Transfer of non-T-DNA portions of the Agrobacterium tumefaciens Ti plasmid pTiA6 from the left terminus of T<SUB>L</SUB>-DNA

We introduced a plant selection marker, nptII, to the left of border A in the Agrobacterium Ti plasmid pTiA6. Infection of tobacco leaf discs with the modified Agrobacterium strain gave rise to kanamycin-resistant calli which grew in a hormone-dependent manner. Southern hybridization analysis of DNA isolated from four transformants indicated initiation of DNA transfer at or near border A and absence of T-DNA sequences. These results demonstrate that DNA transfer events starting at a left border on a native Ti plasmid and moving away from the T-DNA region occur and that they can be detected by designing a suitable selection strategy

Analysis of octopine left border-directed DNA transfer from Agrobacterium to plants

We constructed a binary plasmid, pVR30, with a neomycin phosphotransferase II (nptII) plant expression cassette flanked by a pTiA6 left border on its right and a pTiA6 right border on its left. This plasmid was used to study transfer of DNA to plants from a left border in the presence of a right border. Infection of tobacco leaf discs with a wild type octopine strain ofAgrobacterium tumefaciens harbouring the binary plasmid resulted in the generation of kanamycin resistant calli at 18 to 26% frequency. Southern hybridization analysis of DNA isolated from eight transformed lines to different probes indicated that left border could mediate DNA transfer to plants in the presence of a right border in cis. Our results also suggest that transfer events corresponding to transfer of T-centre DNA of octopine Ti plasmid pTiA6 do occur. We have shown the relevance of left border-initiated T-DNA transfer by specifically selecting for such events and have confirmed it by Southern hybridization analysis. We also found that a border could be skipped in a few T-DNA transsfer events

Some Problematic "Channels" In the Teaching of Critical Thinking in Current LI Composition Textbooks: Implications for L2 Student-Writers

Advanced writing courses in manyfreshman composition programs stress the importance of teaching critical thinking skills where students—both LI and L2—are encouraged to examine and question the social world they inhabit. Derived from an analysis of 12 current freshman composition textbooks, we identify three common "channels" through which student-writers are inducted into the critical thinking practice. These three channels are: (1) using informal logic as a way of developing students' reasoning strategies, (2) developing and refining students' problem solving skills, and (3) developing students' ability to analyze hidden assumptions in 'everyday arguments. ' This study calls attention to the problematic nature ofthese "channels " and to some implications oftransferring these channels in L2 writing classrooms. We believe that critical thinking is largely a sociocognitive practice that draws significantly on shared cultural practices and norms that mainstream students have (had) access to. ESL student-writers, however, given their diverse sociocultural backgrounds, have not necessarily been socialized in ways that would make induction into critical thinking a (relatively) smooth process (Atkinson &amp; Ramanathan, 1995). Using critical thinking textbooks (written by and large for LI students) then, in L2 writing classrooms has complex consequences. Based on our current examination and previous study (Ramanathan &amp; Kaplan, 1996a), we propose a discipline oriented approach to teaching writing, especially for non-native student-writers