Calling for a role of salicylic acid signalling also in basal immunity
The pathway activated by aluminium activates some features that, in grapevine cells, usually are associated with cell-death related immunity, such as actin remodelling, or activation of RboH (Chang et al., 2015). On the other side aluminium die not induce cell death. We wondered, therefore, to what extent this would be reflected in activation of salicylic acid signalling, as salicylic acid is associated with defence against biotrophic pathogens, i.e. a condition, where cell-death related defence would provide an evolutionary advantage, while PTI is associated with jasmonate signalling (reviewed in Glazebrook, 2005). Also for grapevine cells, jasmonate signalling seems to be exclusively linked to PTI (Chang et al., 2017). Surprisingly, aluminium not only induced the transcripts for phytoalexin genes, but also activated isochorismate synthase , a key enzyme of salicyclic-acid biosynthesis (Figure 5B ), as well as the salicylic-acid responsive PR1 (Figure 5A ). In both cases, the induction by aluminium can be inhibited by Latrunculin B, providing evidence for the involvement of the actin-dependent pathway. A link between actin and salicylic-acid signalling is apparently not confined to grapevine: In tobacco leaves, the marker genes PR1and PR2 were significantly increased by treatment with cytochalasins (Kobayashi & Kobayashi, 2007). A similar effect could be observed in Arabidopsis seedlings, when they were treated with actin filaments drugs latrunculin B and cytochalasin E. Here, the expression of PR1 , PR2 and WRKY38 was increased after 6 h (Matoušková et al., 2014), mimicking the patterns seen for salicylic acid.
The co-existence of salicylic-acid signalling with a defence pattern not accompanied by programmed cell death, indicates that salicylic acid, contradicting the current prevailing concept, can be recruited for basal immunity. Also this conclusion is in line with previous observations, where salicylic acid can activate a specific stilbene synthase promoter allele from a wild Chinese grapevine species, V. pseudoreticulata(Jiao et al., 2016).