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 .