A fitness cost. A rapid evolutionary response within the

A component of the SAR signalling pathway, NPR1, has been identified as a candidate
gene affecting drought tolerance. A future aim is to investigate its role in
drought tolerance, via gene knockout experiments, since its gene product has
been shown to suppress genes that synthesise or respond to the phytohormone
abscisic acid (ABA) (Yasuda et al., 2008). ABA

accumulation could significantly impact drought
tolerance through mechanisms such as altering
stomatal development (Franks and Farquhar,
2001). Therefore, inhibition of ABA would explain why no alteration in stomatal development occurs after
the infection of a resistant host (Murray et al., 2016). This hormonal antagonism (Yasuda et al., 2008) could
cause the fitness trade-off in resistant hosts proposed by this study. This trade-off
may only occur under drought stress, since watered N. glutinosa plants do not exhibit a decreased drought tolerance
after viral infection (see Figures).

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By comparison, viral infection in a
susceptible host results in ABA accumulation (Whenham et al., 1985),
uninhibited in the absence of the SAR pathway (Ross, 1961). For reasons stated
above, this accumulation of ABA could increase drought tolerance in susceptible
hosts (Xu et al., 2008) in both watered and
drought-treated susceptible hosts (see Figures).

This study then sought to characterise the interaction
between susceptible host-virus. The systemic infection rate of the virus can be
used as a measure of its fitness (Sacristán et
al., 2011). Hence, we can conclude from the results that drought imposes
a fitness cost on TMV. This has been shown in other viruses (Cordoba et al., 1991).
Water may be required for viral movement if the virus infects systemically via
the xylem (Séron and Haenni, 1996). Drought
may therefore act as a selection pressure on the viral population to manipulate
the host by increasing its water use efficiency (Parkhurst and Loucks, 1972). The
virus may have induced this, via ABA accumulation (Whenham et al., 1985), as an
evolutionary response to minimise a potential drought-imposed fitness cost.

A rapid evolutionary response within the viral
population is likely considering the evolution rate of RNA viruses (viruses
that have ribonucleic acid, RNA, as their genetic material) such as TMV (Holland
et al., 1982). Genetic drift would have been a strong influence on the viral
populations in the study, due to the initial population bottleneck from
inoculation (Sacristán et al., 2011). Combined
with drought stress, these influencing factors may explain the large amount of variation
in viral proportion within systemic leaves in Experiment 1, as indicated by its
R2 value.

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