The mechanisms and applications of symbiotic opportunistic plant symbionts

Abstract

A number of fungi have evolved a symbiotic life style with plants, including some organisms that include similar strains or species that are plant pathogens. Some are obligate symbionts such as ecto- or endomycorrhizal fungi, while others are endophytes that have free-living capabilities. Still others are highly competitive in soil and proliferate there. These are the opportunistic plant symbionts. Fungi in the genus Trichoderma have long been considered as biocontrol agents, but they are highly successful plant symbionts as well. The critical step for establishment of the symbiotic life style begins with root colonization and infection of outer cortical layers. A zone of chemical interaction is established; some of the Trichoderma signaling molecules are known. As a result of this interaction, the fungus is walled off; in rare cases where components of this communication are lacking, Trichoderma can become a pathogen. The results of this interaction include induced systemic resistance, increased growth responses and yields, and increased nutrient uptake and fertilizer use efficiency. The interaction induces substantial changes in plant physiology. In the maize-T. harzianum strain T22 interaction, more than 300 proteins have altered expression, with a number of them being up-regulated. Included in this group are, most notably, enzymes of carbohydrate metabolism and proteins associated with pathogen resistance and stress. Multiple forms of several proteins are upregulated, including numerous examples of chitinases, β-glucosidases, proteins with nucleotide binding sites and leucine rich repeats associated with resistance to disease, sucrose synthase, and methionine synthase. The substantial increases in several of these are highly suggestive of changes in metabolic pathways or regulation. © 2007 Springer.