Actin-dependent and PTI signalling merge between calcium
influx and MAPK
Activation of calcium influx is a hallmark for PTI (Gómez-Gómez &
Boller, 2002). However, while aluminium can trigger
phytoalexin-synthesis transcripts, it does not produce any extracellular
alkalinisation (Supp. Figure S3 ), indicating that this
transcript induction is independent of calcium influx. On the other
hand, in presence of the specific MAPK inhibitor PD98059, the transcript
response to aluminium is blocked completely (Figure 6A ). Thus,
the most straightforward working model would place the merge the merge
between actin-dependent and PTI signalling downstream of calcium influx,
but upstream of MAPK activation. This model leads to two implications
that are both supported by the literature record: First, MAPK signalling
should be activated by oxidative burst. In fact, this has been reported
for a number of cases (for a recent review see Liu & He, 2017), for
instance mediated by the serine-threonine kinase OXI1 (Rentel et
al. , 2004). Second, the MAPK cascade should be activated by
modulations of actin dynamics. Also this implication has been
experimentally confirmed (reviewed in Šamaj et al. , 2004). For
instance, the activity of SIMK and SAMK, a MAP kinase from alfalfa were
activated, when actin filaments were eliminated by Latrunculin B (Šamaj
et al. , 2002; Sangwan et al. , 2002), in case of SIMK, also
suppression of actin turnover by Jaspaklinolide was activating (Šamaj et
al. , 2002). These authors also had observed that a MEK1 inhibitor
induced remodelling of plant actin, indicative of a feedback of MAPK
signalling upon actin (consistent with our findings using PD98059,Figure 6 ).