Trichoderma for plant disease management: A reality or myth?

Abstract

Trichoderma Spp. are the most widely used biofungicides the world over. These mycoparasitic fungi are effective against several pathogens that cause seed and seedling rot, foliar infections as well as post-harvest decay. We have been working on a particular species of Trichoderma {Trichoderma virens) for more than three decades and found it to be one of the most effective biocontrol agents against soil-borne pathogens like Sclerotium folfsii, Rhizoctonia solani, Fusarium oxysporum and Pythium spp. In this brief review, I will discuss the progress and potential of biocontrol of Trichoderma virens with special emphasis on the strain (IMI 304061) isolated and developed by our group. Two distinct strains of T. virens have been reported - the "Q" strains that produce copious amounts of the antibiotic gliotoxin and the "P" strains that do not produce gliotoxin, but instead produce gliovirin. Our strain belongs to "P" group and is a very rapid colonizer of the sclerotia of S. rolfsii and R. solani, and this mode of antagonism plays an important role in bringing down the inoculum potential. This strain is also effective as an inducer of systemic resistance in plants. Recent advances in molecular genetics have enabled understanding the molecular mechanism of biocontrol and our strain has emerged as a model system in fungal-fungal interactions. Several cDNA, cosmid and BAC libraries have been constructed, ESTs sequences deposited. The role of two mitogen-activated protein kinase (MAPKs) has been studied. The TmkA pathogenicity MAPK was found to be involved in repression of conidiation, parasitism of sclerotia and induced resistance response in plants. Another MAPK, the TmkB cell integrity kinase was also involved in repression of conidiation in addition to antagonistic properties and cell wall integrity. Similarly, a G-protein alpha subunit TgaA was found to be involved in antagonism, though in a host selective manner. Interestingly, even though the two G-proteins studied had no major effects on growth of the fungus, deletion of Tacl, the adenylate cyclase, drastically affected the growth. Deletion of Tad also affected the production of antifungal compounds viridin and viridiol. In addition to functional studies, using suppression subtractive hybridization (SSH), genes have been identified that are regulated during MAPK signaling as well as secondary metabolism. These data have opened up enormous potential for genetic improvements of these strains.