Plant defense strategies

Abstract

Plants are primary food sources for many organisms and therefore require effective defense mechanisms. Some plants produce defense compounds such as toxic alkaloids, terpenoids, phenolic compounds, or saponins which instantly act against herbivore attack (constitutive defense). Also mechanical barriers, sticky secretions, a pungent, bitter taste, or extreme pH may deter the organisms immediately from feeding.Instead, some plants upregulate their defense mechanisms only after provocation (induced defense). A complex recognition and signaling system including jasmonic acid and salicylate ensures an adequate response. Induced defense has the drawback that some time passes from recognition to defense, yet if not attacked all resources can be invested in growth. The production of toxins (phytoalexins) or proteinase inhibitors is dramatically increased. Together with nutrient allocation, the latter contribute to lowering of the feeding success of herbivores. Included in the induced defense is the production of volatiles in order to warn neighboring plants as well as to attract help from predatory insects. Besides, the nectar flow of extrafloral nectaries is induced providing a reward to insects that remove herbivores.As a third general strategy, some plants release highly reactive toxins quickly by enzymatic activation from inactive storage forms such as benzoxazinoids, iridoids, cyanogenic glycosides, or glucosinolates (activated defense).