The Receptor Kinase IMPAIRED OOMYCETE SUSCEPTIBILITY1 Attenuates Abscisic Acid Responses in Arabidopsis    

Abstract

(Al.A., L.M.V.) In plants, membrane-bound receptor kinases are essential for developmental processes, immune responses to pathogens and the establishment of symbiosis. We previously identified the Arabidopsis (Arabidopsis thaliana) receptor kinase IMPAIRED OOMYCETE SUSCEPTIBILITY1 (IOS1) as required for successful infection with the downy mildew pathogen Hyaloperonospora arabidopsidis. We report here that IOS1 is also required for full susceptibility of Arabidopsis to unrelated (hemi)biotrophic filamentous oomycete and fungal pathogens. Impaired susceptibility in the absence of IOS1 appeared to be independent of plant defense mechanism. Instead, we found that ios1-1 plants were hypersensitive to the plant hormone abscisic acid (ABA), displaying enhanced ABA-mediated inhibition of seed germination, root elongation, and stomatal opening. These findings suggest that IOS1 negatively regulates ABA signaling in Arabidopsis. The expression of ABA-sensitive COLD REGULATED and RESISTANCE TO DESICCATION genes was diminished in Arabidopsis during infection. This effect on ABA signaling was alleviated in the ios1-1 mutant background. Accordingly, ABA-insensitive and ABA-hypersensitive mutants were more susceptible and resistant to oomycete infection, respectively, showing that the intensity of ABA signaling affects the outcome of downy mildew disease. Taken together, our findings suggest that filamentous (hemi)biotrophs attenuate ABA signaling in Arabidopsis during the infection process and that IOS1 participates in this pathogen-mediated reprogramming of the host.

Retraction

“…Consistent with this, Arabidopsis overexpressing IOS1 demonstrated increased accumulation of callose and potentiated expression levels of FRK1 upon MAMP elicitation. By contrast, both FRK1 expression and callose deposition were not affected in the ios1-1 mutant after inoculation with filamentous pathogens (Hok et al, 2014). These discrepancies may be explained by early (this work) versus late time point analyses (Hok et al, 2014).…”

“…IOS1 could be a direct or indirect target of oomycete effectors necessary for pathogen virulence. IOS1 absence in ios1-1 would not allow H. arabidopsidis or P. parasitica to fully repress Arabidopsis PTI (Hok et al, 2014). Taken together, these observations suggest that IOS1 is involved in Arabidopsis immunity to various microbial pathogens.…”

“…Three independent insertion lines in the malectin-like/LRR-RLK IMPAIRED OOMY-CETE SUSCEPTIBILITY1 (IOS1) (Hok et al, 2011) were found to be hypersusceptible to bacterial pathogens. IOS1 is known to contribute to disease caused by filamentous (hemi)biotrophs and to attenuate abscisic acid (ABA) responses in Arabidopsis (Hok et al, 2011(Hok et al, , 2014. Through loss-and gain-of-function analyses and biochemical approaches, we show that IOS1 is an important modulator of Arabidopsis PTI that associates with the LRR-RLKs FLS2, EFR, and BAK1 in a ligand-independent manner, notably controlling the complex formation between FLS2 and BAK1.…”

“…Lines overexpressing IOS1 demonstrated a wild-type resistance to A. brassicicola, suggesting that Arabidopsis resistance to this necrotrophic fungus occurs independently of IOS1. The mutant ios1-1 is known to be more resistant to the filamentous oomycete pathogens Hyaloperonospora arabidopsidis and Phytophthora parasitica (Hok et al, 2011(Hok et al, , 2014. IOS1 could be a direct or indirect target of oomycete effectors necessary for pathogen virulence.…”

“…One of these genes is the Malectin-like/LRR-RLK IOS1 (At1g51800) (Hok et al, 2011(Hok et al, , 2014. For our analyses, we used ios1-1, a transcriptional knockout Ds transposon insertion line in Ler-0 background (GT_5_22250) recently isolated (Hok et al, 2011), and ios1-2 (Salk_137388) and ios1-3 (SAIL_343_B11), two independent T-DNA insertion lines in Col-0 background still producing truncated IOS1 transcripts (Supplemental Figure 1).…”