Kinases and protein motifs required for AZI1 plastid localization and trafficking during plant defense induction

Lipid peroxidation and stress-induced signalling molecules in systemic resistance mediated by azelaic acid/AZELAIC ACID INDUCED1: signal initiation and propagation

Systemic acquired resistance protects plants against a broad spectrum of secondary infections by pathogens. A crucial compound involved in the systemic spread of the threat information after primary pathogen infection is the C9 oxylipin azelaic acid (AZA), a breakdown product of unsaturated C18 fatty acids. AZA is generated during lipid peroxidation in the plastids and accumulates in response to various abiotic and biotic stresses. AZA stimulates the expression of AZELAIC ACID INDUCED1 (AZI1), and a pool of AZI1 accumulates in the plastid envelope in association with AZA. AZA and AZI1 utilize the symplastic pathway to travel through the plasmodesmata to neighbouring cells to induce systemic stress resistance responses in distal tissues. Here, we describe the synthesis, travel and function of AZA and AZI1 and discuss open questions of signal initiation and propagation.

Chemical priming of plant defense responses to pathogen attacks

Plants can acquire an improved resistance against pathogen attacks by exogenous application of natural or artificial compounds. In a process called chemical priming, application of these compounds causes earlier, faster and/or stronger responses to pathogen attacks. The primed defense may persist over a stress-free time (lag phase) and may be expressed also in plant organs that have not been directly treated with the compound. This review summarizes the current knowledge on the signaling pathways involved in chemical priming of plant defense responses to pathogen attacks. Chemical priming in induced systemic resistance (ISR) and systemic acquired resistance (SAR) is highlighted. The roles of the transcriptional coactivator NONEXPRESSOR OF PR1 (NPR1), a key regulator of plant immunity, induced resistance (IR) and salicylic acid signaling during chemical priming are underlined. Finally, we consider the potential usage of chemical priming to enhance plant resistance to pathogens in agriculture.

Lipid transfer proteins involved in plant-pathogen interactions and their molecular mechanisms

Nonspecific lipid transfer proteins (LTPs) are small, cysteine-rich proteins that play numerous functional roles in plant growth and development, including cutin wax formation, pollen tube adhesion, cell expansion, seed development, germination, and adaptation to changing environmental conditions. LTPs contain eight conserved cysteine residues and a hydrophobic cavity that provides a wide variety of lipid-binding specificities. As members of the pathogenesis-related protein 14 family (PR14), many LTPs inhibit fungal or bacterial growth, and act as positive regulators in plant disease resistance. Over the past decade, these essential immunity-related roles of LTPs in plant immune processes have been documented in a growing body of literature. In this review, we summarize the roles of LTPs in plant-pathogen interactions, emphasizing the underlying molecular mechanisms in plant immune responses and specific LTP functions.

Identification and analysis of proline-rich proteins and hybrid proline-rich proteins super family genes from Sorghum bicolor and their expression patterns to abiotic stress and zinc stimuli

Systematic genome-wide analysis of Sorghum bicolor revealed the identification of a total of 48 homologous genes comprising 21 proline-rich proteins (PRPs) and 27 hybrid proline-rich proteins (HyPRPs). Comprehensive scrutiny of these gene homologs was conducted for gene structure, phylogenetic investigations, chromosome mapping, and subcellular localization of proteins. Promoter analysis uncovered the regions rich with phosphorous- (BIHD), ammonium-, sulfur-responsive (SURE), and iron starvation-responsive (IRO2) along with biotic, abiotic, and development-specific cis-elements. Further, PRPs exhibit more methylation and acetylation sites in comparison with HyPRPs. miRNAs have been predicted which might play a role in cleavage and translation inhibition. Several of the SbPRP genes were stimulated in a tissue-specific manner under drought, salt, heat, and cold stresses. Additionally, exposure of plants to abscisic acid (ABA) and zinc (Zn) also triggered PRP genes in a tissue-dependent way. Among them, SbPRP17 has been found upregulated markedly in all tissues irrespective of the stress imposed. The expressions of SbHyPRPs, especially SbHyPRP2, SbHyPRP6, and SbHyPRP17 were activated under all stresses in all three tissues. On the other hand, SbHyPRP8 (root only) and SbHyPRP12 (all three tissues) were highly responsive to cold stress and ABA while SbHyPRP26 was induced by drought and Zn in the stem. Taken together, this study indicates the critical roles that SbPRPs and SbHyPRPs play during diverse abiotic stress conditions and notably the plausible roles that these genes play upon exposure to zinc, the crucial micronutrient in plants.

Friend or foe: Hybrid proline-rich proteins determine how plants respond to beneficial and pathogenic microbes

Plant plastids generate signals, including some derived from lipids, that need to be mobilized to effect signaling. We used informatics to discover potential plastid membrane proteins involved in microbial responses in Arabidopsis (Arabidopsis thaliana). Among these are proteins co-regulated with the systemic immunity component AZELAIC ACID INDUCED 1, a hybrid proline-rich protein (HyPRP), and HyPRP superfamily members. HyPRPs have a transmembrane domain, a proline-rich region (PRR), and a lipid transfer protein domain. The precise subcellular location(s) and function(s) are unknown for most HyPRP family members. As predicted by informatics, a subset of HyPRPs has a pool of proteins that target plastid outer envelope membranes via a mechanism that requires the PRR. Additionally, two HyPRPs may be associated with thylakoid membranes. Most of the plastid- and nonplastid-localized family members also have pools that localize to the endoplasmic reticulum, plasma membrane, or plasmodesmata. HyPRPs with plastid pools regulate, positively or negatively, systemic immunity against the pathogen Pseudomonas syringae. HyPRPs also regulate the interaction with the plant growth-promoting rhizobacteria Pseudomonas simiae WCS417 in the roots to influence colonization, root system architecture, and/or biomass. Thus, HyPRPs have broad and distinct roles in immunity, development, and growth responses to microbes and reside at sites that may facilitate signal molecule transport.

Threat at One End of the Plant: What Travels to Inform the Other Parts?

Adaptation and response to environmental changes require dynamic and fast information distribution within the plant body. If one part of a plant is exposed to stress, attacked by other organisms or exposed to any other kind of threat, the information travels to neighboring organs and even neighboring plants and activates appropriate responses. The information flow is mediated by fast-traveling small metabolites, hormones, proteins/peptides, RNAs or volatiles. Electric and hydraulic waves also participate in signal propagation. The signaling molecules move from one cell to the neighboring cell, via the plasmodesmata, through the apoplast, within the vascular tissue or-as volatiles-through the air. A threat-specific response in a systemic tissue probably requires a combination of different traveling compounds. The propagating signals must travel over long distances and multiple barriers, and the signal intensity declines with increasing distance. This requires permanent amplification processes, feedback loops and cross-talks among the different traveling molecules and probably a short-term memory, to refresh the propagation process. Recent studies show that volatiles activate defense responses in systemic tissues but also play important roles in the maintenance of the propagation of traveling signals within the plant. The distal organs can respond immediately to the systemic signals or memorize the threat information and respond faster and stronger when they are exposed again to the same or even another threat. Transmission and storage of information is accompanied by loss of specificity about the threat that activated the process. I summarize our knowledge about the proposed long-distance traveling compounds and discuss their possible connections.