Arabidopsis Restricts Sugar Loss to a Colonizing Trichoderma harzianum Strain by Downregulating SWEET11 and -12 and Upregulation of SUC1 and SWEET2 in the Roots.

Phosphate (Pi) availability has a strong influence on the symbiotic interaction between Arabidopsis and a recently described root-colonizing beneficial Trichoderma harzianum strain. When transferred to media with insoluble Ca3(PO4)2 as a sole Pi source, Arabidopsis seedlings died after 10 days. Trichoderma grew on the medium containing Ca3(PO4)2 and the fungus did colonize in roots, stems, and shoots of the host. The efficiency of the photosynthetic electron transport of the colonized seedlings grown on Ca3(PO4)2 medium was reduced and the seedlings died earlier, indicating that the fungus exerts an additional stress to the plant. Interestingly, the fungus initially alleviated the Pi starvation response and did not activate defense responses against the hyphal propagation. However, in colonized roots, the sucrose transporter genes SWEET11 and -12 were strongly down-regulated, restricting the unloading of sucrose from the phloem parenchyma cells to the apoplast. Simultaneously, up-regulation of SUC1 promoted sucrose uptake from the apoplast into the parenchyma cells and of SWEET2 sequestration of sucrose in the vacuole of the root cells. We propose that the fungus tries to escape from the Ca3(PO4)2 medium and colonizes the entire host. To prevent excessive sugar consumption by the propagating hyphae, the host restricts sugar availability in its apoplastic root space by downregulating sugar transporter genes for phloem unloading, and by upregulating transporter genes which maintain the sugar in the root cells.

An Endophytic Trichoderma Strain Promotes Growth of Its Hosts and Defends Against Pathogen Attack.

Plants host numerous endophytic microbes which promote plant performance, in particular under stress. A new endophytic fungus was isolated from the leaves of a deciduous wood tree Leucas aspera. Morphological inspection and multilocus phylogeny identified the fungus as a new Trichoderma strain. If applied to Arabidopsis thaliana and Nicotiana attenuata, it mainly colonizes their roots and strongly promotes initial growth of the plants on soil. The fungus grows on high NaCl or mannitol concentrations, and shows predatory capability on the pathogenic fungus Alternaria brassicicola. Colonized Arabidopsis plants tolerate higher salt stress and show lower A. brassicicola spread in roots and shoots, while arbuscular mycorrhiza formation in N. attenuata is not affected by the Trichoderma strain. These beneficial features of the novel Trichoderma strain are important prerequisites for agricultural applications.