Keith Hanson

Soil Fungal Resources in Annual Cropping Systems and Their Potential for Management

Soil fungi are a critical component of agroecosystems and provide ecological services that impact the production of food and bioproducts. Effective management of fungal resources is essential to optimize the productivity and sustainability of agricultural ecosystems. In this review, we (i) highlight the functional groups of fungi that play key roles in agricultural ecosystems, (ii) examine the influence of agronomic practices on these fungi, and (iii) propose ways to improve the management and contribution of soil fungi to annual cropping systems. Many of these key soil fungal organisms (i.e., arbuscular mycorrhizal fungi and fungal root endophytes) interact directly with plants and are determinants of the efficiency of agroecosystems. In turn, plants largely control rhizosphere fungi through the production of carbon and energy rich compounds and of bioactive phytochemicals, making them a powerful tool for the management of soil fungal diversity in agriculture. The use of crop rotations and selection of optimal plant genotypes can be used to improve soil biodiversity and promote beneficial soil fungi. In addition, other agronomic practices (e.g., no-till, microbial inoculants, and biochemical amendments) can be used to enhance the effect of beneficial fungi and increase the health and productivity of cultivated soils.

Intertwined Existence: The Life of Plant Symbiotic Fungi in Agricultural Soils

Arbuscular mycorrhizal (AM) fungi are root endophytes that have evolved with land plants for over 400 million years (Pirozinsky and Dalpe 1992). Through time, fungal endophytes and plants have become interdependent. Plants rely largely on fungi for soil nutrient uptake and some fungi even became obligate biotrophs unable to exist without a living host plant. Plants and fungi have coevolved within relatively stable ecosystems, in very successful symbiosis at times, as illustrated by the widespread occurrence of AM mycorrhizae. Cropping systems are typically highly disturbed systems in which cropping practices often harm symbiotic fungi or their host plants. We know that the full potential of the AM symbiosis is rarely realized in cultivated fields, leading to inefficiencies in the function of these ecosystems. Other fungal endophytes of crop roots were overlooked and their role in plant fitness remains largely obscure, although they may be important in ecosystem function. Disruption in the proper function of plant symbioses may lead to inefficiencies in the function of cultivated soils. With the impoverishment of cultivated soils in poor countries and environmental quality degradation due to the loss of residual fertilizer to the environment in economically favored countries, it becomes urgent to improve the nutrient use efficiency of crop production. Improving the effectiveness of the AM symbiosis in cultivated fields would certainly be an important step toward this goal. Improving plant symbiosis effectiveness requires a good understanding of the conditions specific to cultivated soils that are influencing beneficial plant symbiosis. Therefore, the goal of this chapter is to consider the factors influencing the symbioses formed between plants roots and soil fungi, most importantly AM fungi.

The H2-oxidizing Rhizobacteria Associated with Field-Grown Lentil Promote the Growth of Lentil Inoculated with Hup+ Rhizobium Through Multiple Modes of Action

Certain H2-oxidizing rhizobacteria promote the growth of legume plants nodulated with rhizobia devoid of an uptake hydrogenase system (Hup−). We demonstrated and assessed the plant growth-promoting ability of H2-oxidizing rhizobacteria naturally associating with lentil roots nodulated by rhizobia possessing an uptake hydrogenase system (Hup+ lentil) in semiarid Canada. The ten H2-oxidizing rhizobacteria isolated were strains of Variovorax paradoxus, Variovorax sp., Rhodococcus sp., Mycobacterium sp., Acinetobacter sp., Acinetobacter calcoaceticus, and Curtobacterium sp. Several of these strains increased Hup+ lentil shoot and root biomasses, and root nodule number in the absence or presence of drought stress. Inoculation with H2-oxidizing rhizobacteria enhanced the growth of Hup+ lentil infected by the fungal root pathogens Fusarium avenaceum, Rhizoctonia solani, and Pythium ultimum. Fusarium avenaceum growth was markedly suppressed by all H2-oxidizing rhizobacteria in vitro, and seven isolates also suppressed the growth of both R. solani and P. ultimum. Siderophore production was detected in nine isolates and one isolate could solubilize phosphate. Indole-3-acetic acid production was found in four isolates, and 1-aminocyclopropane-1-carboxylate deaminase activity in six isolates. Most H2-oxidizing rhizobacterial isolates exhibited multiple plant growth-promoting attributes and all isolates exhibited at least one. Our results suggest that the H2-oxidizing rhizobacteria naturally associating with lentil roots in semiarid Canada are beneficial in an Hup+ environment.