The Role of Nitric Oxide-Induced ATILL6 in Growth and Disease Resistance in Arabidopsis thaliana

Nitric oxide (NO) is a signaling molecule that regulates various processes, including plant growth and development, immunity, and environmental interactions. Using high throughput RNA-seq data, we explored the role of the NO-induced ATILL6 gene in plant growth and defense using functional genomics. The atill6 mutant and wild-types were challenged with either oxidative (H2O2, MV) or nitro-oxidative (CySNO, GSNO) stress conditions, and the phenotypic results showed that ATILL6 gene differentially regulates cotyledon development frequency (CDF) as well as the root and shoot lengths of the plants. To investigate whether ATILL6 plays a role in plant basal or resistance (R)-gene-mediated defense, the plants were challenged with either virulent or avirulent strains of Pseudomonas syringae pathovar tomato (Pst) DC3000. The atill6 line showed a susceptible phenotype, higher pathogen growth, and highly reduced transcript accumulation of PR1 and PR2 genes. These results suggested that ATILL6 positively regulates plant basal defense. Furthermore, after the inoculation of atill6 with avirulent Pst (DC3000), the expressions of the PR1 and PR2 genes decreased, suggesting a positive role in R-gene-mediated resistance in protecting the plant from further spread of disease. We also investigated the role of ATILL6 in systemic acquired resistance (SAR), and the results showed that ATILL6 positively regulates SAR, as the mutant line atill6 has significantly (p ≤ 0.05) lower transcript accumulation of PR, G3DPH, and AZI genes. Overall, these results indicate that the NO-induced ATILL6 gene differentially regulates plant growth and positively regulates plant basal defense, R-gene-mediated resistance, and SAR.

Salicylic Acid: Biosynthesis and Signaling

Salicylic acid (SA) is an essential plant defense hormone that promotes immunity against biotrophic and semibiotrophic pathogens. It plays crucial roles in basal defense and the amplification of local immune responses, as well as the establishment of systemic acquired resistance. During the past three decades, immense progress has been made in understanding the biosynthesis, homeostasis, perception, and functions of SA. This review summarizes the current knowledge regarding SA in plant immunity and other biological processes. We highlight recent breakthroughs that substantially advanced our understanding of how SA is biosynthesized from isochorismate, how it is perceived, and how SA receptors regulate different aspects of plant immunity. Some key questions in SA biosynthesis and signaling, such as how SA is produced via another intermediate benzoic acid and how SA affects the activities of its receptors in the transcriptional regulation of defense genes, remain to be addressed. Expected final online publication date for the Annual Review of Plant Biology, Volume 72 is May 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

A gain of function mutation in SlNRC4a enhances basal immunity resulting in broad-spectrum disease resistance

Plants rely on innate immunity to perceive and ward off microbes and pests, and are able to overcome the majority of invading microorganisms. Even so, specialized pathogens overcome plant defenses, posing a persistent threat to crop and food security worldwide, raising the need for agricultural products with broad, efficient resistance. Here we report a specific mutation in a tomato (S. lycopersicum) helper nucleotide-binding domain leucine-rich repeat H-NLR, SlNRC4a, which results in gain of function constitutive basal defense activation, in absence of PRR activation. Knockout of the entire NRC4 clade in tomato was reported to compromise Rpi-blb2 mediated immunity. The SlNRC4a mutant reported here possesses enhanced immunity and disease resistance to a broad-spectrum of pathogenic fungi, bacteria and pests, while lacking auto-activated HR or negative effects on plant growth and crop yield, providing promising prospects for agricultural adaptation in the war against plant pathogens that decrease productivity.

WRKY10 transcriptional regulatory cascades in rice are involved in basal defense and Xa1-mediated resistance

WRKY proteins play essential roles as negative or positive regulators of pathogen defense. This study explored the roles of different OsWRKY proteins in basal defense and Xa1-mediated resistance to Xanthomonas oryzae pv. oryzae (Xoo) infection in rice. Assays of disease in OsWRKY10KD and OsWRKY88KD lines following infection with an incompatible Xoo race, which induced Xa1-mediated resistance in wild-type plants, showed that OsWRKY10 and OsWRKY88 were positive regulators of Xa1-mediated resistance. OsWRKY10 also acted as a positive regulator in basal defense by directly or indirectly activating transcription of defense-related genes. OsWRKY10 activated the OsPR1a promoter by binding to specific WRKY binding sites. Two transcriptional regulatory cascades of OsWRKY10 were identified in basal defense and Xa1-mediated resistance. In the first transcriptional regulatory cascade, OsWRKY47 acted downstream of OsWRKY10 whereas OsWRKY51 acted upstream. OsWRKY10 activated OsPR1a in two distinct ways: by binding to its promoter and, at the same time, by indirect activation through OsWRKY47. In the second transcriptional regulatory cascade, OsWRKY47 acted downstream of OsWRKY10, and OsWRKY88 acted upstream. These OsWRKY10 transcriptional regulatory cascades played important roles in basal defense and Xa1-mediated resistance to enable the mounting of a rapid immune response against pathogens.

Seed Priming with Jasmonic Acid Counteracts Root Knot Nematode Infection in Tomato by Modulating the Activity and Expression of Antioxidative Enzymes

The environmental stress, biotic as well as abiotic, is the main cause of decreased growth and crop production. One of the stress-causing agents in plants are parasitic nematodes responsible for crop loss. Jasmonic acid (JA) is recognized as one of signaling molecules in defense-related responses in plants, however, its role under nematode infestation is unclear. Therefore, the present study was planned to traverse the role of JA in boosting the activities of antioxidative enzymes in tomato seedlings during nematode inoculation. Application of JA declined oxidative damage by decreasing O2•− content, nuclear and membrane damage under nematode stress. JA treatment elevated the activities of SOD, POD, CAT, APOX, DHAR, GPOX, GR, and PPO in nematode-infested seedlings. Seed soaking treatment of JA upregulated the expression of SOD, POD, CAT, and GPOX under nematode stress. Various amino acids were found in tomato seedlings and higher content of aspartic acid, histidine, asparagine, glutamine, glutamic acid, glycine, threonine, lysine, arginine, B-alanine, GABA, phenylalanine, proline, and ornithine was observed in seeds soaked with JA (100 nM) treatment during nematode inoculation. The results suggest an indispensable role of JA in basal defense response in plants during nematode stress.

A coevolved EDS1-SAG101-NRG1 module mediates cell death signaling by TIR-domain immune receptors

Plant intracellular nucleotide-binding/leucine-rich repeat (NLR) immune receptors are activated by pathogen effectors to trigger host defenses and cell death. Toll-Interleukin1-receptor (TIR)-domain NLRs (TNLs) converge on the Enhanced Disease Susceptibility1 (EDS1) family of lipase-like proteins for all resistance outputs. In Arabidopsis TNL immunity,AtEDS1 heterodimers with Phytoalexin Deficient4 (AtPAD4) transcriptionally boost basal defense pathways.AtEDS1 uses the same surface to interact with PAD4-related Senescence-Associated Gene101 (AtSAG101), but the role ofAtEDS1-AtSAG101 heterodimers was unclear. We show thatAtEDS1-AtSAG101 function together withAtNRG1 coiled-coil domain helper NLRs as a coevolved TNL cell death signaling module.AtEDS1-AtSAG101-AtNRG1 cell death activity is transferable to the solanaceous species,Nicotiana benthamiana, and cannot be substituted byAtEDS1-AtPAD4 withAtNRG1 orAtEDS1-AtSAG101 with endogenousNbNRG1. Analysis of EDS1-family evolutionary rate variation and heterodimer structure-guided phenotyping ofAtEDS1 variants orAtPAD4-AtSAG101 chimeras identify closely aligned α-helical coil surfaces in theAtEDS1-AtSAG101 partner C-terminal domains that are necessary for TNL cell death signaling. Our data suggest that TNL-triggered cell death and pathogen growth restriction are determined by distinctive features of EDS1-SAG101 and EDS1-PAD4 complexes and that these signaling machineries coevolved with further components within plant species or clades to regulate downstream pathways in TNL immunity.

Multiple quantitative trait loci contribute tolerance to bacterial canker incited by Pseudomonas syringae pv. actinidiae in kiwifruit (Actinidia chinensis)

Pseudomonas syringae pv. actinidiae (Psa) Biovar 3, a virulent, canker-inducing pathogen is an economic threat to the kiwifruit (Actinidia spp.) industry worldwide. The commercially grown diploid (2x) A. chinensis var. chinensis is more susceptible to Psa than tetraploid and hexaploid kiwifruit. However information on the genetic loci modulating Psa resistance in kiwifruit is not available. Here we report mapping of quantitative trait loci (QTLs) regulating tolerance to Psa in a diploid kiwifruit population, derived from a cross between an elite Psa-susceptible ‘Hort16A’ and a tolerant male breeding parent P1. Using high-density genetic maps and intensive phenotyping, we identified a single QTL for Psa tolerance on Linkage Group (LG) 27 of ‘Hort16A’ revealing 16-19% phenotypic variance and candidate alleles for susceptibility and tolerance at this loci. In addition, six minor QTLs were identified in P1 on distinct LGs, exerting 4-9% variance. Complete tolerance in the F1 population is attained by additive effects from ‘Hort16A’ and P1 QTLs providing evidence that divergent genetic pathways fend-off virulent Psa strain. Two different bioassays further identified new QTLs for tissue-specific responses to Psa. Transcriptome analysis of Psa-tolerant and susceptible genotypes in field revealed hallmarks of basal defense and provided candidate RNA-biomarkers for screening Psa tolerance.