I think that would depend on the tissue to be inoculted and the method used (for example, if vaccum or wounding are involved), so the effect has to be tested for each particular case. For instance, in this modification of the floral dip method for the transformation of Arabidopsis, they observe more than two-fold increase in the transformation efficiency when using OD >2.0 compared to OD 0.8:
http://link.springer.com/article/10.1007%2FBF02773350
The following protocol i used for transformation in rice callus. Hope it will be helpful.
Primary culture of Agrobacterium was prepared by inoculating single colony from a freshly streaked plate, in 5 ml of autoclaved liquid YEP medium (10 g/l bactopeptone, 10 g/l yeast extract, 5 g/l sodium chloride, pH 7.0) supplemented with 25 mg/l streptomycin, 10 mg/l rifampicin, 50 mg/l kanamycin. The culture was incubated for 16-20 h on a rotatory incubator shaker at 200 rpm in dark at 28°C. Secondary culture was prepared in a 500 ml baffled flask containing 100 ml YEP medium (supplemented with same antibiotics as used for primary culture) by adding 0.4% of the primary culture and grown under similar conditions. Once the O.D. 600 reached ~1.0, Agrobacterium cells were pelleted by centrifugation at 8000 × g for 15 min at 4°C. The cells were resuspended in MS resuspension medium containing 150 μM acetosyringone (MS salts, 68 g/l sucrose, 36 g/l glucose, 3 g/l KCl, 4 g/l MgCl2, pH 5.2) to adjust the O.D.600 of the bacterial suspension to 0.3.
OD of the culture is measure of cell numer. so it is important to know before going to infect your tissue and again it is species specific. It is also related to the method used. so all the time it is better to go for piolet project to optimize conditions for agrobacterium mediated transformation.
Actually, the OD measurement record of Agrobacterium depends on the type of tissue or explant and method employed, but mostly OD600 from 0.2-0.6 can be used.
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