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Pattern-triggered immunity and effector-triggered immunity are two primary forms of innate immunity in land plants. The molecular components and connecting nodes of pattern-triggered immunity and effector-triggered immunity are not fully understood. Here, we report that the Arabidopsis calcium-dependent protein kinase CPK3 is a key regulator of both pattern-triggered immunity and effector-triggered immunity. In vitro and in vivo phosphorylation assays, coupled with genetic and cell biology-based analyses, show that actin-depolymerization factor 4 (ADF4) is a physiological substrate of CPK3, and that phosphorylation of ADF4 by CPK3 governs actin cytoskeletal organization associated with pattern-triggered immunity. CPK3 regulates stomatal closure induced by flg22 and is required for resistance to Pst DC3000. Our data further demonstrates that CPK3 is required for resistance to Pst DC3000 carrying the effector AvrPphB. These results suggest that CPK3 is a missing link between cytoskeleton organization, pattern-triggered immunity and effector-triggered immunity.

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Within natural ecosystems, most plants are resistant to most pathogens. At a fundamental level, this seemingly simply truth may hold the key to our understanding of how plants have evolved to survive under a myriad of environmental conditions and their associated stresses. Indeed, in defining how plants evolve, adapt and maintain broad spectrum resistance to most pathogens – typically referred to as non‐host resistance – we may not only reveal the mechanisms that underpin plant resistance signalling but also the precise manner in which plants regulate these processes under various environmental conditions. Herein lies the greatest challenge and unanswered question in the field of agriculture today: How do we feed 9 billion people by the year 2050? To address this, one of the first hurdles that must be overcome is a full understanding of the processes that regulate stress (i.e. abiotic and biotic) signalling in plants, as well as the processes that define pathogen and host specificity, including the performance of these processes under rapidly changing environmental conditions. In the case of pathogen infection, plants utilise a broad suite of innate and inducible mechanisms to resist invasion. In large part, these processes are governed by the activity of resistance (R) proteins, which are evolutionarily conserved and highly evolved proteins that function not only in pathogen recognition but also in the activation of the cellular processes necessary to defend against proliferation and the elicitation of disease. Furthermore, recent data supports the hypothesis that numerous processes, such as the balance between growth and defence, also contribute to the host resistance and pathogen virulence. Key Concepts Most plants are resistant to most pathogens. Modern agriculture practices positively impact crop yield and durability. These practices can also have a negative impact on the unintended selection and enrichment of virulent pathogens. Plants defend against pathogen invasion using a suite of highly conserved resistance ( R ) genes. Pathogens have evolved to recognise and respond to the activity of plant R proteins through the deployment of secreted virulence factors. Immune signalling in plants utilises various pre‐formed and inducible processes to defend against pathogen infection. Many basic physiological processes, including response to light, temperature and water availability, are associated with, and required for, immune signalling. The development of advanced genome sequencing technologies has increased the speed at which the development and selection of elite breeding lines are deployed into cropping systems. Current plant breeding approaches utilise a hybrid of molecular genomics and classical breeding techniques to identify and introduce desirable traits into crops to enhance plant performance (e.g. resistance) and yield.

NON-RACE-SPECIFIC DISEASE RISISTANCE1 (NDR1) is a key component of plant immune signaling, required for defense against the bacterial pathogen Pseudomonas syringae. Plant stress responses have overlapping molecular, physiological, and cell biology signatures, and given the central role of NDR1 during biotic stress perception and signaling, we hypothesized that NDR1 also functions in abiotic stress responses, including in a role that mediates signaling at the plasma membrane (PM) - cell wall (CW) continuum. Here, we demonstrate that NDR1 is required for the induction of drought stress responses in plants, a role that couples stress signaling in an abscisic acid-dependent manner. We show that NDR1 physically associates with the PM-localized H+-ATPases AHA1, AHA2 , and AHA5 and is required for proper regulation of H+-ATPase activity and stomatal guard cell dynamics, providing a mechanistic function of NDR1 during drought responses. In the current study, we demonstrate that NDR1 functions in signaling processes associated with both biotic and abiotic stress response pathways, a function we hypothesize represents NDR1's role in the maintenance of cellular homeostasis during stress. We propose a role for NDR1 as a core transducer of signaling between cell membrane processes and intercellular stress response activation.