Beneficial microbes in the microbiome of plant roots improve plant health. Induced systemic resistance (ISR) emerged as an important mechanism by which selected plant growth–promoting bacteria and fungi in the rhizosphere prime the whole plant body for enhanced defense against a broad range of pathogens and insect herbivores. A wide variety of root-associated mutualists, including Pseudomonas, Bacillus, Trichoderma, and mycorrhiza species sensitize the plant immune system for enhanced defense without directly activating costly defenses. This review focuses on molecular processes at the interface between plant roots and ISR-eliciting mutualists, and on the progress in our understanding of ISR signaling and systemic defense priming. The central role of the root-specific transcription factor MYB72 in the onset of ISR and the role of phytohormones and defense regulatory proteins in the expression of ISR in aboveground plant parts are highlighted. Finally, the ecological function of ISR-inducing microbes in the root microbiome is discussed.

/pdf/induced-systemic-resistance-by-beneficial-microbes-9uemo31ql5.pdf

Induced systemic resistance by beneficial microbes

Jasmonates: biosynthesis, perception, signal transduction and action in plant stress response, growth and development. An update to the 2007 review in Annals of Botany

Systemic acquired resistance (SAR) is an induced immune mechanism in plants. Unlike vertebrate adaptive immunity, SAR is broad spectrum, with no specificity to the initial infection. An avirulent pathogen causing local programmed cell death can induce SAR through generation of mobile signals, accumulation of the defense hormone salicylic acid, and secretion of the antimicrobial PR (pathogenesis-related) proteins. Consequently, the rest of the plant is protected from secondary infection for a period of weeks to months. SAR can even be passed on to progeny through epigenetic regulation. The Arabidopsis NPR1 (nonexpresser of PR genes 1) protein is a master regulator of SAR. Recent study has shown that salicylic acid directly binds to the NPR1 adaptor proteins NPR3 and NPR4, regulates their interactions with NPR1, and controls NPR1 protein stability. However, how NPR1 interacts with TGA transcription factors to activate defense gene expression is still not well understood. In addition, redox regulators, the m...

Systemic Acquired Resistance: Turning Local Infection into Global Defense

https://www.cell.com/molecular-plant/pdf/S1674-2052(14)60932-1.pdf

Growth-defense tradeoffs in plants: a balancing act to optimize fitness.

The coordinated expression of highly related homoeologous genes in polyploid species underlies the phenotypes of many of the world's major crops. Here we combine extensive gene expression datasets to produce a comprehensive, genome-wide analysis of homoeolog expression patterns in hexaploid bread wheat. Bias in homoeolog expression varies between tissues, with ~30% of wheat homoeologs showing nonbalanced expression. We found expression asymmetries along wheat chromosomes, with homoeologs showing the largest inter-tissue, inter-cultivar, and coding sequence variation, most often located in high-recombination distal ends of chromosomes. These transcriptionally dynamic genes potentially represent the first steps toward neo- or subfunctionalization of wheat homoeologs. Coexpression networks reveal extensive coordination of homoeologs throughout development and, alongside a detailed expression atlas, provide a framework to target candidate genes underpinning agronomic traits in wheat.

/pdf/the-transcriptional-landscape-of-polyploid-wheat-51ipbhfwws.pdf

The transcriptional landscape of polyploid wheat

Salicylic acid (SA) is a plant immune signal produced after pathogen challenge to induce systemic acquired resistance. It is the only major plant hormone for which the receptor has not been firmly identified. Systemic acquired resistance in Arabidopsis requires the transcription cofactor nonexpresser of PR genes 1 (NPR1), the degradation of which acts as a molecular switch. Here we show that the NPR1 paralogues NPR3 and NPR4 are SA receptors that bind SA with different affinities. NPR3 and NPR4 function as adaptors of the Cullin 3 ubiquitin E3 ligase to mediate NPR1 degradation in an SA-regulated manner. Accordingly, the Arabidopsis npr3 npr4 double mutant accumulates higher levels of NPR1, and is insensitive to induction of systemic acquired resistance. Moreover, this mutant is defective in pathogen effector-triggered programmed cell death and immunity. Our study reveals the mechanism of SA perception in determining cell death and survival in response to pathogen challenge. Plant resistance to pathogen challenge is thought to be mediated through salicylic acid (SA) signalling; here NPR3 and NPR4, paralogues of the transcription cofactor NPR1, are identified as receptors of SA. Salicylic acid is the only major plant hormone for which a receptor has not been firmly identified. It is produced in plants in response to pathogen challenge, and induces systemic acquired resistance against secondary infection. This process requires the transcription cofactor NPR1, which indicated that NPR1 might be a salicylic acid receptor, but NPR1 alone does not bind to the hormone. Here, Xinnian Dong and colleagues identify the NPR1 paralogues NPR3 and NPR4 as salicylic acid receptors with different binding affinities. The authors propose a model for the regulation of NPR1 by NPR3 and NPR4 in response to different levels of salicylic acid.

/pdf/npr3-and-npr4-are-receptors-for-the-immune-signal-salicylic-44vkoqe0db.pdf

NPR3 and NPR4 are receptors for the immune signal salicylic acid in plants

The HSF-like transcription factor TBF1 is a major molecular switch for plant growth-to-defense transition.

Howard Hughes Medical Institute, Duke University,Durham, NC 27708, USASummaryBackground: Induction of plant immune responses involvessigniﬁcant transcription reprogramming that prioritizesdefense over growth-related cellular functions. Despite inten-sive forward genetic screens and genome-wide expression-proﬁling studies, a limited number of transcription factorshave been found that regulate this transition.Results: Using the endoplasmic-reticulum-resident genesrequired for antimicrobial protein secretion as markers, weidentiﬁed a heat-shock factor-like transcription factor thatspeciﬁcally binds to the TL1 (GAAGAAGAA) cis elementrequired for the induction of these genes. Surprisingly, plantslacking this TL1-binding factor, TBF1, respond normally toheat stress but are compromised in immune responsesinduced by salicylic acid and by microbe-associated molec-ular pattern, elf18. Genome-wide expression proﬁling indi-cates that TBF1 plays a key role in the growth-to-defensetransition. Moreover, the expression of TBF1 itself is tightlyregulated at both the transcriptional and translational levels.Two upstream open reading frames encoding multiplearomatic amino acidswere found 5

https://www.cell.com/current-biology/pdf/S0960-9822(11)01390-X.pdf

Article The HSF-like Transcription Factor TBF1 Is a Major Molecular Switch for Plant Growth-to-Defense Transition

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NPR3 and NPR4 are receptors for the immune signal salicylic acid in plants

The HSF-like transcription factor TBF1 is a major molecular switch for plant growth-to-defense transition.

Article The HSF-like Transcription Factor TBF1 Is a Major Molecular Switch for Plant Growth-to-Defense Transition