What is the relationship of aluminium-triggered signalling
with basal defence?
The fact that SA signalling can be activated by aluminium and is acting
in the context of basal immunity provokes a new question: Is it possible
to define the event, where the actin-dependent pathway merges with the
signalling deployed in the context of PTI? The operational criterion for
an event located downstream of this merging point would be that
inhibition of this event would block both, gene expression induced by
flg22, and gene expression induced by aluminium. It seems that MAPK
signalling meets this criterion, because treatment of grapevine cells
with the MAPK inhibitor PD98059 can fully suppress the induction of all
three tested genes involved in phytoalexin synthesis (i.e. PAL ,RS , and STS , Figure 6A ), and the same was
observed for the induction of STS by both flg22 and by harpin (Chang &
Nick, 2012). This inhibitor acts at the MAPKK level of the cascade
(Cohen, 1997). Specifically for defence signalling, MKK1 has been
suggested as target (Mészáros et al., 2006). Thus, at the second tier of
MAPK signalling, flg22-triggered, harpin-triggered, and
aluminium-triggered signal transduction seem to be confluent already.
Since aluminium, in contrast to flg22, fails to induce extracellular
alkalinisation (Suppl. Figure S3 ), the merging point must be
downstream of calcium influx. Furthermore, it must be located downstream
of actin, since Latrunculin B can block the induction of gene expression
by aluminium (Figure 3A ).
The fact that aluminium activates a key gene of SA synthesis
(isochorismate synthase) and a marker gene for SA response (PR1) does
not mean that SA is part of the signalling responsible for the
activation of phytoalexin synthesis genes. This is rather unlikely, due
to the time course. However, SA might act in a feedback loop
contributing to the persistence of defence. A straightforward mechanism
would be the accumulation of intracellular ROS. The original idea that
this might be caused by direct inhibition of catalase through salicylic
acid (Chen et al., 1993; Klessig et al. , 2019) has meanwhile been
discarded after decades of controversy, and replaced by a model, where
SA is acting through a signalling network that regulates expression and
activities of the enzymatic and non-enzymatic redox homeostasis (for a
recent review see Klessig et al. , 2019). The link with actin
might come through phospholipase D: by binding to phospholipids, the
substrates of phospholipid D, aluminium inhibits the formation of
phosphatidic acids, as concluded from the fact that exogenous
phosphatidic acids can mitigate the effect of aluminium in Arabidopsis
seedlings (Matoušková et al., 2014).
There is a seemingly paradox observation, though: In our study, several
of the genes that are activated by Al3+, are also
activated by Latrunculin B significantly to the same extent. While the
combination of Al3+ and Latrunculin B eliminates this
activation. Interestingly, the transcription factor MYB14 does not show
this activation by Latrunculin B. Although it appears paradox that two
compounds that act antagonistically upon actin, should exert the same
effect on gene activation, this is not the first time that this has been
reported for phytoalexin synthesis genes in grapevine (Qiao et al.,
2010). Also for MAPK signalling, the activation of the MAP kinase SIMK
by both Latrunculin B and Jaspaklinolide has revealed a similar paradox
(Šamaj et al. , 2002). Activation of signalling by elimination of
actin could be achieved by releasing an activator that had been tethered
to actin, as it has been proposed for the MAP kinases SIMK and SAMK
(Šamaj et al. , 2002; Sangwan et al. , 2002). How signalling
might be stimulated by suppression of actin turnover, is more
challenging to explain. One link might be Capping Protein that can
either be sequestered to the plasma membrane by phosphatidic acids, the
products of the phospholipase D signalling hub, or block the elongation
of actin filaments by blocking the barbed end (Li et al. , 2015).
Stabilisation of actin might repartition Capping Protein from the
membrane towards bundling actin cables, such that phosphatidic acid
would be released for other signalling functions, such as activation of
the Respiratory burst oxidase Homologue (Eggenberger et al. ,
2017), leading to a self-amplifying signalling loop between actin and
reactive oxygen species. The fact that some genes (PAL ,RS , STS ) show both, the activation by
Al3+ and by Latrunculin B, while others (MYB14 )
are not activated by Latrunculin B, indicates that the signal chains
initiating from actin (one possibly through MAPK signalling, the other
possibly through phosphatidic acid signalling) might be activated
differentially and exert differing impact on different target genes.