I partly disagree with the answer by Liu (respectfully). To my knowledge sugar is neither a nutrient nor a osmoprotectant in infiltration experiment but was shown a long time ago to have the capacity to trigger vir genes synergistically with phenolics (in fact our lab systematically uses both in our dipping solution). 

Here is the reference of the original publication.

Proc Natl Acad Sci U S A. 1990 Sep;87(17):6708-12.

Sugars induce the Agrobacterium virulence genes through a periplasmic binding protein and a transmembrane signal protein.

Cangelosi GA1, Ankenbauer RG, Nester EW.

The sucrose (or glucose) in the media for floral dipping transformation is a minimal nutrient for the agrobacteria, it also keep the right medium osmosis and protect the agro from the surfactant to live longer in the plant surfaces. For a successful transformation, the agro need to enter the plant tissue through the plant stomatas or other small holes in the surfaces, as these are very small holes and plants usually have strong surface attention which prevent the agro to pass through these holes, the surfactant is used to reduce the surface attention to allow the agro inter the plant tissue like the vacuum method that uses the vacuum to help the agro to pass through these holes. However only very mild surfactants are suitable for this purpose. The lined publication might help to get more information  http://onlinelibrary.wiley.com/doi/10.1046/j.1365-313x.1998.00343.x/epdf.

I partly disagree with the answer by Liu (respectfully). To my knowledge sugar is neither a nutrient nor a osmoprotectant in infiltration experiment but was shown a long time ago to have the capacity to trigger vir genes synergistically with phenolics (in fact our lab systematically uses both in our dipping solution). 

Here is the reference of the original publication.

Proc Natl Acad Sci U S A. 1990 Sep;87(17):6708-12.

Sugars induce the Agrobacterium virulence genes through a periplasmic binding protein and a transmembrane signal protein.

Cangelosi GA1, Ankenbauer RG, Nester EW.

@  Hugo Germain,

Thanks for pointing out the paper.

Huanting Liu and Hugo Germain, Thank you very much. Hugo, special thanks for the paper which actually I needed.

@  Hugo Germain,

But, why do people choose 'sucrose' over 'glucose'? Both sugars were tested and included in the paper you attached.