Effect of Trichoderma colonization on auxin-mediated regulation of root elongation

The biocontrol fungus Trichoderma harzianum 1295-22 increases root growth in addition to roles in suppressing disease. Its agricultural use could be expanded if the mechanism of growth enhancement were known. Among the proposed mechanisms of growth enhancement is that the fungus counteracts auxin inhibition of root-cell elongation. We tested whether there was evidence for a secreted auxin inhibitor, for enhanced auxin degradation, or for altered auxin sensitivity. Our results provide no support for any of these mechanisms. Trichoderma secretions inhibited growth, whereas an auxin inhibitor would increase growth. Auxin inhibited growth to the same extent in colonized and uncolonized roots, indicating no change in auxin sensitivity. Endogenous auxin levels maintained growth closer to the maximum in uncolonized roots, indicating stronger auxin limitation of growth in colonized roots. These tests indicated that Trichoderma-colonized roots had a faster maximum growth rate, but an unchanged response to auxin.