Small proteins modulate ion-channel-like ACD6 to regulate immunity in Arabidopsis thaliana.

The multi-pass transmembrane protein ACCELERATED CELL DEATH 6 (ACD6) is an immune regulator in Arabidopsis thaliana with an unclear biochemical mode of action. We have identified two loci, MODULATOR OF HYPERACTIVE ACD6 1 (MHA1) and its paralog MHA1-LIKE (MHA1L), that code for ∼7 kDa proteins, which differentially interact with specific ACD6 variants. MHA1L enhances the accumulation of an ACD6 complex, thereby increasing the activity of the ACD6 standard allele for regulating plant growth and defenses. The intracellular ankyrin repeats of ACD6 are structurally similar to those found in mammalian ion channels. Several lines of evidence link increased ACD6 activity to enhanced calcium influx, with MHA1L as a direct regulator of ACD6, indicating that peptide-regulated ion channels are not restricted to animals.

Increasing the resilience of plant immunity to a warming climate.

Extreme weather conditions associated with climate change affect many aspects of plant and animal life, including the response to infectious diseases. Production of salicylic acid (SA), a central plant defence hormone1-3, is particularly vulnerable to suppression by short periods of hot weather above the normal plant growth temperature range via an unknown mechanism4-7. Here we show that suppression of SA production in Arabidopsis thaliana at 28 °C is independent of PHYTOCHROME B8,9 (phyB) and EARLY FLOWERING 310 (ELF3), which regulate thermo-responsive plant growth and development. Instead, we found that formation of GUANYLATE BINDING PROTEIN-LIKE 3 (GBPL3) defence-activated biomolecular condensates11 (GDACs) was reduced at the higher growth temperature. The altered GDAC formation in vivo is linked to impaired recruitment of GBPL3 and SA-associated Mediator subunits to the promoters of CBP60g and SARD1, which encode master immune transcription factors. Unlike many other SA signalling components, including the SA receptor and biosynthetic genes, optimized CBP60g expression was sufficient to broadly restore SA production, basal immunity and effector-triggered immunity at the elevated growth temperature without significant growth trade-offs. CBP60g family transcription factors are widely conserved in plants12. These results have implications for safeguarding the plant immune system as well as understanding the concept of the plant-pathogen-environment disease triangle and the emergence of new disease epidemics in a warming climate.

Crops of the future: building a climate-resilient plant immune system.

A grand challenge facing plant scientists today is to find innovative solutions to increase global crop production in the context of an increasingly warming climate. A major roadblock to global food sufficiency is persistent loss of crops to plant diseases and insect infestations. The United Nations has declared 2020 as the International Year of Plant Health. For historical reasons, molecular studies of plant-biotic interactions in the past several decades have not paid enough attention to how variable climate conditions affect plant-biotic interactions. Here, we highlight a few recent studies that begin to reveal how major climatic drivers impact the plant immune system, particularly secondary messenger and defense hormone signaling, and discuss possible approaches toward engineering climate-resilient plant immunity as part of an ongoing global effort to design 'dream' crops of the future.

Tonoplast-localized Ca2+ pumps regulate Ca2+ signals during pattern-triggered immunity in Arabidopsis thaliana.

One of the major events of early plant immune responses is a rapid influx of Ca2+ into the cytosol following pathogen recognition. Indeed, changes in cytosolic Ca2+ are recognized as ubiquitous elements of cellular signaling networks and are thought to encode stimulus-specific information in their duration, amplitude, and frequency. Despite the wealth of observations showing that the bacterial elicitor peptide flg22 triggers Ca2+ transients, there remain limited data defining the molecular identities of Ca2+ transporters involved in shaping the cellular Ca2+ dynamics during the triggering of the defense response network. However, the autoinhibited Ca2+-ATPase (ACA) pumps that act to expel Ca2+ from the cytosol have been linked to these events, with knockouts in the vacuolar members of this family showing hypersensitive lesion-mimic phenotypes. We have therefore explored how the two tonoplast-localized pumps, ACA4 and ACA11, impact flg22-dependent Ca2+ signaling and related defense responses. The double-knockout aca4/11 exhibited increased basal Ca2+ levels and Ca2+ signals of higher amplitude than wild-type plants. Both the aberrant Ca2+ dynamics and associated defense-related phenotypes could be suppressed by growing the aca4/11 seedlings at elevated temperatures. Relocalization of ACA8 from its normal cellular locale of the plasma membrane to the tonoplast also suppressed the aca4/11 phenotypes but not when a catalytically inactive mutant was used. These observations indicate that regulation of vacuolar Ca2+ sequestration is an integral component of plant immune signaling, but also that the action of tonoplast-localized Ca2+ pumps does not require specific regulatory elements not found in plasma membrane-localized pumps.

Sense and sensibility: the use of fluorescent protein-based genetically encoded biosensors in plants.

Fluorescent protein-based biosensors are providing us with an unprecedented, quantitative view of the dynamic nature of the cellular networks that lie at the heart of plant biology. Such bioreporters can visualize the spatial and temporal kinetics of cellular regulators such as Ca2+ and H+, plant hormones and even allow membrane transport activities to be monitored in real time in living plant cells. The fast pace of their development is making these tools increasingly sensitive and easy to use and the rapidly expanding biosensor toolkit offers great potential for new insights into a wide range of plant regulatory processes. We suggest a checklist of controls that should help avoid some of the more cryptic issues with using these bioreporter technologies.

Systemic signaling in response to wounding and pathogens.

Plants possess systemic signaling networks that allow the perception of local stresses to be translated into plant-wide responses. Although information can be propagated via a variety of molecules such as hormones and RNAs moving within the bulk flow of the phloem or in the transpiration stream, the vasculature also appears to be a major pathway whereby extremely rapid signals move bi-directionally throughout the plant. In these cases, the movement mechanisms are not dependent on redistribution through bulk flow. For example, self-reinforcing systems based around changes in Ca2+ and reactive oxygen species, coupled to parallel electrical signaling events appear able to generate waves of information that can propagate at hundreds of µm/s. These signals then elicit distant responses that prime the plant for a more effective defense or stress response in unchallenged tissues. Although ion channels, Ca2+, reactive oxygen species and associated molecular machineries, such as the NADPH oxidases, have been identified as likely important players in this propagation system, the precise nature of these signaling networks remains to be defined. Critically, whether different stimuli are using the same rapid, systemic signaling network, or whether multiple, parallel pathways for signal propagation are operating to trigger specific systemic outputs remains a key open question.

Rapid, Long-Distance Electrical and Calcium Signaling in Plants.

Plants integrate activities throughout their bodies using long-range signaling systems in which stimuli sensed by just a few cells are translated into mobile signals that can influence the activities in distant tissues. Such signaling can travel at speeds well in excess of millimeters per second and can trigger responses as diverse as changes in transcription and translation levels, posttranslational regulation, alterations in metabolite levels, and even wholesale reprogramming of development. In addition to the use of mobile small molecules and hormones, electrical signals have long been known to propagate throughout the plant. This electrical signaling network has now been linked to waves of Ca(2+) and reactive oxygen species that traverse the plant and trigger systemic responses. Analysis of cell type specificity in signal propagation has revealed the movement of systemic signals through specific cell types, suggesting that a rapid signaling network may be hardwired into the architecture of the plant.

Salt stress-induced Ca2+ waves are associated with rapid, long-distance root-to-shoot signaling in plants.

Their sessile lifestyle means that plants have to be exquisitely sensitive to their environment, integrating many signals to appropriate developmental and physiological responses. Stimuli ranging from wounding and pathogen attack to the distribution of water and nutrients in the soil are frequently presented in a localized manner but responses are often elicited throughout the plant. Such systemic signaling is thought to operate through the redistribution of a host of chemical regulators including peptides, RNAs, ions, metabolites, and hormones. However, there are hints of a much more rapid communication network that has been proposed to involve signals ranging from action and system potentials to reactive oxygen species. We now show that plants also possess a rapid stress signaling system based on Ca(2+) waves that propagate through the plant at rates of up to ∼ 400 µm/s. In the case of local salt stress to the Arabidopsis thaliana root, Ca(2+) wave propagation is channeled through the cortex and endodermal cell layers and this movement is dependent on the vacuolar ion channel TPC1. We also provide evidence that the Ca(2+) wave/TPC1 system likely elicits systemic molecular responses in target organs and may contribute to whole-plant stress tolerance. These results suggest that, although plants do not have a nervous system, they do possess a sensory network that uses ion fluxes moving through defined cell types to rapidly transmit information between distant sites within the organism.