We all care about our food. We wish to be informed about the practices that underpin their production and have adequate supplies at affordable prices. The factors that affect food availability, however, (also termed ‘food security’) are generally not considered, let alone the scientific research addressing threats to food production.
Diverse microbial communities continuously challenge plants, but disease is the exception rather than the rule (Boller and Felix, 2009; Jones and Dangl, 2006; Dodds and Rathjen, 2010). Physical (e.g. cell walls) and/or chemical barriers block infection by potential pathogens in most cases by denying access to the host interior.
Not surprisingly, some microbes have acquired and evolved enzymes that help degrade physical barriers or detoxify their host environment. Because of the high-end stakes of plant-pathogen interactions (life vs death), plants have adopted additional defensive strategies to help keep pathogens at bay. Genetic, biochemical and functional studies in model plants and crop species have firmly demonstrated mechanisms of adaptive immunity that fend off microbial attacks (Jones and Dangl 2006). These include local induced responses (site of infection or offence) and systemic responses that are produced by hormones that originate from infection sites (Figure 2). These signals travel throughout the plant, alerting distant tissues of pathogen presence and thereby priming the plant for a faster and stronger response
Early genetic studies have identified a great number of mutants in Arabidopsis, that are either unable to mount systemically induced responses or have lost the ability to limit the expression of defence associated genes (constitutive defence mutants with enhanced resistance). These and subsequent studies have facilitated both the dissection of distinct defence signalling pathways, as well as the identification of gene complements whose expression is defence associated. Not surprisingly, many of the proteins encoded by defence-associated genes have enzymatic functions (e.g. lytic enzymes that break down pathogen cell walls, produce anti-microbial metabolites) or act as inhibitors of pathogen-encoded enzymes (Tian et al., 2007; Dong et al., 2014).
From this, the overall view emerging from the literature is that:
I Plants have evolved inducible defences to deal with microbes that can break down physical barriers or take advantage of wounding events.
II The proteins produced by the host during these defence responses are often secreted to combat infection.
III That these weapons are not only directed against the pathogen cell itself (lytic enzymes) but also against pathogen proteins that facilitate infection (breaking down barriers or detoxify plant compounds).
These observations point to an ongoing evolutionary conflict (driven by the life vs death stakes game) that drives the acquisition, adaptation, and selection of new molecular weaponry in both plants and pathogens. Consequently, host-microbe interactions are considered evolutionary arms races. The idea of co-evolutionary arms races also conforms with the “Red Queen Hypothesis” (Figure 3, a reference to the running red queen in “Alice Through the Looking Glass” proposed by Leigh van Valen (1973)). Much like the red queen who must run on a moving chessboard to stand still, organisms must continuously adapt (run) to survive (stand still) in their ever-changing natural environment (moving chessboard).
The Red Queen explaining the running game to Alice. Image Source
The notion of a co-evolutionary arms race only holds if both the host and the pathogen are “running” to survive. Consistent with this idea, numerous studies have identified a range of evolutionary strategies, pathogens have adapted to keep up with evolving host plants. These fascinating and scintillating observations will be described in a future piece. The co-evolutionary arms race idea demands a re-think on how we cultivate large monoculture crops. The use of elite crop varieties that are genetically identical and grown together will impact these arms races.