 The functions of plant catecholamines have not been clearly established, but there is evidence that they play a role in the response to stressors such as bacterial infection, act as growth-promoting factors in some situations, and modify the way that sugars are metabolized.The receptors that mediate these actions have not yet been identified, nor have the intracellular mechanisms that they activate.Dopamine consumed in food cannot act on the brain, because it cannot cross the blood–brain barrier.