I have GV3101 Agrobacterium Tumefaciens strain with Strep resistance. I want to transform them with a plasmid with the same resistance. Can I use this strain for the plasmid transformation with more antibiotic concentration or any similar approach?
provided is correct. In theory what you propose might work if the chromosomal resistance marker was low level resistance. But generally the chromosomal strep resistance alleles are pretty high level resistance so practically speaking it won't be useful. However many strep-resistance cassettes on plasmids also encode spectinomycin resistance. You might check whether your plasmid encodes spec-R and whether your strain is Spec sensitive or not (it probably depends upon the allele making it Strep resistance).
Yes.. You can't use same resistance for obtaining both wild type and mutant ones. Streptomycin resistance plasmids are high resistance ones. So. It's better to use a plasmid with resistance to a different drug altogether.