Screening of short-day onions for resistance to Stemphylium leaf blight in the seed-to-bulb stage (stage I) and bulb-to-seed stage (stage II).

Stemphylium leaf blight, caused by Stemphylium vesicarium, is a very important fungal disease in onions since its epidemics are able to affect both the bulb yield and the seed quality. The aim of this study was to screen onion genotypes at stage I (seed to bulb) and further screen the identified resistant and susceptible genotypes at stage II (bulb to seed). One hundred and fifty-seven genotypes were screened against SLB under artificially inoculated field conditions. Results revealed a significant variation among the morphological and biochemical traits studied. Correlation studies revealed a significant and negative correlation between percent disease incidence (PDI), pseudostem width, neck thickness, and dry matter. Fifteen genotypes were identified as moderately resistant, and the rest were categorized as susceptible. Bulbs of the genotypes, identified as moderately resistant, were again screened for resistance in stage II. All the genotypes were categorized as moderately susceptible. Biochemical analysis revealed that total foliar phenol content, pyruvic acid, catalase, and peroxidase increased up to 20 days after inoculation (DAI) and thereafter declined. Protein content was highest in the initial stage and declined at 10, 20, and 30 DAI. The higher biochemical activity was observed in moderately resistant category genotypes compared with the susceptible ones. Correlation analysis showed a highly significant and negative correlation of PDI with total foliar phenol content (TFPC), pyruvic acid, catalase, peroxidase, and protein content. To conclude, it was observed that screening against SLB should be done at both the stages (stage I and Stage II) to identify resistant onion genotypes. Direction selection for genotypes with high dry matter, higher phenols, and enzymes may be an alternative pathway to select genotypes for a robust resistance breeding program.

Molecular marker-based characterization of cytoplasm and restorer of male sterility (Ms) locus in commercially grown onions in India.

BACKGROUND: The cytoplasmic-genic male-sterility system has been extensively employed for the production of onion hybrids. Molecular marker-assisted characterization of the cytotypes and genotyping at the restorer-of male-fertility (Ms) locus is important for the accelerated breeding of onion hybrids. Indian onion breeding has focussed more on open-pollinated varieties than hybrids. To accelerate the breeding efforts, marker-assisted selection (MAS) plays a pivotal role. METHODS AND RESULTS: This study aimed to characterize the Indian breeding lines, varieties, hybrids, and exotic accessions for cytotype and Ms locus. For cytoplasm, cytotype markers, accD, and MKFR and for Ms locus identification, PCR markers AcPMS1 and AcSKP1 were employed. Bulk strategy to identify cytoplasm and Ms locus was tested. Sequencing of PCR products amplified by accD was also tried. Both the accD and MKFR were synonymous in cytoplasm identification except in T821 where T cytoplasm was identified. AcPMS1 was more reliable than AcSKP1 for Ms locus identification. Sequencing proved that N and T cytoplasm are identical. Bulking strategy can be used for cytotype identification but not for Ms locus. CONCLUSIONS: Indian onions have a predominance of normal (N) cytoplasm and homozygous recessive (msms) locus. This might be beneficial for hybrid development. S cytoplasm was identified in exotic varieties. For the first time, T cytoplasm has been reported from India. These findings will assist Indian onion breeders to develop MAS strategies for accelerating hybrid development programs. And for the release of onion hybrids with high productivity and uniformity.

Induction of shoot regeneration in cotyledon explants of the oilseed crop Sesamum indicum L.

Sesamum indicum is an ancient oilseed crop known for its high quality edible oil and its medicinally important lignans. The crop is said to be recalcitrant to plant tissue culture thus limiting the use of modern biotechnology for its genetic improvement. We present here a protocol describing plant regeneration through adventitious shoot formation from cotyledons dissected from sesame seeds soaked for four hours in water. Subculturing of the cotyledons after two weeks of culture on to a fresh Murashige and Skoog medium leads to differentiation of adventitious shoots from the proximal cut end of the explant. Culture of cotyledons on a medium containing 9% sucrose for a couple of weeks prior to transfer to MS medium supplemented with 3% sucrose induced a higher frequency of shoot regeneration. The highest frequency of 25% adventitious shoot regeneration was observed for S. indicum variety UMA. This variety also turned out to be the best among the ten genotypes tested for shoot regeneration through tissue culture. While addition of IAA marginally improved regeneration, silver nitrate was found essential for enhancing the frequency of shoot regeneration. The regenerated shoots formed roots on full strength MS medium supplemented with 1 mg/l IBA and the rooted plants were established in soil.