Maize nicotinate N-methyltransferase interacts with the NLR protein Rp1-D21 and modulates the hypersensitive response.

Most plant intracellular immune receptors belong to nucleotide-binding, leucine-rich repeat (NLR) proteins. The recognition between NLRs and their corresponding pathogen effectors often triggers a hypersensitive response (HR) at the pathogen infection sites. The nicotinate N-methyltransferase (NANMT) is responsible for the conversion of nicotinate to trigonelline in plants. However, the role of NANMT in plant defence response is unknown. In this study, we demonstrated that the maize ZmNANMT, but not its close homolog ZmCOMT, an enzyme in the lignin biosynthesis pathway, suppresses the HR mediated by the autoactive NLR protein Rp1-D21 and its N-terminal coiled-coil signalling domain (CCD21 ). ZmNANMT, but not ZmCOMT, interacts with CCD21 , and they form a complex with HCT1806 and CCoAOMT2, two key enzymes in lignin biosynthesis, which can also suppress the autoactive HR mediated by Rp1-D21. ZmNANMT is mainly localized in the cytoplasm and nucleus, and either localization is important for suppressing the HR phenotype. These results lay the foundation for further elucidating the molecular mechanism of NANMTs in plant disease resistance.

Genome-wide identification of the maize 2OGD superfamily genes and their response to Fusarium verticillioides and Fusarium graminearum.

In maize, eat rot and stalk rot caused by Fusarium verticillioides and Fusarium graminearum lead to contamination of moldy grains to produce mycotoxins. Identification of resistance genes against these pathogens for maize breeding is an effective way for disease control. Several 2-oxoglutarate-dependent dioxygenase (2OGD) proteins have been found to confer resistance to different pathogens in diverse plant species. However, little is known about the 2OGD superfamily in maize. Here, we identified 103 putative 2OGD genes in maize from a genome-wide analysis, and divided them into three classes - DOXA, DOXB, and DOXC. We further comprehensively investigated their gene structure, chromosome distribution, phylogenetic tree, gene-function enrichment, and expression profiles among different tissues. The genes encoding three 2OGD proteins, ACO, F3H, and NCS involved in ethylene biosynthesis, flavonoids biosynthesis, and alkaloids biosynthesis pathways, respectively, were identified to be induced by F. verticillioides and F. graminearum. The promoters of the three genes contain the binding sites for the transcription factor ZmDOF and ZmHSF, which are also induced by the two pathogens. The results imply that the three 2OGDs and the two transcription factors might be involved in the resistance to the two pathogens. This study provided a comprehensive understanding of the 2OGD superfamily in maize and laid the foundation for the further functional analysis of their roles in maize resistance to eat rot and stalk rot.

Maize metacaspases modulate the defense response mediated by the NLR protein Rp1-D21 likely by affecting its subcellular localization.

Plants usually employ resistance (R) genes to defend against the infection of pathogens, and most R genes encode intracellular nucleotide-binding, leucine-rich repeat (NLR) proteins. The recognition between R proteins and their cognate pathogens often triggers a rapid localized cell death at the pathogen infection sites, termed the hypersensitive response (HR). Metacaspases (MCs) belong to a cysteine protease family, structurally related to metazoan caspases. MCs play crucial roles in plant immunity. However, the underlying molecular mechanism and the link between MCs and NLR-mediated HR are not clear. In this study, we systematically investigated the MC gene family in maize and identified 11 ZmMCs belonging to two types. Further functional analysis showed that the type I ZmMC1 and ZmMC2, but not the type II ZmMC9, suppress the HR-inducing activity of the autoactive NLR protein Rp1-D21 and of its N-terminal coiled-coil (CCD21 ) signaling domain when transiently expressed in Nicotiana benthamiana. ZmMC1 and ZmMC2 physically associate with CCD21 in vivo. We further showed that ZmMC1 and ZmMC2, but not ZmMC9, are predominantly localized in a punctate distribution in both N. benthamiana and maize (Zea mays) protoplasts. Furthermore, the co-expression of ZmMC1 and ZmMC2 with Rp1-D21 and CCD21 causes their re-distribution from being uniformly distributed in the nucleocytoplasm to a punctate distribution co-localizing with ZmMC1 and ZmMC2. We reveal a novel role of plant MCs in modulating the NLR-mediated defense response and derive a model to explain it.

Fine-Tuning Immunity: Players and Regulators for Plant NLRs.

Plants have evolved a sophisticated innate immune system to defend against pathogen infection, and intracellular nucleotide-binding, leucine-rich repeat (NLR or NB-LRR) immune receptors are one of the main components of this system. NLR activity is fine-tuned by intra- and intermolecular interactions. We survey what is known about the conservation and diversity of NLR-interacting proteins, and divide them into seven major categories. We discuss the molecular mechanisms by which NLR activities are regulated and how understanding this regulation has potential to facilitate the engineering of NLRs for crop improvement.

Maize ZmFNSI Homologs Interact with an NLR Protein to Modulate Hypersensitive Response.

Nucleotide binding, leucine-rich-repeat (NLR) proteins are the major class of resistance (R) proteins used by plants to defend against pathogen infection. The recognition between NLRs and their cognate pathogen effectors usually triggers a rapid localized cell death, termed the hypersensitive response (HR). Flavone synthase I (FNSI) is one of the key enzymes in the flavone biosynthesis pathway. It also displays salicylic acid (SA) 5-hydroxylase (S5H) activity. A close homolog of FNSI/S5H displays SA 3-hydroxylase (S3H) activity. Both FNSI/S5H and S3H play important roles in plant innate immunity. However, the underlying molecular mechanisms and the relationship between S5H and S3H with the NLR-mediated HR are not known in any plant species. In this study, we identified three genes encoding ZmFNSI-1, ZmFNSI-2 and ZmS3H that are significantly upregulated in a maize line carrying an autoactive NLR Rp1-D21 mutant. Functional analysis showed that ZmFNSI-1 and ZmFNSI-2, but not ZmS3H, suppressed HR conferred by Rp1-D21 and its signaling domain CCD21 when transiently expressed in N. benthamiana. ZmFNSI-1 and ZmFNSI-2 physically interacted with CCD21. Furthermore, ZmFNSI-1 and ZmFNSI-2 interacted with HCT, a key enzyme in lignin biosynthesis pathway, which can also suppress Rp1-D21-mediated HR. These results lay the foundation for the further functional analysis of the roles of FNSI in plant innate immunity.