From a BBC News, July 14, 2010, feature:
“When we shone the light on the plant for one hour and then infected it [with a virus or with bacteria] 24 hours after that light exposure, it resisted the infection,” explained Professor Stanislaw Karpinski from the Warsaw University of Life Sciences in Poland. “But when we infected the plant before shining the light, it could not build up resistance.
“[So the plant] has a specific memory for the light which builds its immunity against pathogens, and it can adjust to varying light conditions.”
He said that plants used information encrypted in the light to immunise themselves against seasonal pathogens.
Read more here.