Rhizosphere competence of selected Trichoderma species

Abstract

Trichoderma is a genus of fungi commonly used as biological control agents and growth promoters of plants. The ability of these fungi to become rhizosphere competent has been linked to their biological control capabilities. However, the extent of rhizosphere competence and factors that affect it are poorly defined in the literature. Rhizosphere competence (RC) is defined as the ability of an organism to colonise, grow and develop in the rhizosphere soil. The aim of this research was to determine RC across a range of Trichoderma species and isolates. A series of experiments were conducted to (1) rank RC, (2) determine if RC is plant species specific, (3) test RC under a range of abiotic conditions, and (4) evaluate RC over time. All experiments were carried out by coating seeds of test plants with Trichoderma spore suspensions and sowing seeds in non-sterile Wakanui silt-loam soil. The ranking of RC was assessed for 22 isolates of 11 of the most common Trichoderma species used for biological control in a non sterile sweetcorn expermental system in which plants were typically grown for 7 days (Chapter 2). Results showed that 82% of the Trichoderma isolates had populations that were significantly higher than the control indicating that rhizosphere competence was widespread within the selected species. The least and most rhizosphere competent isolates belonged to the same species, indicating that RC was not species specific. The three least and most RC isolates from the ranking experiment were tested on six plant species (sweetcorn, ryegrass, cauliflower, carrots, onion and white clover) in Chapter 3. Ryegrass and cauliflower were the most receptive plants to Trichoderma species, and clover was the least receptive. The least rhizosphere competent isolate on sweetcorn was the only one able to colonise onion roots and had significantly higher RC than all the other isolates, suggesting host plant specificity of RC. On average, Trichoderma atroviride LU132 was the most rhizosphere competent isolate across the six plant species. Rhizosphere competence of T. atroviride LU132 was assessed under a range of three soil moistures (16, 20 and 24% gravimetric water content, GWC), three pH levels (5.5, 6.5 and 7.5), and three soil available nitrogen concentrations (75, 150 and 300 kg N/ha) (Chapter4). This isolate was rhizosphere competent on sweetcorn and ryegrass, regardless of the abiotic conditions. Trichoderma atroviride LU132 also endophytically colonised sweetcorn and ryegrass shoots and roots in all test conditions. Rhizosphere competence over time was assessed for a strong (T. atroviride LU132) and weak (T. virens LU556) sweetcorn coloniser at 7, 21, 35 and 56 days post planting (Chapter 5). The influence of both isolates on the rhizosphere microbial communities was assessed by denaturing gradient gel electrophoresis (DGGE). Trichoderma atroviride LU132 was more RC than T. virens LU556 at every assessment point. Whilst T. virens LU556 populations continued to increase over time, T. atroviride LU132 populations reached a maximum after 35 days. Shoots and roots of sweetcorn were endophytically colonised by both isolates regardless of harvest time. Trichoderma atroviride LU132 and T. virens LU556 changed the bacterial, fungal and arbuscular mycorrhizal fungal communities of the rhizosphere compared to untreated plants. Rhizosphere competence and endophytic colonisation of sweetcorn was also assessed at different root depths on 35 day old plants (Chapter 6). Results showed that regardless of the isolate, the top portion of sweetcorn roots had significantly higher Trichoderma populations than the middle and bottom parts. Trichoderma atroviride LU132 was more rhizosphere competent than T. virens LU556 regardless of the root portion. More endophytic Trichoderma colonies were isolated from plants treated with T. atroviride LU132, further indicating the greater RC of this isolate. Overall, this research showed that the relationship between Trichoderma and the plant is dependant on the Trichoderma isolate and the plant species. However, some isolates are more broadly RC than others, and may have greater utility as plant protection agents. Greater RC appeared to be associated with enhanced endophytic colonisation.