4.3 AR156 induces phytoalexin secretion via ABC transporter protein to resist broad-spectrum pathogens
The ATP-binding cassette (ABC) transporter can transport a large number of substrates in Arabidopsis , such as secondary metabolites (Verrier et al., 2008). The PDR subfamily is classified as an ABC transporter subfamily. PEN3 and PDR12 are members of the PDR subfamily (Crouzet et al., 2006; Stein et al.). PEN3 and PDR12 are redundant in function and responsible for transporting camalexin from intracellular to extracellular regions upon B. cinerea infection (He et al., 2019). In the current experiment, we found that AR156 induced up-regulated expression of PEN3 and PDR12 (Figure 2 A, H, and I). Consistent with previous studies, inoculation with PstDC3000 only, pen3-3 showed enhanced sensitivity to PstDC3000 compared to Col-0 (Xin et al., 2013). In line with previous studies, we found that pen3-3/pdr12-2 double mutant showed remarkably less resistance to B. cinerea compared to Col-0,pen3-3 , and pdr12-2 when inoculated with B. cinereaonly (Figure 6 B) (He et al., 2019). PEN3 has been reported to mediate the translocation of numerous substances while participating in the defense of pathogens (Kang et al., 2011; Strader and Bartel, 2009), similarly to PDR12 (Campbell et al., 2003; Kang et al., 2010; Lee et al., 2005). We found that PEN3 is required for AR156 induction of ISR against the three pathogens, while PDR12 acts in AR156 elicitation against fungi and oomycetes, but notPst DC3000 (Figure 6). This may result from the different functions of PEN3 and PDR12 in Arabidopsis responding to different pathogens infections. Further studies are necessary to investigate whether other secondary metabolites are involved in the AR156-induced ISR and how AR156 regulates the transport of different secondary metabolites by PEN3 and PDR12 in response to pathogens with different lifestyles.