What effect does vitamin a have on plants that are injected with a
carcinagen called agrobacterium tumefacians?

Vitamin A, when given as provitamin A (beta-carotene), has been shown
to protect sunflowers against the development of crown-gall disease (a
plant cancer) caused by the soil bacterium Agrobacterium tumefaciens.

Two accounts of this experiment:

Usra Khanzada
"The Effect of Concentrated Beta-Carotene Solutions on Crown Gall
Disease"
"three groups of sunflower plants were fed different concentrations of
beta-carotene vitamin mixed with water through the soil after being
inoculated with Agrobacterium tumefaciens... The results reveal that
the beta-carotene enriched groups showed better resistance to the
disease than the other groups that received little or no
beta-carotene."
http://www.research.att.com/~kbl/njrsf/htmlabs02.html (North Jersey
Regional Science Fair, Abstracts for March 2002).

Ramina J Nouri
"Does beta-carotene prevent cancer in plants?"
Beta-carotene was protective, although more in the "Lemon Queen"
strain of sunflower than in the "Mammoth" strain.
http://www.usc.edu/CSSF/Current/Projects/J1429.pdf (California State
Science Fair 2002)

Search strategy: "Agrobacterium tumefaciens" sunflowers