Trichoderma harzianum Protects the Arabidopsis Salt Overly Sensitive 1 Mutant Against Salt Stress

Salt stress is one of the major environmental factors that limits crop productivity. To mount an effective response to cope with salt stress, plants rely on the salt overly sensitive (SOS) pathway. The SOS1, SOS2 and SOS3 proteins are crucial for the maintenance of ion homeostasis and the sos1 mutant is hypersensitive to salt stress. Trichoderma harzianum, a beneficial fungus, increases the tolerance of plants to abiotic stresses. We examined the effect of the Trichoderma strain on the performance of the salt overly sensitive (sos1) mutant of Arabidopsis under salt stress. Compared to the isogenic glabra1 (gl1) control seedlings, the fresh weight, chlorophyll fluorescence, photosynthetic pigment content and transcript level of genes involved in ROS scavenging were increased in Trichoderma-inoculated sos1 plants under 150 mM salt stress. Trichoderma also enhanced the accumulation of the osmolytes proline, alanine, as well as the sucrose and glucose in the salt-stressed sos1, but not gl1 mutants, and the accumulation of Na⁺ was restricted in the sos1 mutant. The beneficial effects of T. harzianum could be attributed to higher colonization rates of the sos1 mutant compared to the gl1 controls. In conclusion, these findings underscore that the Trichoderma strain activates stronger salt protective responses in the salt-sensitive sos1 mutant than in control gl1 plants. Therefore, the Trichoderma strain is a valuable tool to investigate how a beneficial endophyte can stimulate salt tolerance responses in the host to promote its performance under stress.