Perception of the novel MAMP eMax from different Xanthomonas species requires the Arabidopsis receptor-like protein ReMAX and the receptor kinase SOBIR

Abstract

As part of their innate immune system plants carry a number of pattern recognition receptors (PRRs) that can detect a broad range of microbe-associated molecular patterns (MAMPs). In a recently published article (1) we described a novel, proteinaceous MAMP termed eMax (enigmatic MAMP of Xanthomonas) that derives from Xanthomonas and gets recognized by the receptor-like protein ReMAX (RECEPTOR OF eMax) of Arabidopsis thaliana. ReMAX has no ortholog in Nicotiana benthamiana and this species does not respond to eMax even when transformed with ReMAX. However, interfamily transfer of eMax perception was successful with a chimeric form of ReMAX where the C-terminal part of the protein was replaced by the corresponding part of the tomato RLP EIX2 (ETHYLENE INDUCING XYLANASE2). In this addendum we describe the difficulties with the purification and identification of the MAMP eMax and we present data demonstrating that functionality of ReMAX, much like that of related RLPs, depends on the presence of the receptor kinase SOBIR (SUPPRESSOR OF BIR1-1).

Keywords: Arabidopsis thaliana; Xanthomonas; microbe-associated molecular pattern; pattern recognition receptor; plant innate immunity.

Figure 1. Testing eMax for contamination with flagellin. Extracellular alkalinization in cell cultures of A. thaliana wild‐type cells to test the response to flg22 and eMax in the presence or absence of the flagellin antagonist flg22‐Δ2.

Figure 2. Ethylene biosynthesis in response to different Xanthomonas strains. Col-0 wild-type and the mutants rlp1–2, fls2 efr and fls2 efr rlp1–2 were treated with extracts of different Xanthomonas strains and the fungal extract Pen (90 µg/ml) as positive control (Xac: Xanthomonas axonopodis pv citri, Xcc: Xanthomonas campestris pv campestris, Xcj: Xanthomonas campestris pv juglandis, Xcm: Xanthomonas campestris pv malvacearum). The bars and error bars represent means and standard deviations of three replicates.

Figure 3.sobir mutants lack responsiveness to eMax. Ethylene biosynthesis in response to eMax (2 µg/ml) and Pen (90 µg/ml) in leaves from Col-0 wild-type and the mutants fls2 efr rlp1–2 and sobir1–12 (SALK_050715¹⁹). The bars and error bars represent means and standard deviations of three replicates.