A novel allele of the Arabidopsis thaliana MACPF protein CAD1 results in deregulated immune signaling

Abstract

Immune recognition in plants is governed by two major classes of receptors: pattern recognition receptors (PRRs) and nucleotide-binding leucine-rich repeat receptors (NLRs). Located at the cell surface, PRRs bind extracellular ligands originating from microbes (indicative of ‘non-self’) or damaged plant cells (indicative of ‘infected-self’), and trigger signaling cascades to protect against infection. Located intracellularly, NLRs sense pathogen-induced physiological changes and trigger localized cell death and systemic resistance. Immune responses are under tight regulation in order to maintain homeostasis and promote plant health. In a forward-genetic screen to identify regulators of PRR-mediated immune signaling, we identified a novel allele of the membrane-attack complex and perforin (MACPF)-motif containing protein CONSTITUTIVE ACTIVE DEFENSE 1 (CAD1) resulting from a missense mutation in a conserved N-terminal cysteine. We show that cad1-5 mutants display deregulated immune signaling and symptoms of autoimmunity dependent on the lipase-like protein ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1), suggesting that CAD1 integrity is monitored by the plant immune system. We further demonstrate that CAD1 localizes to both the cytosol and plasma membrane using confocal microscopy and subcellular fractionation. Our results offer new insights into immune homeostasis and provide tools to further decipher the intriguing role of MACPF proteins in plants.

Plant autoimmunity—fresh insights into an old phenomenon

“…NLR protein might be involved. These comprise a recessive gain-of-function mutation in the CYCLIC NUCLEOTIDE-GATED CHANNEL 20 ( CNGC20 ) gene ( Zhao et al, 2021 ) and a missense mutation in the CONSTITUTIVE ACTIVE DEFENSE 1 ( CAD1 ) gene ( Holmes et al, 2021 ). Interestingly, double mutants of the genes coding for the common ETI-associated microbe-associated molecular pattern (MAMP) co-receptor BAK1 and its paralog BAK1-LIKE 1 (BKK1) exhibit spontaneous cell death reminiscent of autoimmunity.…”

Overexpression of the Arabidopsis MACPF Protein AtMACP2 Promotes Pathogen Resistance by Activating SA Signaling

“…In this study, we demonstrated that macp2 knockout mutants and MACP2-OE s display altered sensitivity to bacterial and fungal pathogens ( Figure 3 and Figure 4 ), suggesting that MACP2 participates in plant immunity responses to external pathogens. Interestingly, the knockout mutant of CAD1 showed similar lesion mimic phenotypes [ 10 , 15 , 36 ], suggesting the activation of immune responses in this knockout mutant. Previous genetic and physiological studies have demonstrated that the cad1-1 mutant is resistant to the virulent bacterial pathogen Pst DC3000 [ 15 ].…”

“…Furthermore, NSL1-mediated PCD triggered by flg22 has been considered a potential PAMP response and is characterized by the accumulation of SA and ROS, which are typical MTI outputs in response to pathogen attacks [ 4 , 45 ]. In contrast, CAD1 has been proposed to induce HR-related cell death by activating NLR signaling [ 36 , 46 ]. In this article, subsequent analysis indicated that EDS1 is downstream of MACP2 to confer plant immune responses ( Figure 3 and Figure 4 ).…”

Non‐seed plants are emerging gene sources for agriculture and insect control proteins

“…These relationships are close enough to suggest HGT events from bacteria into the non‐seed plant lineages. While some proteins in seed plants have MACPF‐like domains, these seed plant proteins appear to be widely diverged in both sequence and function (Fukunaga et al., 2017; Holmes et al., 2021) and may not have a direct common origin with the IPD079‐type MACPF proteins. Unlike for non‐seed plants and bacteria, direct isolation of MACPF‐like insecticidal proteins activity from seed plants is not reported.…”