Chloroplasts serve as cold priming hubs modulating the transcriptional response of Arabidopsis thaliana to a second cold stimulus for several days by post-cold accumulation of thylakoid ascorbate peroxidases (tAPX). In an attempt to investigate cross-priming effects of cold on plant pathogen protection, we show here that such a single 24 h cold treatment at 4°C decreased the susceptibility of Arabidopsis to virulent Pseudomonas syringae pv. tomato DC3000 (Pst), but did not alter resistance against the avirulent strains Pst avrRPM1 and Pst avrRPS4 or the effector-deficient Pst hrcC- strain. The effect of cold priming against Pst was active immediately after cold exposure and memorized for at least 5 days. The priming benefit was established independent of the immune regulator EDS1 (Enhanced Disease Susceptibility 1) or activation of the immune-related genes NHL10, FRK1, ICS1 and PR1, but required thylakoid-bound as well as stromal ascorbate peroxidase activities as the effect was absent or weak in corresponding knock-out-lines. Suppression of tAPX post-cold regulation in a conditional-inducible tAPX-RNAi line lead to increased bacterial growth numbers. This highlights that the plant immune system benefits from post-cold regeneration of the protective chloroplast peroxidase system.

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Short title: Cold priming memory reduces plant susceptibility 15 16 One-sentence summary: 24 hour cold exposure reduces plant susceptibility against virulent 17 pathogens dependent on chloroplast ascorbate peroxidases. 18 19 2 ABSTRACT 28 29Chloroplasts, as recently shown, serve as cold priming hubs in modulating the transcriptional 30 response of Arabidopsis thaliana to a second cold stimulus after a stress-free interphase of 31 several days. Here, we studied if such a single 24 h cold stress at 4 °C also alters the 32 susceptibility of Arabidopsis to virulent Pseudomonas syringae pv. tomato DC3000. Our data 33show that cold priming did not only increase resistance of Arabidopsis to a subsequent infection 34 immediately, but also after a memory phase of 5 days, during which thylakoid ascorbate 35 peroxidases accumulate. Contrasting to susceptibility, the prior cold exposure did not alter 36 resistance against avirulent and effector-triggered immunity-inducing Pseudomonas syringae 37 strains. The effect of cold priming on the plant pathogen susceptibility was independent of the 38 central nucleo-cytoplasmic immune regulator EDS1 (Enhanced Disease Susceptibility 1) and 39 uncoupled from classical immune gene activation. The priming benefits against pathogens 40 required thylakoid and stromal ascorbate peroxidase activity. Combinatorial priming of 41 Arabidopsis pathogen susceptibility by metastable regulation of stromal ascorbate peroxidase 42 activity and post-cold expression of thylakoid ascorbate peroxidase guarantees immediate 43 protection without latency time and prolonged protection by the memory element that regulates 44 future cold responses. 45 47Plants have evolved sophisticated molecular networks that respond differently to simultaneous 48 or sequentially experienced stress events than to single stress situations (Zhang and 49 Sonnewald, 2017; Saijo and Loo, 2019). The concept of priming describes a combination of two 50 sequential and transient stress situations in which the exposure to a prior stress leads to earlier, 51 faster, stronger, and/or more specific responses during the subsequent triggering stress (Crisp 52 et al., 2016; Hilker et al., 2016). Although plants lack a nervous system and an antibody-based 53 adaptive immune system, the plant capacity for a (stress) memory is evident and well described 54 (Conrath, 2011; Crisp et al., 2016; Hilker et al., 2016; Gourbal et al., 2018). The molecular 55 priming memory is formed, while primary stress responses are lost during a stress-free 56 interphase (lag or memory phase), and subsequently modifies the response to a later triggering 57 stimulus (Hilker et al., 2016). The priming memory can result of chromatin modifications, but 58 also be imprinted by preparatory formation or persistence of key signalling metabolites and 59 proteins which are kept in an inactive form during the stress-free memory phases (Conrath, 60 2011; Crisp et al., 2016; Baier et al., 2019). 61The priming and the subsequent stress events can be of same nature (cis-p...

Plant acclimation to low temperatures occurs through system-wide mechanisms that include proteome shifts, some of which occur at the level of protein synthesis. All proteins are synthesised by ribosomes. Rather than being monolithic, transcript-to-protein translation machines, ribosomes can be selective and cause effective proteome shifts required for successful temperature acclimation. Here, we use apical root meristems of germinating seedlings of the monocotyledonous plant barley as a model to study changes in protein abundance and synthesis rates during cold acclimation. We measure metabolic and physiological parameters that allow us to compare protein synthesis rates in different physiological states, e.g., in cold acclimation compared to the optimal temperature state. We show that ribosomal proteins are independently synthesised and assembled into ribosomal complexes in root proliferative tissue, and assess how the ribo-proteome shifts during cold may be associated with changes in synthesis and accumulation of macromolecular complexes. We demonstrate that translation initiation is the limiting step during cold acclimation and based on our data propose a model of a ribosomal code that depends on a reconfigured ribosome population, where as a mode of cold acclimation, specific ribosomal protein compositions may confer selective association capabilities between 60S subunits and 48S initiation complexes.