Impaired defense reactions in apple replant disease-affected roots of Malus domestica ‘M26’

Plants are hosts to a wide array of pathogens from all kingdoms of life. In the absence of an active immune system or combinatorial diversifications that lead to recombination-driven somatic gene flexibility, plants have evolved different strategies to combat both individual pathogen strains and changing pathogen populations. The receptor-like kinase (RLK) gene-family expansion in plants was hypothesized to have allowed accelerated evolution among domains implicated in signal reception, typically a leucine-rich repeat (LRR). Under that model, the gene-family expansion represents a plant-specific adaptation that leads to the production of numerous and variable cell surface and cytoplasmic receptors. More recently, it has emerged that the LRR domains of RLK interact with a diverse group of proteins leading to combinatorial variations in signal response specificity. Therefore, the RLK appear to play a central role in signaling during pathogen recognition, the subsequent activation of plant defense mechanisms, and developmental control. The future challenges will include determinations of RLK modes of action, the basis of recognition and specificity, which cellular responses each receptor mediates, and how both receptor and kinase domain interactions fit into the defense signaling cascades. These challenges will be complicated by the limited information that may be derived from the primary sequence of the LRR domain. The review focuses upon implications derived from recent studies of the secondary and tertiary structures of several plant RLK that change understanding of plant receptor function and signaling. In addition, the biological functions of plant and animal RLK-containing receptors were reviewed and commonalities among their signaling mechanisms identified. Further elucidated were the genomic and structural organizations of RLK gene families, with special emphasis on RLK implicated in resistance to disease and development.

Plant acclimation to environmental stress is controlled by a complex network of regulatory genes that compose distinct stress-response regulons. In contrast to many signaling and regulatory genes that are stress specific, the zinc-finger protein Zat12 responds to a large number of biotic and abiotic stresses. Zat12 is thought to be involved in cold and oxidative stress signaling in Arabidopsis (Arabidopsis thaliana); however, its mode of action and regulation are largely unknown. Using a fusion between the Zat12 promoter and the reporter gene luciferase, we demonstrate that Zat12 expression is activated at the transcriptional level during different abiotic stresses and in response to a wound-induced systemic signal. Using Zat12 gain- and loss-of-function lines, we assign a function for Zat12 during oxidative, osmotic, salinity, high light, and heat stresses. Transcriptional profiling of Zat12-overexpressing plants and wild-type plants subjected to H(2)O(2) stress revealed that constitutive expression of Zat12 in Arabidopsis results in the enhanced expression of oxidative- and light stress-response transcripts. Under specific growth conditions, Zat12 may therefore regulate a collection of transcripts involved in the response of Arabidopsis to high light and oxidative stress. Our results suggest that Zat12 plays a central role in reactive oxygen and abiotic stress signaling in Arabidopsis.

The phytohormone ethylene is a principal modulator in many aspects of plant life, including various mechanisms by which plants react to pathogen attack. Induced ethylene biosynthesis and subsequent intracellular signaling through a single conserved pathway have been well characterized. This leads to a cascade of transcription factors consisting of primary EIN3-like regulators and downstream ERF-like transcription factors. The latter control the expression of various effector genes involved in various aspects of systemic induced defense responses. Moreover, at this level significant cross-talk occurs with other defense response pathways controlled by salicylic acid and jasmonate, eventually resulting in a differentiated disease response.