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.

The Role of Ethylene in Host-Pathogen Interactions

Elicitors of induced disease resistance in postharvest horticultural crops: a brief review

Strawberry, a small fruit crop of great importance throughout the world, has been considered a model plant system for Rosaceae, and is susceptible to a large variety of phytopathogenic organisms. Most components and mechanisms of the strawberry defense network remain poorly understood. However, from current knowledge, it seems clear that the ability of a strawberry plant to respond efficiently to pathogens relies first on the physiological status of injured tissue (pre-formed mechanisms of defense) and secondly on the general ability to recognize and identify the invaders by surface plant receptors, followed by a broad range of induced mechanisms, which include cell wall reinforcement, production of reactive oxygen species, phytoalexin generation and pathogenesis-related protein accumulation. Dissection of these physiological responses at a molecular level will provide valuable information to improve future breeding strategies for new strawberry varieties and to engineer strawberry plants for durable and broad-spectrum disease resistance. In turn, this will lead to a reduction in use of chemicals and in environmental risks. Advances in the understanding of the molecular interplay between plant (mainly those considered model systems) and various classes of microbial pathogens have been made in the last two decades. However, major progress in the genetics and molecular biology of strawberry is still needed to uncover fully the way in which this elaborate plant innate immune system works. These fundamental insights will provide a conceptual framework for rational human intervention through new strawberry research approaches. In this review, we will provide a comprehensive overview and discuss recent advances in molecular research on strawberry defense mechanisms against pathogens.

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The Strawberry Plant Defense Mechanism: A Molecular Review

Ethylene (ET) plays a critical role in the activation of plant defenses against different biotic stresses through its participation in a complex signaling network that includes jasmonic acid (JA), salicylic acid (SA), and abscisic acid (ABA). Pathogen attack, wounding, and herbivory trigger asymmetric activation of this defense signaling network, thereby affecting the final balance of interactions between its components and establishing a targeted response to the initial threat. Ethylene’s contribution to the modulation of this defense network relies on the complexity of the regulation of multigene families involved in ET biosynthesis, signal transduction, and crosstalk and enables the plant to fine-tune its response. The function of the members of these multigene families is tightly regulated at transcriptional, post-transcriptional, and post-translational levels. It is generally accepted that ET cooperates with JA in the activation of defenses against necrotrophic pathogens and antagonizes SA-dependent resistance against biotrophic pathogens. However, this is likely an oversimplified view, because cooperative interactions between ET and SA pathways have been reported and ET has been implicated in the activation of defenses against some biotrophic and hemibiotrophic pathogens. Therefore, deciphering ET’s place in this hormonal network is essential to understanding how the cell orchestrates an optimal response to a specific biotic stress.

Modulation of Plant Defenses by Ethylene

Curcuminoid biosynthesis by two type III polyketide synthases in the herb Curcuma longa.

Changes in the phytoalexin content in unripe fruit of banana, Musa acuminata, were analyzed after various treatments. The results show that level of hydroxyanigorufone started to increase 1-2 day after either wounding or inoculation with conidia of Colletotrichum musae. Inoculation followed by wounding induced the formation of many other phenylphenalenones. The accumulation of hydroxyanigorufone decreased, after its transient maximum, on ripening by exposure of the wounded fruit to ethylene. The level of production of hydroxyanigorufone in ripe fruit treated by wounding and/or by inoculation was much lower than that in unripe fruit. 2-Aminooxyacetic acid, an inhibitor of phenylalanine ammonia-lyase (PAL), inhibited the accumulation of hydroxyanigorufone in wounded fruit, and the PAL activity increased after wounding and ethylene treatment, respectively. Feeding experiments with [1-13C] and [2-13C]cinnamic acids, and [2-13C]malonate show that two molecules of cinnamic acid and one of malonate were incorpor...

Changes in the content and biosynthesis of phytoalexins in banana fruit.

Fourteen phenylphenalenone-type phytoalexins (1-14), including three new compounds, were isolated from the peel of unripe Musa acuminata [AAA] cv. Bungulan fruit which had been injured and then inoculated with conidia of Colletotrichum musae. These new phytoalexins were identified as (+)-cis-2,3-dihydro-2,3-dihydroxy-4-(4'-hydroxyphenyl)phenalen-1-one (12), 9-(3',4'-dimethoxyphenyl)-2-methoxyphenalen-1-one (13) and 9-(4'-hydroxyphenyl)-2-methoxyphenalen-1-one (14). The ratios of the relative intensities of the [M](+)/[M-H](+) ions or [M-H2O](+)/[M-H2O-H](+) ions in the EI mass spectra were applied to discriminate between 4- and 9-phenylphenalenones. An antifungal test on the phytoalexins showed that a phenolic hydroxyl group was essential for the activity.

Phenylphenalenone-type Phytoalexins from Unripe Buñgulan Banana Fruit.

New phenylphenalenones from banana fruit

Seven phytoalexins (1-7), including a new compound, were isolated from the peel of unripe kiwi fruit (Actinidia deliciosa cv. Golden King) that had been wounded and inoculated with Colletotrichum musae. The new phytoalexin (1) was identified as 2α,3β,23-trihydroxy-12,20(30)-ursadien-28-oic acid, and named actinidic acid. Phytoalexins 2-6 are known triterpenes but have not previously been described as phytoalexins. Phytoalexin 7 is the same triterpene as the phytoalexin of nectarine fruit.

Actinidic acid, a new triterpene phytoalexin from unripe kiwi fruit.

Strawberry cv. Houkouwase is resistant to infection by Colletotrichum fragariae. The formation of antifungal compounds was observed in unripe fruit which had been wounded and inoculated with conidia of C. musae. Three antifungal compounds were isolated and identified as euscaphic acid, tormentic acid and myrianthic acid. Myrianthic acid inhibited the growth of C. musae at 3 μg, and euscaphic and tormentic acids showed inhibitory effects at 100 μg. A quantitative analysis of their contents showed that the triterpenes increased in wounded fruit, and in wounded and inoculated fruit, but not in non-treated fruit. These findings indicate that unripe fruit of Houkouwase produced the triterpenes as phytoalexins. The triterpene phytoalexins seem to be involved in the resistance of strawberry to the fungus.

Tsunashi Kamo

Papers

Changes in the content and biosynthesis of phytoalexins in banana fruit.

Phenylphenalenone-type Phytoalexins from Unripe Buñgulan Banana Fruit.

New phenylphenalenones from banana fruit

Actinidic acid, a new triterpene phytoalexin from unripe kiwi fruit.

Triterpene phytoalexins from strawberry fruit.