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 ).