Root traits
Plant roots, like leaves, are also thought to follow a root economic
spectrum that influences soil pathogen susceptibility, but root systems
are less well studied than aboveground traits and some evidence suggests
that the root economic spectrum is only partially correlated with the
leaf economic spectrum
(Kramer-Walter et
al. 2016; Laliberté 2017). Larger root systems can improve disease
tolerance of plants when roots serve as storage organs for regrowth
after damage/defoliation (Erbet al. 2009). As seen with leaves, faster growing roots with
high turnover rates are less susceptible to pathogen attack
(Yanai & Eissenstat
2002; Atucha et al. 2014), and root surface area and secondary
root diameter have been shown to be positively correlated with disease
resistance (Sollaet al. 2011; Singh et al. 2019). There is limited
information on the mechanisms driving these correlations and studies
across a broad range of species are needed to determine the full range
of variability in disease susceptibility.
It is well documented that arbuscular mycorrhizal (AM) colonization can
protect plants from soil-borne pathogens
(Borowicz
2001; Whipps 2004; Pozo & Azcón-Aguilar 2007; Wehner et al.2011; Lewandowski et al. 2013). Potential mechanisms for
mycorrhizal mediated pathogen protection include; 1) direct competition
with or inhibition of pathogens
(Newsham et al.1995); 2) promotion or changes in plant growth, nutrition, and
morphology (Wehner et
al. 2011); 3) modulation of biochemical plant defense
(Van Wees et al.2008); and 4) facilitation of pathogen suppressive microbiomes
(Whipps 2004). These
mechanisms can be influenced by root architecture such as root branching
(Sikes et al.2009; Maherali 2014). Ectomycorrhizal (ECM) fungi can also protect
their hosts from pathogens and are thought to provide better protection
against pathogens than AM fungi because of the sheath, hyphal network,
and antimicrobial substances associated with ECM fungi
(Garrett 1956;
Marx 1972; Stack 1975; Duchesne et al. 1989). Evidence has been
accumulating to suggest that ECM fungal protection against soil-borne
pathogens can be a major mechanism behind more positive plant-soil
feedbacks among ECM driving mono-specific stands of ECM compared to AM
species (Connell &
Lowman 1989; Laliberté et al. 2015; Liang et al. 2020).
These findings suggest that ECM invasive plants may be less susceptible
to pathogens than AM or non-mycorrhizal invaders, provided they
encounter compatible ECM fungi.