Agrobacterium-mediated transformation of embryogenic calluses of Ponkan mandarin and the regeneration of plants containing the chimeric ribonuclease gene

Ponkan (Citrus reticulata Blanco), one of the most important commercial cultivars of mandarin, is very seedy. In this study, the chimeric ribonuclease gene (barnase) driven by an anther tapetum-specific promoter (pTA29) was introduced into embryogenic callus of Ponkan by Agrobacterium-mediated transformation using the bar gene as a selectable marker. In contrast to previous reports, embryogenic calluses were used as the explant for Agrobacterium infection and transgenic plant regeneration. Selection of transformed callus was accomplished using basta. After 3 days of co-culture, calluses were transferred to MT medium with 50 mg/l basta and 400 mg/l cefotaxime. Resistant calluses were recovered and proliferated after three to four subcultures and then regenerated plantlets. A total of 52 resistant plants were recovered, of which 43 were verified to be transformants by polymerase chain reaction amplification of a fragment of the transgene. Southern hybridization of seven randomly selected transformed plants further confirmed their transgenic nature. The potential of this strategy for breeding citrus seedless types is discussed.