Function, Evolution, and Ecology of Type VI Secretion Systems of Plant-Associated Bacteria.

Intense competition for resources among microorganisms imposes strong selective pressure for traits that provide a competitive advantage, including traits that harm others. The type VI secretion system (T6SS) is a versatile contractile injection apparatus encoded by many Gram-negative bacteria. This system is best known for its lethal use in deploying effectors toxic to neighboring bacteria. However, T6SSs can also be used to secrete effectors into the environment to influence nutrient acquisition. Additionally, for some bacteria, T6SSs deploy effectors toxic to eukaryotic hosts and are involved in virulence, which, however, has not been demonstrated for plant-associated bacteria. Here, we review the diverse functions and evolutionary basis of T6SSs. We discuss the potential ecological impacts of T6SSs in plant-associated communities. Understanding outcomes is important for finding the best approaches for using bacteria in sustainable management of plant agricultural systems.