The Nicotiana attenuata GLA1 lipase controls the accumulation of Phytophthora parasitica-induced oxylipins and defensive secondary metabolites

Antagonistic and plant growth promotion of rhizobacteria against Phytophthora colocasiae in taro

Taro leaf blight caused by Phytophthora colocasiae adversely affects the growth and yield of taro. The management of this disease depends heavily on synthetic fungicides. These compounds, however, pose potential hazards to human health and the environment. The present study aimed to investigate an alternative approach for plant growth promotion and disease control by evaluating seven different bacterial strains (viz., Serratia plymuthica, S412; S. plymuthica, S414; S. plymuthica, AS13; S. proteamaculans, S4; S. rubidaea, EV23; S. rubidaea, AV10; Pseudomonas fluorescens, SLU-99) and their different combinations as consortia against P. colocasiae. Antagonistic tests were performed in in vitro plate assays and the effective strains were selected for detached leaf assays and greenhouse trials. Plant growth-promoting and disease prevention traits of selected bacterial strains were also investigated in vitro. Our results indicated that some of these strains used singly (AV10, AS13, S4, and S414) and in combinations (S4+S414, AS13+AV10) reduced the growth of P. colocasiae (30-50%) in vitro and showed disease reduction ability when used singly or in combinations as consortia in greenhouse trials (88.75-99.37%). The disease-suppressing ability of these strains may be related to the production of enzymes such as chitinase, protease, cellulase, and amylase. Furthermore, all strains tested possessed plant growth-promoting traits such as indole-3-acetic acid production, siderophore formation, and phosphate solubilization. Overall, the present study revealed that bacterial strains significantly suppressed P. colocasiae disease development using in vitro, detached leaf, and greenhouse assays. Therefore, these bacterial strains can be used as an alternative strategy to minimize the use of synthetic fungicides and fertilizers to control taro blight and improve sustainable taro production.

Developing a platform for production of the oxylipin KODA in plants

KODA (9-hydroxy-10-oxo-12(Z),15(Z)-octadecadienoic acid) is a plant oxylipin involved in recovery from stress. As an agrichemical, KODA helps maintain crop production under various environmental stresses. In plants, KODA is synthesized from α-linolenic acids via 9-lipoxygenase (9-LOX) and allene oxide synthase (AOS), although the amount is usually low, except in the free-floating aquatic plant Lemna paucicostata. To improve KODA biosynthetic yield in other plants such as Nicotiana benthamiana and Arabidopsis thaliana, we developed a system to overproduce KODA in vivo via ectopic expression of L. paucicostata 9-LOX and AOS. The transient expression in N. benthamiana showed that the expression of these two genes is sufficient to produce KODA in leaves. However, stable expression of 9-LOX and AOS (with consequent KODA production) in Arabidopsis plants succeeded only when the two proteins were targeted to plastids or the endoplasmic reticulum/lipid droplets. Although only small amounts of KODA could be detected in crude leaf extracts of transgenic Nicotiana or Arabidopsis plants, subsequent incubation of the extracts increased KODA abundance over time. Therefore, KODA production in transgenic plants stably expressing 9-LOX and AOS requires specific sub-cellular localization of these two enzymes and incubation of crude leaf extracts, which liberates α-linolenic acid via breakdown of endogenous lipids.

Transcriptome and Oxylipin Profiling Joint Analysis Reveals Opposite Roles of 9-Oxylipins and Jasmonic Acid in Maize Resistance to Gibberella Stalk Rot

Gibberella stalk rot caused by Fusarium graminearum is one of the devastating diseases of maize that causes significant yield losses worldwide. The molecular mechanisms regulating defense against this pathogen remain poorly understood. According to recent studies, a major oxylipin hormone produced by 13-lipoxygenases (LOX) namely jasmonic acid (JA) has been associated with maize susceptibility to GSR. However, the specific roles of numerous 9-LOX-derived oxylipins in defense against Gibberella stalk rot (GSR) remain unexplained. In this study, we have shown that disruption of a 9-LOX gene, ZmLOX5, resulted in increased susceptibility to GSR, indicating its role in defense. To understand how ZmLOX5 regulates GSR resistance, we conducted transcriptome and oxylipin profiling using a zmlox5-3 mutant and near-isogenic wild type B73, upon infection with F. graminearum. The results showed that JA biosynthetic pathway genes were highly up-regulated, whereas multiple 9-LOX pathway genes were down-regulated in the infected zmlox5-3 mutant. Furthermore, oxylipin profiling of the mutant revealed significantly higher contents of several jasmonates but relatively lower levels of 9-oxylipins in zmlox5-3 upon infection. In contrast, B73 and W438, a more resistant inbred line, displayed relatively lower levels of JAs, but a considerable increase of 9-oxylipins. These results suggest antagonistic interaction between 9-oxylipins and JAs, wherein 9-oxylipins contribute to resistance while JAs facilitate susceptibility to F. graminearum.

Integrated Proteomic and Metabolomic Profiling of Phytophthora cinnamomi Attack on Sweet Chestnut (Castanea sativa) Reveals Distinct Molecular Reprogramming Proximal to the Infection Site and Away from It

Phytophthora cinnamomi is one of the most invasive tree pathogens that devastates wild and cultivated forests. Due to its wide host range, knowledge of the infection process at the molecular level is lacking for most of its tree hosts. To expand the repertoire of studied Phytophthora-woody plant interactions and identify molecular mechanisms that can facilitate discovery of novel ways to control its spread and damaging effects, we focused on the interaction between P. cinnamomi and sweet chestnut (Castanea sativa), an economically important tree for the wood processing industry. By using a combination of proteomics, metabolomics, and targeted hormonal analysis, we mapped the effects of P. cinnamomi attack on stem tissues immediately bordering the infection site and away from it. P. cinnamomi led to a massive reprogramming of the chestnut proteome and accumulation of the stress-related hormones salicylic acid (SA) and jasmonic acid (JA), indicating that stem inoculation can be used as an easily accessible model system to identify novel molecular players in P. cinnamomi pathogenicity.

Plant-Pathogen Interactions: Underestimated Roles of Phyto-oxylipins

Plant (or phyto-) oxylipins (POs) are produced under a wide range of stress conditions and although they are well known to activate stress-related signalling pathways, the nonsignalling roles of POs are poorly understood. We describe oxylipins as direct biocidal agents and propose that structure-function relationships play here a pivotal role. Based on their chemical configuration, POs, such as reactive oxygen and electrophile species, activate defence-related gene expression. We also propose that their ability to interact with pathogen membranes is important, but still misunderstood, and that they are involved in cross-kingdom communication. Taken as a whole, the current literature suggests that POs have a high potential as biocontrol agents. However, the mechanisms underlying these multifaceted compounds remain largely unknown.

Functional diversity of glycerolipid acylhydrolases in plant metabolism and physiology

Most current knowledge about plant lipid metabolism has focused on the biosynthesis of lipids and their transport between different organelles. However, lipid composition changes during development and in response to environmental cues often go beyond adjustments of lipid biosynthesis. When lipids have to be removed to adjust the extent of membranes during down regulation of photosynthesis, or lipid composition has to be adjusted to alter the biophysical properties of membranes, or lipid derived chemical signals have to be produced, lipid-degrading enzymes come into play. This review focuses on glycerolipid acylhydrolases that remove acyl groups from glycerolipids and will highlight their roles in lipid remodeling and lipid-derived signal generation. One emerging theme is that these enzymes are involved in the dynamic movement of acyl groups through different lipid pools, for example from polar membrane lipids to neutral lipids sequestered in lipid droplets during de novo triacylglycerol synthesis. Another example of acyl group sequestration in the form of triacylglycerols in lipid droplets is membrane lipid remodeling in response to abiotic stresses. Fatty acids released for membrane lipids can also give rise to potent signaling molecules and acylhydrolases are therefore often the first step in initiating the formation of these lipid signals.

Jasmonates: biosynthesis, metabolism, and signaling by proteins activating and repressing transcription

The lipid-derived phytohormone jasmonate (JA) regulates plant growth, development, secondary metabolism, defense against insect attack and pathogen infection, and tolerance to abiotic stresses such as wounding, UV light, salt, and drought. JA was first identified in 1962, and since the 1980s many studies have analyzed the physiological functions, biosynthesis, distribution, metabolism, perception, signaling, and crosstalk of JA, greatly expanding our knowledge of the hormone's action. In response to fluctuating environmental cues and transient endogenous signals, the occurrence of multilayered organization of biosynthesis and inactivation of JA, and activation and repression of the COI1-JAZ-based perception and signaling contributes to the fine-tuning of JA responses. This review describes the JA biosynthetic enzymes in terms of gene families, enzymatic activity, location and regulation, substrate specificity and products, the metabolic pathways in converting JA to activate or inactivate compounds, JA signaling in perception, and the co-existence of signaling activators and repressors.

Oil is on the agenda: Lipid turnover in higher plants

Lipases hydrolyze ester bonds within lipids. This process is called lipolysis. They are key players in lipid turnover and involved in numerous metabolic pathways, many of which are shared between organisms like the mobilization of neutral or storage lipids or lipase-mediated membrane lipid homeostasis. Some reactions though are predominantly present in certain organisms, such as the production of signaling molecules (endocannabinoids) by diacylglycerol (DAG) and monoacylglycerol (MAG) lipases in mammals and plants or the jasmonate production in flowering plants. This review aims at giving an overview of the different functional classes of lipases and respective well-known activities, with a focus on the most recent findings in plant biology for selected classes. Here we will put an emphasis on the physiological role and contribution of lipases to the turnover of neutral lipids found in seed oil and other vegetative tissue as candidates for increasing the economical values of crop plants. This article is part of a Special Issue entitled: Plant Lipid Biology edited by Kent D. Chapman and Ivo Feussner.

Jasmonic acid signalling mediates resistance of the wild tobacco Nicotiana attenuata to its native Fusarium, but not Alternaria, fungal pathogens

We recently characterized a highly dynamic fungal disease outbreak in native populations of Nicotiana attenuata in the southwestern United States. Here, we explore how phytohormone signalling contributes to the observed disease dynamics. Single inoculation with three native Fusarium and Alternaria fungal pathogens, isolated from diseased plants growing in native populations, resulted in disease symptoms characteristic for each pathogen species. While Alternaria sp.-infected plants displayed fewer symptoms and recovered, Fusarium spp.-infected plants became chlorotic and frequently spontaneously wilted. Jasmonic acid (JA) and salicylic acid (SA) levels were differentially induced after Fusarium or Alternaria infection. Transgenic N. attenuata lines silenced in JA production or JA conjugation to isoleucine (JA-Ile), but not in JA perception, were highly susceptible to infection by F. brachygibbosum Utah 4, indicating that products derived from the JA-Ile biosynthetic pathway, but not their perception, is associated with increased Fusarium resistance. Infection assays using ov-nahG plants which were silenced in pathogen-induced SA accumulations revealed that SA may increase N. attenuata's resistance to Fusarium infection but not to Alternaria. Taken together, we propose that the dynamics of fungal disease symptoms among plants in native populations may be explained by a complex interplay of phytohormone responses to attack by multiple pathogens.

Lipases and the biosynthesis of free oxylipins in plants

The production of free oxylipins in plants is exquisitely controlled by cellular mechanisms that respond to environmental factors such as mechanical damage, insect herbivory and pathogen infection. One of the main targets of these cellular mechanisms are glycerolipases class A (GLA); acyl-hydrolyzing enzymes that upon their biochemical activation release unsaturated fatty acids or acylated oxylipins from glycerolipids. Recent studies performed in the wild tobacco species Nicotiana attenuata have started to reveal the complexity and specificity of GLA-regulated free oxylipin production. I present a model in which individual GLA lipases associate with individual lipoxygenases (LOX) in chloroplast membranes and envelope to define the initial committed steps of distinct oxylipin biosynthesis pathways. The unravelling of the mechanisms that activate GLAs and LOXs at the biochemical level and that control the interaction between these enzymes and their association with membranes will prove to be fundamental to understand how plants control free oxylipin biogenesis.