Physiological and molecular implications of plant PA metabolism during biotic interactions

Abstract

During ontogeny, plants interact with a wide variety of microorganisms. The association with mutualistic microbes results in benefits for the plant. By contrast, pathogens may cause a remarkable impairment of plant growth and development. Both types of interactions provoke notable changes in the PA metabolism of the host and/or the microbe, being each interaction a complex and dynamic process. Regarding this, it has been well documented that the levels of PA s, mainly putrescine, spermidine, and spermine, and their conjugated forms suffer profound changes in plant tissues interacting with microorganisms. In general, this is correlated with the induction of biosynthetic as well as catabolic enzymes. Interestingly, some evidences suggest that the relative importance of these metabolic pathways may depend on the nature of the microorganism. This concept stems from the fact that these amines, in particular spermine, mediate the activation of plant defense mechanisms. This effect is not only exerted by the PA itself, but its catabolism has been likewise demonstrated to make a remarkable contribution to defense. Thus, apoplastic H2O2 generated by the activity of PA oxidases induces defense gene expression and elicits plant cell death during the hypersensitive response. In this trend, it was shown that PA oxidation favors the infections originated by necrotrophic microorganisms, which fed from dead matter, but protect plants from biotrophic pathogens colonizing living plant tissues. In turn, a precise coordination of general PA metabolism would also be required to allow mutualistic interactions to occur. In the last years, a great deal of effort has been devoted to profile plant gene expression in response to invading microorganisms. In addition, the phenotypes of transgenic and mutant plants in PA metabolic genes have been assessed. In this review, we integrated the current knowledge on this field and analyze the possible roles of these amines during the interaction of plants with microbes.