Infection of plants by necrotizing pathogens or colonization of plant roots with certain beneficial microbes causes the induction of a unique physiological state called "priming." The primed state can also be induced by treatment of plants with various natural and synthetic compounds. Primed plants display either faster, stronger, or both activation of the various cellular defense responses that are induced following attack by either pathogens or insects or in response to abiotic stress. Although the phenomenon has been known for decades, most progress in our understanding of priming has been made over the past few years. Here, we summarize the current knowledge of priming in various induced-resistance phenomena in plants.

A decade-long investigation of nitric oxide (NO) functions in plants has led to its characterization as a biological mediator involved in key physiological processes. Despite the wealth of information gathered from the analysis of its functions, until recently little was known about the mechanisms by which NO exerts its effects. In the past few years, part of the gap has been bridged. NO modulates the activity of proteins through nitrosylation and probably tyrosine nitration. Furthermore, NO can act as a Ca(2+)-mobilizing messenger, and researchers are beginning to unravel the mechanisms underlying the cross talk between NO and Ca(2+). Nonetheless, progress in this area of research is hindered by our ignorance of the pathways for NO production in plants. This review summarizes the basic concepts of NO signaling in animals and discusses new insights into NO enzymatic sources and molecular signaling in plants.

Contents Introduction 250 Increases in free cellular [Ca2+]: an early and key event in plant defence signalling 251 Relationships between [Ca2+]cyt increases and defence‐signalling pathways 256 Calcium targets involved in plant defence reactions 258 Concluding remarks 262 Acknowledgements 263 References 263 Summary In plant cells, the calcium ion is a ubiquitous intracellular second messenger involved in numerous signalling pathways. Variations in the cytosolic concentration of Ca2+ ([Ca2+]cyt) couple a large array of signals and responses. Here we concentrate on calcium signalling in plant defence responses, particularly on the generation of the calcium signal and downstream calcium‐dependent events participating in the establishment of defence responses with special reference to calcium‐binding proteins.

Plant pathogen attacks are perceived through pathogen-issued compounds or plant-derived molecules that elicit defense reactions. Despite the large variety of elicitors, general schemes for cellular elicitor signaling leading to plant resistance can be drawn. In this article, we review early signaling events that happen after elicitor perception, including reversible protein phosphorylations, changes in the activities of plasma membrane proteins, variations in free calcium concentrations in cytosol and nucleus, and production of nitric oxide and active oxygen species. These events occur within the first minutes to a few hours after elicitor perception. One specific elicitor transduction pathway can use a combination or a partial combination of such events which can differ in kinetics and intensity depending on the stimulus. The links between the signaling events allow amplification of the signal transduction and ensure specificity to get appropriate plant defense reactions. This review first describes the early events induced by cryptogein, an elicitor of tobacco defense reactions, in order to give a general scheme for signal transduction that will be use as a thread to review signaling events monitored in different elicitor or plant models.

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