Introduction
Although static with respect to their location, plants live in a dynamic
environment, which means that stress adaptation is crucial for their
survival (Mittler, 2006). A central aspect of stress resilience is the
immunity against pathogens. While plants lack the adaptable immunity,
found in mammals, they have evolved two tiers of innate immunity (Jones
& Dangl, 2006). For the first tier, plants sense and respond to
microbes including non-pathogens via conserved Pathogen, Microbe and
Damage-Associated Molecular Patterns (PAMPs, MAMPs, or DAMPs,
respectively). Binding of the eliciting molecule to specific pattern
recognition receptors (PRRs triggers
a broadband basal immunity (Bigeard et al. , 2015) termed as PAMP
triggered immunity (PTI). Since PAMPs are essential for the survival of
the pathogen, selective pressure by the host towards the loss of these
treacherous molecules is buffered by the selective pressure to maintain
these molecules, because they are essential. This balance of
antagonistic selective pressures favoured the evolution of effectors
that can inhibit PTI and re-install pathogenicity. In response, plants
have acquired specific receptors recognising these effectors leading to
the second tier of immune system called effector-triggered immunity
(ETI), which is often specific for a certain pathogen, or even to a
given strain of a pathogen (Tsuda & Katagiri, 2010).
The recognition of PAMPs by PRRs is known to trigger calcium influx,
followed by activation of mitogen-activated protein kinase (MAPK)
cascades, in parallel with apoplastic oxidative burst. In consequence,
transcription factors are activated that will stimulate the expression
of defence genes (Withers & Dong, 2017). Some of these events are
shared with ETI (Tsuda & Katagiri, 2010), but the relative timing seems
to differ. For instance, while calcium influx precedes oxidative burst
during flagellin-triggered PTI of grapevine cells, the temporal order of
these two stress inputs is reversed after elicitation with harpin, a
bacterial trigger of an ETI-like response culminating in programmed cell
death (Chang & Nick, 2012).
The two levels of immunity differ also with respect to regulation by
phyto-hormones. While salicylic acid (SA) seems to activate ETI as
effective strategy against biotrophic
pathogens,
jasmonic acid (JA) is involved in the basal defence against necrotrophic
pathogens. The antagonistic role of the two phyto-hormones is reflected
as antagonistic relationship of PTI versus ETI (Ramirez-Prado et al.,
2018). In suspension cells from the North American grape speciesVitis rupestris , JA accumulated in response to PTI triggered by
flg22, but not in response to harpin, a trigger of ETI-like defence
(Chang et al., 2017). It should be noted, however, that during ETI, this
antagonism of JA and SA signalling can sometimes be replaced by a
synergistic interaction. Activation of the SA receptors NPR3 and NPR4 inArabidopsis can also activate JA response and synthesis genes
through degradation of JAZ proteins (Liu et al., 2016), a mechanism
proposed to prevent the spread of necrotrophic pathogens in organs,
where biotrophic pathogens have been warded off by hypersensitive cell
death.
As described above, the early signalling, and also the regulation of PTI
and ETI by phyto-hormones can overlap to a certain extent, leading to
the question, whether there are early cellular events that are
associated with the dichotomy of the two immunity layers. Comparative
studies, where the responses to flg22 (triggering PTI), and harpin (a
bacterial elicitor triggering an ETI-like type of defence) were compared
side by side in suspension cells from either grapevine (Chang & Nick,
2012), or tobacco BY-2 (Guan et al., 2013), suggest that rapid
reorganisation of actin filaments seems to qualify as early marker for
cell-death related immunity. During harpin-triggered ETI-like response,
the NADPH-dependent oxidase Respiratory burst oxidase Homologue (RboH)
is activated and generates superoxide. How this primary signal is
transduced into cell death and/or gene activation, has remained elusive.
A few hints exist, however: A role of superoxide, and phospholipase D
(PLD) for actin remodelling in response to harpin has been inferred from
inhibitor studies (Chang et al. , 2015). Furthermore,
pharmacological modulation of actin filaments is accompanied by elevated
expression of defence genes (Qiao et al. , 2010). Modulations of
actin dynamics have been shown to alter SA synthesis and signalling
(Matoušková et al., 2014). Although these findings indicate that actin
filaments participate in defence signalling, the functional context is
far from clear. Is actin remodelling necessary and sufficient for the
activation of phytoalexin genes? Is actin bundling inevitably linked
with cell death, or is it possible to separate these events?
A strategy to address these questions would be to trigger actin
remodeling in the absence of a pathogen-related signal and to test,
whether this would activate defence responses. Actin filaments respond
to numerous intracellular or extracellular signals (reviewed in
Wasteneys & Yang 2004). In particular, abiotic stress factors that
induce oxidative burst often cause actin bundling, and actin is also
involved in tolerance to these factors. For instance, aluminium as
abundant metal, and therefore of agricultural impact, is able to cause
oxidative burst (reviewed in Panda et al. , 2009), and can induce
actin bundling in tobacco seedlings (Ahad & Nick, 2007). Moreover,
tobacco mutants generated by activation tagging that are tolerant to
aluminium showed constitutive bundling of actin, even in absence of
aluminium as stressor (Ahad & Nick, 2007). Therefore, we used in the
current study the rationale to trigger actin bundling by
Al3+. To follow actin responses, we used a grapevine
cell line expressing the fluorescent actin marker GFP-AtFABD2(Akaberi et al., 2018), and we investigated the resulting defence
responses in cells and plants of grapevine. We show that
Al3+ causes actin reorganisation dependent on RboH,
but does not induce programmed cell death. This actin reorganisation in
response to Al3+ activates genes for the synthesis of
SA and phytoalexins supporting a model, where actin participates in a
pathway involved in basal immunity by connecting the input from
oxidative burst with the activation of defence genes.