Plant-Microbe Interactions and Secondary Metabolites with Antibacterial, Antifungal and Antiviral Properties

Plants have developed effective defence strategies to protect themselves from phytopathogenic microbes and herbivores in their environment. Disease resistance in plants depends on the activation of coordinated, multicomponent defence mechanisms. One mechanism for disease resistance in plants is their ability to accumulate low-molecular-weight compounds (secondary metabolites) with high antimicrobial activities, such as alkaloids, coumarins, isoflavonoids, polyacetylenes, quinones, tannins and terpenes. Based on this knowledge, there is every reason to believe that the plant kingdom is an important source of new antimicrobial agents with special biological targets. Thus, in the last two decades, hundreds of different new secondary metabolites were screened for their potential antibacterial, antifungal and antiviral activities. For instance, several secondary metabolites with antiviral properties have exhibited competitive in vitro and in vivo activities with those found for synthetic antiviral drugs. It has been shown that phyto-antiviral agents interfere with many viral targets, ranging from adsorption of the virus to the host cell via the inhibition of virus-specific enzymes (e.g. reverse transcriptase, protease) to release virus from the cells. It is generally accepted that bioactive plant-derived secondary metabolites are useful leads to synthesize new and more active antimicrobial agents as well as substances with new pharmacological effects by repeated structural modification. It is expected that structurally modified natural products will exhibit increased potency, selectivity, duration of action, bioavailability and reduced toxicity.