Plant Defense Signaling Mechanisms and Evolution

Abstract

Plant innate immunity has been classified into two layers of defense systems. Nucleotide-binding domain leucine-rich repeat (NLR) protein complexes activated by pathogen effectors launches effector-triggered immunity (ETI). A forward genetic screen using the ETI marker gene WRKY46 promoter fused with a firefly luciferase gene (pWRKY46::LUC) as a reporter identified five ARABIDOPSIS GENES GOVERNING IMMUNE GENE EXPRESSION (aggie) mutants, named as aggie4-8. Though with elevated pWRKY46::LUC activity, aggie4 showed enhanced resistance to avirulent bacterial infections, yet delayed hypersensitive response (HR), while aggie5 exhibited compromised disease resistance and delayed HR. Map-based cloning coupled with next generation sequencing (NGS) suggests that causal mutations of aggie4 and aggie5 locate in different regions of chromosome 5. In addition, pWRKY46::LUC activity is elevated in aggie6 and aggie7, while suppressed in aggie8.

Perception of microbe-associated molecular patterns (MAMPs) by pattern recognition receptors (PRRs) triggers another tier of innate immunity, termed as pattern-triggered immunity (PTI). The Arabidopsis SOMATIC EMBRYOGENESIS RECEPTOR KINASE (SERK) family plays important roles in plant defense responses. It remains unknown how SERK members have evolved. Here, three SERK homologs, Pp1s35_219V6.1, Pp1s96_90V6.1 and Pp1s118_79V6.1 were identified in P. Patens with 60%-80% amino acid identify to AtSERKs and were named as PpSERK1.1, PpSERK1.2 and PpSERK2 respectively. In vitro kinase assay revealed that PpSERK1.1 and PpSERK1.2, but not PpSERK2, possess strong kinase activity. Functional complementation analysis suggested that PpSERK1.1 and PpSERK1.2 but not PpSERK2 regulate FLS2-mediated plant defense, and PpSERK1.2 but not PpSERK1.1 also regulates cell death in Arabidopsis.

PTI induces a rapid and profound transcriptional reprograming via concerted actions of specific transcription factors and general transcription machinery. Arabidopsis SH4-related 3 (ASR3), a plant specific trihelix family of transcription factors, is rapidly phosphorylated by MPK4 at Threonine 189 residue upon multiple MAMP treatments but not upon elicitation of ETI. Genetic and biochemical data suggests that phosphorylation of ASR3 enhances its DNA binding activity to suppress immune genes expression. Importantly, the asr3 mutant shows higher immune gene activation and enhanced disease resistance, while transgenic plants overexpressing ASR3 exhibit compromised PTI responses. This study reveals that ASR3 functions as a transcription repressor regulated by MPK4 to fine-tune plant defense.