Bacterial predation of a fungal wheat pathogen: Prelude to experimental evolution of enhanced biocontrol agents

<p><span>Research to identify appropriate biological agents for controlling pathogens might exploit experimental evolution to select for enhanced antagonism against pathogens. We report development of an experimental regime facilitating this approach featuring the broad-spectrum bacterial predator <em>Myxococcus</em> <em>xanthus</em> and the fungal wheat pathogen <em>Zymoseptoria</em> <em>tritici</em>. We demonstrate that <em>Z. tritici</em>, a major cause of crop loss and fungicide use worldwide, both fuels growth of and is killed by <em>M. xanthus</em> on wheat straw, a likely source of vernal seedling infection. <em>M. xanthus</em> is first found capable of density-dependent growth on moistened straw alone, unaided by <em>Z. tritici</em> as prey, growth that also depends on how long straw was moistened before <em>M. xanthus</em> inoculation. Such growth of non-cellulolytic myxobacteria on plant detritus may have implications for understanding their roles in nutrient cycling. However, straw with <em>Z. tritici </em>is found to fuel greater <em>M. xanthus </em>growth than straw alone under many conditions. After shorter moistening, growth by low-density <em>M. xanthus</em> populations is found to be fully dependent on <em>Z. tritici</em>. Such pathogen-dependent <em>M. xanthus</em> populations could be sustained with population replacement over five one-week growth cycles, indicating feasibility of conducting long-term experimental evolution with this system. Further, <em>M. xanthus</em> is found to kill majorities of <em>Z. tritici</em> populations on both buffered agar and straw.  Our results suggest that myxobacteria may serve as effective biocontrol agents of <em>Z. tritici</em> and are amenable to long-term experimental selection for enhanced killing of this pathogen, an approach broadly applicable to many potential biocontrol agents and target pathogens.</span></p>