Study Of The Biocontrol Strain Trichoderma Asperellum And Its Mechanism On Plant Growth Promotion Under Salt Stress

With the global environmental deterioration, soil desertification and salinizationproblems are increasingly threats to the limited land resources for human survival.Approximately 7% of the global land surface is covered with saline soil. Moreover,there exist wide areas of secondary soil salinization in China, which effects theagricultural production and the ecological environment, due to poor irrigation systemwithout proper drainage and inorganic fertilizer management. These cultivated landsaffected by salts may adversely cause oxidative damage to the soil which in turnaffects the plant growth by reducing plant’s nutrient uptake and metabolic andphotosynthetic activities. In addition, most of the saline soils have alkaline pH valueand most iron exists in an insoluble form (Fe3+), which prevents plants from optimalgrowth and even leads to leaf chlorosis. Thus plants growing on this kind of soils areoften simultaneously subject to salinity and iron deficiency.Trichoderma spp. are versatile beneficial fungi which can stimulate growth andplant resistance to biotic and abiotic stresses. They have long been recognized asagents for their plant disease control and plant growth promotion, but there are fewreports on their ability to improve salt tolerance of plant and the specific knowledgeabout the underlying mechanisms remain to be explored. For further research of thepotential of Trichoderma in promoting the cucumber growth under salt stress and itspossible mechanisms, we isolated a biocontrol fungus Trichoderma isolate Q1 withplant growth-promoting activities and evaluated its potential in promoting cucumbergrowth and the role of siderophore produced by the strain in alleviating negativeeffect of salinity. In addition, the siderophore was purified by using immobilizedmetal affinity chromatography (IMAC) and identified the type. The specific results srepresented as follows:1. Strain Q1 was isolated from the rhizosphere in greenhouse and identified as Trichoderma asperellum based on its morphological features and the molecularphylogenetic analyses. The Trichoderma asperellum Q1 showed antagonism againstmany phytopathogenic fungi, such as F. oxysporum f. sp. Cucumerinum, F. oxysporumf. sp. Niveum and Fusarium solani. In addition, it exhibited some plantgrowth-promoting attributes of phosphate solubilization,1-aminocyclopropane-1-carboxylate (ACC) deaminase activity, auxin and siderophoreproduction. These results indicated that Trichoderma asperellum Q1 as biologicalcontrol agents are able to produce potential growth-promoting metabolites tostimulate plant growth.2. The pot trial was conducted to study the effects of Trichoderma asperellum Q1on the growth of cucumber seedlings by assaying the changes in physiological andbiochemical parameters and photosynthetic efficiency under salt stress. Fourtreatments were set up as follows: cucumber seedlings inoculated with strain Q1 (Q1),strain Q1 and NaCl (Q1+salt), uninoculated but NaCl-treated (salt), uninoculated andwithout salt stress (control). Cucumber seedlings were inoculated with sporesuspension of strain Q1 with or without exposure to salt conditions, and changes ingrowth and biochemical parameters such as plant height, chlorophyll content, theosmosis molecules soluble sugar, soluble protein, malondialdehyde (MDA) and theactivities of antioxidant enzyme superoxide dismutase (SOD), peroxidase (POD) incucumber leaves as well as several plant physiological parameters like netphotosynthesis (PN), transpiration rate (Evap), stomatal conductance (GS),intercellular CO2concentration (C Int) were evaluated at 25 days after inoculation.The results indicated that application of strain Q1 could significantly promoteseedlings growth and alleviated growth suppression induced by salt stress comparedwith the uninoculated and salt stressed controls of cucumber seedlings.3. Furthermore, the role of siderophores was proved through solution cultureexperiments. In culture solution with limiting iron and 60 mmol l-1NaCl stress, fourtreatments were set up as follows: SCF with and without 30μmol l–1Fe3+(SCF±Fe),uninoculated MM9 medium with and without 30μmol l–1Fe3+(Non-SCF±Fe), andthe treatment of Non-SCF-Fe was used as control. The result indicted that the biomass of cucumber seedlings is maximum under the treatment of siderophore culture filtrate(SCF) of strain Q1 with insoluble Fe3+in salt stress, compared to the other treatments.Meanwhile, cucumber shoot or root length and first euphylla leaf area were increased,and the percentage of wilted cucumber seedlings was decreased greatly. This clearlydemonstrated that siderophores may play an important role in alleviating thedeleterious effects of salinity to cucumber seedlings, especially under iron deficientconditions.4. For identified the type of siderophore produced by Trichoderma asperellumstrain Q1, the siderophore-containing culture filtrate is purificated by usingimmobilized metal affinity chromatography (IMAC). It was identified as hydroxamatetype siderophore and no fluorescence based on MTT test. This result supplementedeffectively study of siderophore types, and may provided theoretical basis for thechemistry structure study and its role in iron transport.