Transcripts of SA response and synthesis are induced by
Al3+ depending on actin
The experiments described above showed that the aluminium induced
defence-related transcripts dependending on actin. Actin remodeling is
often observed as hallmark for ensuing programmed cell death. In the
context of grapevine cells, this has been observed, for instance, during
the responses to harpin (Qiao et al., 2010), to the stilbene aglycon
resveratrol (Chang et al., 2011), or to the oxylipin derivativecis -3-hexenal (Akaberi et al. 2018). Since defence-related
programmed cell death is usually deployed to encounter infection by
biotrophic pathogens, a defence type commonly regulated by salicylic
acid (SA), we investigated the transcription of PR-1(pathogenesis related 1) (Kobayashi & Kobayashi, 2007) as readout for
SA-dependent gene expression. As shown in Figure 5A , the
steady-state transcript levels for PR1 went up significantly in
response to Al3+, and this induction was strongly (by
a factor of almost 3) suppressed by Latrunculin B (Figure 5A ).
The effect of aluminium could be efficiently mimicked by Phalloidin
(Figure S2 ), a compound stabilising filamentous actin (F-actin)
by effectively suppressing actin dynamics. Latrunculin B alone yielded
only a minor response (less than 25% of the response seen with
Al3+). While these data would place PR1downstream of actin, in a similar way as the phytoalexins synthesis
genes (PAL , RS , and STS ), and the transcriptional
regulator MYB14 , they also lead to the next question:
If PR1 as SA responsive gene is activated by aluminium through
actin-dependent signaling, there should be also a response of
SA-synthesis genes. Two biosynthetic pathways, leading to SA, are known
(reviewed in Chen et al., 2009) – one pathway uses cinnamonic acid as
substrate, and, therefore, depends on the induction ofphenylalanine ammonium lyase (PAL ), while the substrate of
the other pathway is isochorismate, and therefore depends on the
induction of isochorismate synthase (ICS ). The induction
pattern for PAL had already been tested (Figure 3A ), but
since this induction might also occur in context with phytoalexins
synthesis itself, it does not conclusively indicate a response of SA
synthesis. We, therefore, analysed the transcript levels for ICSas well to test, whether they follow the same activation pattern as the
phytoalexins genes. As shown in Figure 5B ,
Al3+ could induce ICS transcripts
significantly, although the induction was only around one fifth of that
seen for PR1 . This aluminium response was completely suppressed
by Latrunculin B, and Latrunculin B alone did not establish the ground
levels, but even decreased ICS significantly. In contrast to the
pattern seen for PR1 and PAL , phalloidin was not able to
mimic aluminium with respect to the induction of of ICS(Figure S2 ).