Book Description

All organisms have evolved to secrete a diverse array of molecules to interact with or affect other organisms and surrounding environment. Volatile compounds (VCs), due to their ability to easily evaporate and diffuse through air, liquids and porous soils, act as info-chemicals that mediate both short- and long-distance organismal interactions in animals, plants and microbes. Accumulating evidence suggests that VCs produced by diverse microbes play multiple critical roles in microbial biology and ecology. However, the focus of research has been on water-soluble molecules that mediate short-distance interactions. The main goal of my thesis research is to explore mostly overlooked roles and mechanisms that underpin VC-mediated fungal interactions with plants and other microbes mainly using Verticillium species and Fusarium oxysporum, two soilborne fungal pathogens that impact agriculture and the environment around the world. In Chapter 1, I reviewed known and suggested roles of microbial VCs in intra- and inter-species interactions with a focus on how fungal VCs affect plant growth, development and stress resistance. Future research need and several technical challenges, which must be overcome in order to expedite studies on VC-mediated plant-fungal interactions, were also discussed. This review was published.In Chapter 2, I present a published study on the role of VCs produced by diverse Verticillium species in plant growth and development, the nature of VCs produced, and the mechanism underlying plant response to Verticillium VCs. VCs produced by 19 strains that represent all known Verticillium species promoted the growth of Arabidopsis thaliana and Nicotiana benthamiana. Using two V. dahliae strains, I showed that their VCs affect multiple plant traits and cause preferential resource allocation for root growth over shoot growth. Using a combination of genetic, histochemical and chemical approaches, I demonstrated the involvement of auxin signaling in controlling plant responses to V. dahliae VCs, with TIR3 playing a critical role. Another published study described in Chapter 3 showed that VCs produced by V. dahliae and F. oxysporum affect plant stress tolerance. Their VCs alleviated plant salt stress, and the potential mechanism underlying their effect involved auxin signaling. In addition, VCs produced by two F. oxysporum strains induced the expression of PR1::GUS in A. thaliana leaves and reduced disease severity caused by the bacterial pathogen Pseudomonas syringae. Co-cultivation of plants and fungi in I plate, a Petri plate with central division, was employed to conduct the studies described in Chapters 2 and 3. To support studies on how fungal VCs affect plants in soils, I developed two novel bioassay systems that provide more ecologically relevant environments to study their roles. Their development and preliminary evaluations are described in Chapter 4. In Chapter 5, I describe an ongoing study designed to explore potential roles of fungal VCs in fungal-fungal interactions using F. oxysporum and four biocontrol Trichoderma species. This study was initiated to test the hypothesis that VC-mediated interactions between biocontrol agents and pathogens play important roles in biocontrol. The long-term goal is to support the development of effective and reliable biocontrol for soilborne fungal diseases. VCs produced by both F. oxysporum and Trichoderma inhibited the growth of the other, suggesting the involvement of VCs as inter-fungal chemical warfare. This study was the first to show that F. oxysporum VCs induced the production/secretion of anti-fungal molecules by Trichoderma in a strain-specific fashion. Interestingly, F. oxysporum also recognized and responded to Trichoderma by sensing its VCs, suggesting the wide involvement of VCs in inter-species interactions. My thesis research suggests that VC-mediated interactions may be a prevalent mechanism employed by diverse microbes, including plant pathogenic fungi, in soils and the rhizosphere and likely play important roles in microbial ecology and pathology. Future studies needed to advance our understanding of the nature and mechanisms underlying VC-mediated plant-fungal and fungal-fungal interactions are described in Chapter 6.