Salt stress is a major environmental factor that inhibits crop growth.  Trichoderma spp. are the most efficient biocontrol fungi and some of the strains can stimulate plant growth.  Phosphate solubilization is known as one of the main mechanisms in promoting plant growth, but the underlying mechanisms of phosphate solubilization in the salinity still need to be explored.  The Trichoderma asperellum Q1 isolated and identified in our lab is a beneficial rhizosphere biocontrol fungus with a high phosphate solubilization activity.  It could produce acid and alkaline phosphatases when using insoluble organic phosphorus as the sole phosphorus source, the salt stress increased the phosphorus-solubilization ability of the strain and the activities of the two enzymes.  Furthermore, an acid phosphatase was purified from the fermentation broth by ammonium sulphate precipitation, ion-exchange, and gel filtration chromatography.  Its molecular weight was 55 kDa as determined by SDS-PAGE.  The purified acid phosphatase was used to investigate growth performance of Arabidopsis thaliana by plate assay and the result showed that it contributed to Arabidopsis growth by transforming organic phosphate into a soluble inorganic form under salt stress.  To our knowledge, this is the first report on acid phosphatase purification from T. asperellum and its function in regulation of plant growth under salt stress.

Salinity is one of the major abiotic stresses limiting crop growth and yield. This study investigated the underlying mechanisms of Trichoderma asperellum Q1 in promoting cucumber growth under salt stress, including the abilities of the strain to solubilize phosphate and to produce phytohormone. The results showed that T. asperellum Q1 could solubilize inorganic or organic phosphate and the activities of phosphatases and phytase could be detected in the culture supernatant. In hydroponic experiments, the growth of cucumber seedlings was increased in the hydroponic system treated by culture filtrate of strain Q1 with tricalcium phosphate or calcium phytate under salt stress. This strain also exhibited the ability to produce indole acetic acid (IAA), gibberellic acid (GA) and abscisic acid (ABA) in liquid medium without any inducers. The levels of those three phytohormones in cucumber seedling leaves also increased after inoculated with this strain, along with increased root growth and root activities of the plant. These results demonstrated the mechanisms of T. asperellum Q1 in alleviating the suppression effect of salt stress involving the change of phytohormone levels in cucumber plant and its ability of phosphate solubilization.