed a separate lineage for IND isolates. Evidence of recombination was found in ORF1 to ORF5 with the maximum number of sites occurring in ORF2. The high incidence of SCYLV recombination suggests that recombination plays an important role in SCYLV evolution.

Sugarcane streak mosaic virus (SCSMV), a member of the family Potyviridae, is an important viral pathogen affecting sugarcane production in India. The variability in the nucleotide (nt) and amino acid (aa) sequences of helper component proteinase (HC-Pro) of SCSMV isolates from India was investigated and compared with those of previously published virus isolates from different Asian countries. Comparison of all of the sequenced virus isolates revealed a high level of diversity in the HC-Pro gene (72-97% nt sequence identity; 83-99% aa sequence identity), and the Indian isolates were found to be the most divergent (up to 12% variation at the amino acid level). Phylogenetic analysis revealed clustering of 16 SCSMV isolates into two groups. Group I included isolates from India and Pakistan, and group II consisted of isolates from Japan and Indonesia. Recombination analysis revealed nine potentially significant recombination events, and putative recombination sites were identified throughout the HC-Pro gene. Analysis of selection pressure indicated that the HC-Pro gene of SCSMV is under strong negative selection. It is likely that recombination, along with strong negative selection, enhances the speed of elimination of deleterious mutations in the HC-Pro gene.

Papaya ringspot virus (PRSV) has a single-stranded RNA genome and causes severe economic losses both in cucurbits and papaya worldwide. The extent to which the genome of PRSV is shaped by recombination provides an understanding of the molecular evolution of PRSV and helps in studying features such as host specificity, geographic distribution, and its emergence as new epidemics. The PRSV-P-Indian isolate was completely sequenced and compared with 14 other isolates reported from the rest of the world for their phylogenetic survey of recombination events. Cistron-by-cistron sequence comparison and phylogenetic analysis based on full-genome polyprotein showed two distinct groupings of Asian and American isolates, although PRSV-P and W-India clustered along with the American isolates. Recombination sites were found throughout the genomes, except in the small 6K1 protein gene. A significant proportion of recombination hotspots was found in the P1 gene, followed by P3, cylindrical inclusion (CI), and helper component proteinase (HcPro). Correlations between the presence of recombination sites, geographic distribution, and phylogenetic relationship provide an opportunity to establish the molecular evolution and geographic route of PRSV.

Pathogenic Behaviour Pattern of Colletotrichum falcatum Isolates of Sugarcane in Sub-tropical India Red rot caused by Colletotrichum falcatum is the most serious disease of sugarcane affecting cane production in different states in India. Since the pathogenic variants very frequently emerge in the field we have assessed pathogenic variation in 12 C. falcatum isolates collected from subtropical India along with previously designated pathotypes on a set of 16 host differentials for the eight seasons. By and large, a variable virulence pattern in C. falcatum isolates was recorded on most of the host differentials. The results showed that the predominant sub-tropical pathotypes (CF09 and CF08) isolated from then the ruling cultivars CoS 767 and CoJ 64, respectively showed less virulence and the pathotype CF02 isolated from Co 7717 showed higher virulence over the period of time on the host differentials. The higher aggressiveness exhibited by CF02 over on the differentials such as Co1148, Co 975 and Khakai and known susceptible differentials Co 997, Co 62399, CoC 671, CoJ 64 and another variety Co 89003 clearly revealed higher virulence of CF11 pathotype. Even though the pathotype CF08 remained virulent on its host cultivar CoJ 64, it could not cause same type of virulence on the recently developed cultivars and it suggests that the pathotype no longer maintains its virulence once a popular cultivar CoJ 64 removed from the cultivation. Among the 11 new C. falcatum isolates evaluated, the isolate Cf8436 Karnal was found to be virulent and the differential CoS 8436 succumbed to this isolate only. Of the different CoJ 64 pathotypes, Cf64-I exhibited higher virulence and showed susceptible to intermediate reaction on different host differentials. This study suggests continuous evolution of red rot pathogen and its adaptation to the new host cultivars in subtropical India....