4.1 Phytoalexin is implicated in AR156-triggered ISR against
broad-spectrum pathogens
Plant secondary metabolites were earlier considered to be the
end-product of metabolic or products of detoxification, and research in
recent decades has demonstrated their role in plant regulation of their
growth and development and response to various biotic and abiotic
environmental stresses in the regulation of plant growth and development
and response to various biotic and abiotic environmental stresses. A
large proportion of them is involved in the defense response of plants
against pathogenic microorganisms, including phytoalexin (Dixon, 2001).
Although it is reported that beneficial bacteria P. fluorescensPTA-CT2 and B. subtilis PTA-271 can induce resistance to B.
cinerea and Pst DC3000 (Nguyen et al., 2022), together with
induction of camalexin accumulation and CYP71A12 expression, many
issues remain to be addressed. For example, could B. cereus AR156
contribute to broad-spectrum pathogens resistance by regulating
phytoalexin-related pathways? How is phytoalexin induced by beneficial
bacteria to accumulate and secrete? To deal with the above concerns, we
analyzed the Arabidopsis RNA-seq results and found that a large
number of genes related to the synthesis and regulation of phytoalexin
(particularly the camalexin) were up-regulated in expression after AR156
treatment compared to the control. Meanwhile, we confirmed the
transcriptome results by qPCR assays (Figure 2 B-H). Known as the
dominant phytoalexin in A. thaliana , camalexin has been
implicated in resistance to a wide range of pathogens including
bacteria, fungi, and oomycetes (Glawischnig, 2007; Jun Tsuji, 1992;
Schlaeppi et al., 2010; Stotz et al., 2011; Thomma et al., 1999). In
addition, we found the content of camalexin modest increase inArabidopsis leaves after AR156 treatment compared to the control
(Figure 2 I). It was reported that
root-colonizing P. fluorescens strain SS101 (Pf.SS101) relies on
camalexin to defend against Pst DC3000 and the insect pestSpodoptera exigua (Van de Mortel et al., 2012). We hypothesize
that AR156 triggers the accumulation of camalexin to defend against
different pathogens upon invasion by different pathogens. As was
expected, when inoculated with different pathogens, camalexin
accumulated markedly in Arabidopsis. At the same time, we found
that the content of camalexin in Arabidopsis Col-0 leaves treated
with AR156 was significantly increased compared to the control
inoculated with P. capsici , B. cinerea, or PstDC3000 only. In this study, we found that enhanced camalexin
accumulation was attributed to the activation of CYP71A13 andPAD3 .