H2O2 sulfenylates CHE linking local infection to establishment of systemic acquired resistance

In plants, a local infection can lead to systemic acquired resistance (SAR) through increased production of salicylic acid (SA). For 30 years, the identity of the mobile signal and its direct transduction mechanism for systemic SA synthesis in initiating SAR have been hotly debated. We found that, upon pathogen challenge, the cysteine residue of transcription factor CHE undergoes sulfenylation in systemic tissues, enhancing its binding to the promoter of SA-synthesis gene, ICS1, and increasing SA production. This occurs independently of previously reported pipecolic acid (Pip) signal. Instead, H2O2 produced by NADPH oxidase, RBOHD, is the mobile signal that sulfenylates CHE in a concentration-dependent manner. This modification serves as a molecular switch that activates CHE-mediated SA-increase and subsequent Pip-accumulation in systemic tissues to synergistically induce SAR.

Circadian redox rhythm gates immune-induced cell death distinctly from the genetic clock

Organisms use circadian clocks to synchronize physiological processes to anticipate the Earth’s day-night cycles and regulate responses to environmental stresses to gain competitive advantage ¹ . While divergent genetic clocks have been studied extensively in bacteria, fungi, plants, and animals, a conserved circadian redox rhythm has only recently been reported and hypothesized to be a more ancient clock ^{2, 3} . However, it is controversial whether the redox rhythm serves as an independent clock and controls specific biological processes ⁴ . Here, we uncovered the coexistence of redox and genetic rhythms with distinct period lengths and transcriptional targets through concurrent metabolic and transcriptional time-course measurements in an Arabidopsis long-period clock mutant ⁵ . Analysis of the target genes indicated regulation of the immune-induced programmed cell death (PCD) by the redox rhythm. Moreover, this time-of-day-sensitive PCD was eliminated by redox perturbation and by blocking the signalling pathway of the plant defence hormones jasmonic acid/ethylene, while remaining intact in a genetic-clock-impaired line. We demonstrate that compared to robust genetic clocks, the more sensitive circadian redox rhythm serves as a signalling hub in regulating incidental energy-intensive processes, such as immune-induced PCD ⁶ , to provide organisms a flexible strategy to prevent metabolic overload caused by stress, a unique role for the redox oscillator.

Comprehensive mapping of abiotic stress inputs into the soybean circadian clock

The plant circadian clock evolved to increase fitness by synchronizing physiological processes with environmental oscillations. Crop fitness was artificially selected through domestication and breeding, and the circadian clock was identified by both natural and artificial selections as a key to improved fitness. Despite progress in Arabidopsis, our understanding of the crop circadian clock is still limited, impeding its rational improvement for enhanced fitness. To unveil the interactions between the crop circadian clock and various environmental cues, we comprehensively mapped abiotic stress inputs to the soybean circadian clock using a 2-module discovery pipeline. Using the "molecular timetable" method, we computationally surveyed publicly available abiotic stress-related soybean transcriptomes to identify stresses that have strong impacts on the global rhythm. These findings were then experimentally confirmed using a multiplexed RNA sequencing technology. Specific clock components modulated by each stress were further identified. This comprehensive mapping uncovered inputs to the plant circadian clock such as alkaline stress. Moreover, short-term iron deficiency targeted different clock components in soybean and Arabidopsis and thus had opposite effects on the clocks of these 2 species. Comparing soybean varieties with different iron uptake efficiencies suggests that phase modulation might be a mechanism to alleviate iron deficiency symptoms in soybean. These unique responses in soybean demonstrate the need to directly study crop circadian clocks. Our discovery pipeline may serve as a broadly applicable tool to facilitate these explorations.

Quantification of the humidity effect on HR by Ion leakage assay

We describe a protocol to measure the contribution of humidity on cell death during the effector-triggered immunity (ETI), the plant immune response triggered by the recognition of pathogen effectors by plant resistance genes. This protocol quantifies tissue cell death by measuring ion leakage due to loss of membrane integrity during the hypersensitive response (HR), the ETI-associated cell death. The method is simple and short enough to handle many biological replicates, which improves the power of test of statistical significance. The protocol is easily applicable to other environmental cues, such as light and temperature, or treatment with chemicals.

Daily humidity oscillation regulates the circadian clock to influence plant physiology

Early circadian studies in plants by de Mairan and de Candolle alluded to a regulation of circadian clocks by humidity. However, this regulation has not been described in detail, nor has its influence on physiology been demonstrated. Here we report that, under constant light, circadian humidity oscillation can entrain the plant circadian clock to a period of 24 h probably through the induction of clock genes such as CIRCADIAN CLOCK ASSOCIATED 1. Under simulated natural light and humidity cycles, humidity oscillation increases the amplitude of the circadian clock and further improves plant fitness-related traits. In addition, humidity oscillation enhances effector-triggered immunity at night possibly to counter increased pathogen virulence under high humidity. These results indicate that the humidity oscillation regulates specific circadian outputs besides those co-regulated with the light-dark cycle.

The CAT(2) Comes Back

Genetic and biochemical evidence supporting CATALASE2 as a salicylic acid (SA) receptor has finally emerged. In this issue of Cell Host & Microbe, Yuan et al. (2017) show that SA binds to CATALASE2 to inhibit auxin and jasmonic acid biosynthetic enzymes as a means to strengthen plant immunity against biotrophic pathogens.

Salicylic acid receptors activate jasmonic acid signalling through a non-canonical pathway to promote effector-triggered immunity

It is an apparent conundrum how plants evolved effector-triggered immunity (ETI), involving programmed cell death (PCD), as a major defence mechanism against biotrophic pathogens, because ETI-associated PCD could leave them vulnerable to necrotrophic pathogens that thrive on dead host cells. Interestingly, during ETI, the normally antagonistic defence hormones, salicylic acid (SA) and jasmonic acid (JA) associated with defence against biotrophs and necrotrophs respectively, both accumulate to high levels. In this study, we made the surprising finding that JA is a positive regulator of RPS2-mediated ETI. Early induction of JA-responsive genes and de novo JA synthesis following SA accumulation is activated through the SA receptors NPR3 and NPR4, instead of the JA receptor COI1. We provide evidence that NPR3 and NPR4 may mediate this effect by promoting degradation of the JA transcriptional repressor JAZs. This unique interplay between SA and JA offers a possible explanation of how plants can mount defence against a biotrophic pathogen without becoming vulnerable to necrotrophic pathogens.

Salicylic acid (SA) and jasmonic acid (JA) often act antagonistically in plant defence. Here, Liu et al. show that during effector-triggered immunity (ETI) against Pseudomonas syringae, JA signalling is activated via a non-canonical pathway involving the SA receptors, NPR3 and NPR4, to positively regulate ETI.

Salicylic acid receptors activate jasmonic acid signalling through a non-canonical pathway to promote effector-triggered immunity

It is an apparent conundrum how plants evolved effector-triggered immunity (ETI), involving programmed cell death (PCD), as a major defence mechanism against biotrophic pathogens, because ETI-associated PCD could leave them vulnerable to necrotrophic pathogens that thrive on dead host cells. Interestingly, during ETI, the normally antagonistic defence hormones, salicylic acid (SA) and jasmonic acid (JA) associated with defence against biotrophs and necrotrophs respectively, both accumulate to high levels. In this study, we made the surprising finding that JA is a positive regulator of RPS2-mediated ETI. Early induction of JA-responsive genes and de novo JA synthesis following SA accumulation is activated through the SA receptors NPR3 and NPR4, instead of the JA receptor COI1. We provide evidence that NPR3 and NPR4 may mediate this effect by promoting degradation of the JA transcriptional repressor JAZs. This unique interplay between SA and JA offers a possible explanation of how plants can mount defence against a biotrophic pathogen without becoming vulnerable to necrotrophic pathogens.