A partially disarmed vir helper plasmid, pKYRT1, in conjunction with 2,4-dichlorophenoxyactic acid promotes emergence of regenerable transgenic somatic embryos from immature cotyledons of soybean

Agrobacterium tumefaciens strain KYRT1 harboring the virulence helper plasmid pKYRT1 induces transgenic somatic embryos (SEs) at high frequency from infected immature soybean cotyledons. KYRT1 is derived from the highly oncogenic strain Chry5. However, pKYRT1 is not completely disarmed and still contains an entire T-right (T_R) and a portion of T-left (T _L). In this report, binary strains, each carrying fully disarmed vir helper plasmids including pKPSF2, which is a fully disarmed version of pKYRT1, were compared to strain KYRT1 for their ability to induce transgenic SEs on immature cotyledons of soybean. Six weeks following cocultivation, histochemical GUS assays of cultured explants indicated that all fully disarmed vir helper plasmids transferred their binary T-DNA, containing a GUS-intron gene, into soybean tissues. However, none of these transformed tissues developed SEs on medium with or without 2,4-dichlorophenoxyactic acid (2,4-D). On the other hand, immature cotyledons cocultivated with strain KYRT1 exhibited high induction of transgenic SEs, but only on medium supplemented with 2,4-D. Derivatives of strain Chry5 harboring other vir helper plasmids did not induce transgenic SEs under any conditions tested, thus suggesting that the chromosomal background of KYRT1 alone was not sufficient to promote somatic embryogenesis. PCR analysis indicated that 55% of transgenic embryogenic cultures and 29% of transgenic T₀ soybean plants derived by transformation using strain KYRT1 contained T_R from pKYRT1 in addition to the uidA gene from the binary construct. None of the transgenic tissues or T₀ plants contained T_L DNA. These results suggest that some function coded for by T_R of pKYRT1 influences somatic embryogenesis in conjunction with exposure of the plant tissues to 2,4-D. Since the co-transformation frequency of the undesirable T-DNA sequences from the vir helper plasmid was relatively low, the partially disarmed strain KYRT1 will likely be very useful for the production of normal transgenic plants of diverse soybean cultivars.