Optimisation of a rapid and efficient transformation protocol for fungal blast-susceptible indica rice cultivars HR-12 and CO-39.

Rice is an important staple crop and fungal blast disease destroys about 10-30% of its global produce, annually. Although genetic manipulation has largely been employed in crop-improvement programmes and agricultural biotechnology, the ease of transformation of several recalcitrant indica cultivars continues to be a challenge. HR-12 and CO-39 are two indica cultivars that are commonly used in breeding programmes, but are susceptible to biotic threats like fungal blast and sheath blight disease. Here in this study, we have optimised a rapid and reproducible transformation protocol for the said cultivars, having compared both the tissue-culture and in-planta methods of transformation. Murashige & Skoog basal media supplemented with maltose and 2.5 mg l-1 2,4-D induced efficient callogenesis in HR-12, while maltose with 3 mg l-1 2,4-D gave optimum results in case of CO-39. The media containing 0.5 mg l-1 NAA, 3 mg l-1 BAP, and 1 mg l-1 kinetin yielded a maximum regeneration efficiency of 62% and 65% in HR-12 and CO-39, respectively. The studies with Agrobacterium tumefaciens, LBA4404 strain harbouring pCAMBIA1303 suggested that although these cultivars demonstrated successful gene-transfer, they failed to regenerate efficiently, post-transformation. Alternatively, our modified in-planta piercing and vacuum infiltration-based protocol resulted in 33-35% transformation efficiency in less than half the time required for tissue-culture based transformation method. As per our knowledge, it is among the highest obtained from existing piercing-based direct transformation protocols in rice, and can also be implemented in genetically manipulating other recalcitrant varieties of rice.