Direct organogenesis from cotyledonary node explants suitable for Agrobacterium-mediated transformation in peanut (Arachis hypogaea L.)

Efficient and genotype-independent in vitro regeneration is an essential prerequisite for incremental trait improvement in peanut (Arachis hypogaea L.) via genetic transformation. We have optimized a facile and rapid method to obtain direct shoot organogenesis from cotyledonary node (CN) explants excised from peanut seedlings germinated on cytokinin-supplemented Murashige and Skoog (MS) basal salt medium. Starting with mature embryos, shoot induction occurred in approximately 7 weeks, followed by 4 weeks for rooting of excised shoots and 3 weeks of acclimatization of regenerated plantlets in soil. The regeneration and transformation system described here is time-efficient, yielding greenhouse-acclimatized plantlets within 14 weeks, in contrast to 12–14 months required for initiating and regenerating somatic embryogenic cultures, currently the most tractable method available for peanut transformation. The highest shoot induction frequency and shoot quality was obtained with 6.66 μM 6-benzylaminopurine, followed by adequate root induction at 5.37 μM α-Naphthaleneacetic acid. New Mexican Valencia A was chosen for Agrobacterium-mediated transformation. Stable GUS expression from pWBvec10a was obtained at a transformation rate of 1.25 %. Furthermore, results from genomic PCR and Southern blot analyses showed that 14 out of 576 putative transgenic regenerants contained transgene pSag12::IPT, therefore yielding a total transformation rate of 2.43 %. The cotyledonary node-based direct regeneration system described here is time-efficient and amenable to Agrobacterium-mediated transformation, and therefore should be further explored for peanut transgenic improvement.