Differential Responses of Vanilla Accessions to Root Rot and Colonization by Fusarium oxysporum f. sp. radicis-vanillae

Comparative analyses reveal a phenylalanine ammonia lyase dependent and salicylic acid mediated host resistance in Zingiber zerumbet against the necrotrophic soft rot pathogen Pythium myriotylum

Little is known about the molecular basis of host defense in resistant wild species Zingiber zerumbet (L.) Smith against the soil-borne, necrotrophic oomycete pathogen Pythium myriotylum Drechsler, which causes the devastating soft rot disease in the spice crop ginger (Zingiber officinale Roscoe). We investigated the pattern of host defense between Z. zerumbet and ginger in response to P. myriotylum inoculation. Analysis of gene expression microarray data revealed enrichment of phenylpropanoid biosynthetic genes, particularly lignin biosynthesis genes, in pathogen-inoculated Z. zerumbet compared to ginger. RT-qPCR analysis showed the robust activation of phenylpropanoid biosynthesis genes in Z. zerumbet, including the core genes PAL, C4H, 4CL, and the monolignol biosynthesis and polymerization genes such as CCR, CAD, C3H, CCoAOMT, F5H, COMT, and LAC. Additionally, Z. zerumbet exhibited the accumulation of the phenolic acids including p-coumaric acid, sinapic acid, and ferulic acid that are characteristic of the cell walls of commelinoid monocots like Zingiberaceae and are involved in cell wall strengthening by cross linking with lignin. Z. zerumbet also had higher total lignin and total phenolics content compared to pathogen-inoculated ginger. Phloroglucinol staining revealed the enhanced fortification of cell walls in Z. zerumbet, specifically in xylem vessels and surrounding cells. The trypan blue staining indicated inhibition of pathogen growth in Z. zerumbet at the first leaf whorl, while ginger showed complete colonization of the pith within 36 h post inoculation (hpi). Accumulation of salicylic acid (SA) and induction of SA regulator NPR1 and the signaling marker PR1 were observed in Z. zerumbet. Silencing of PAL in Z. zerumbet through VIGS suppressed downstream genes, leading to reduced phenylpropanoid accumulation and SA level, resulting in the susceptibility of plants to P. myriotylum. These findings highlight the essential role of PAL-dependent mechanisms in resistance against P. myriotylum in Z. zerumbet. Moreover, our results suggest an unconventional role for SA in mediating host resistance against a necrotroph. Targeting the phenylpropanoid pathway could be a promising strategy for the effective management of P. myriotylum in ginger.

First Vanilla planifolia High-Density Genetic Linkage Map Provides Quantitative Trait Loci for Resistance to Fusarium oxysporum

Fusarium oxysporum f. sp. radicis-vanillae (Forv), the causal agent of root and stem rot disease, is the main pathogen affecting vanilla production. Sources of resistance have been reported in Vanilla planifolia G. Jackson ex Andrews, the main cultivated vanilla species. In this study, we developed the first high-density genetic map in this species with 1,804 genotyping-by-sequencing (GBS)-generated single nucleotide polymorphism (SNP) markers using 125 selfed progenies of the CR0040 traditional vanilla cultivar. Sixteen linkage groups (LG) were successfully constructed, with a mean of 113 SNPs and an average length of 207 cM per LG. The map had a high density with an average of 5.45 SNP every 10 cM and an average distance of 1.85 cM between adjacent markers. The first three LG were aligned against the first assembled chromosome of CR0040, and the other 13 LG were correctly associated with the other 13 assembled chromosomes. The population was challenged with the highly pathogenic Forv strain Fo072 using the root-dip inoculation method. Five traits were mapped, and 20 QTLs were associated with resistance to Fo072. Among the genes retrieved in the CR0040 physical regions associated with QTLs, genes potentially involved in biotic resistance mechanisms, coding for kinases, E3 ubiquitin ligases, pentatricopeptide repeat-containing proteins, and one leucine-rich repeat receptor underlying the qFo72_08.1 QTL have been highlighted. This study should provide useful resources for marker-assisted selection in V. planifolia.

The Use of Spectral Imaging to Follow the Iron and pH-Dependent Accumulation of Fluorescent Coumarins

Plants challenged with iron deficiency produce in their roots and secrete into the rhizosphere several small molecules named coumarins that derive from the phenylpropanoid pathway. Coumarins are biosynthesized in different root cell types and transported to the root epidermis prior to their secretion in the surrounding media. Taking advantage of the natural fluorescence of most coumarins glycosides when exposed to UV light, we developed a method to uncover their individual cellular localization and accumulation. This approach couples spectral imaging acquisition and linear unmixing analysis. In this protocol, we describe guidelines, experimental setup, and conditions for the analysis of coumarins localization and accumulation in Arabidopsis thaliana root seedlings grown in control and iron deficiency conditions, at both acidic and alkaline pH.

Variations in Arbuscular Mycorrhizal Colonization Associated with Root Diameter and Hypodermis Passages Cells across Temperate and Tropical Woody Species

Root hypodermis passage cells (PCs) lack suberin and lignin deposition, responsible for nutrient absorption and arbuscular mycorrhizal colonization, which are crucial for root resource acquisition. Nevertheless, their quantitative variability across diverse woody species and their relationships with root morphology and anatomy, as well as arbuscular mycorrhizal colonization, are still not well understood. Herein, the number and proportion of PCs in the root cross-section, root morphology, anatomy, and arbuscular mycorrhizal colonization rate were quantified across 10 temperate and 12 tropical woody species. The objectives of this study were to determine how PCs vary across contrasting environmental conditions and to explore their relationships with arbuscular mycorrhizal colonization rate and other root functional traits. The results showed that tropical species possessed 56% more PCs than temperate species; by contrast, they had similar proportions of PCs. In both biomes, the number of PCs had a tightly positive correlation with arbuscular mycorrhizal colonization rate (R2 = 0.35–0.87), root diameter (R2 = 0.84–0.93), and cortex thickness (R2 = 0.87–0.89), but the proportion of PCs was mostly independent of root morphological and anatomical traits. Our results suggest that variation in passage cells could well explain the tight linkage between arbuscular mycorrhizal colonization and root diameter across species and biomes, which provides insight into the collaboration gradient between plant roots and mycorrhizal fungi that dominates the root economics space.

The endodermal passage cell - just another brick in the wall?

The endodermis surrounds and protects the vasculature partly by depositing hydrophobic suberin in the cell walls. Yet, some cells remain unsuberised. These historically termed 'passage cells' are assumed to provide a low-resistance pathway to the xylem. Only recently have we started to gain molecular insights into these cells, which allow us to probe how roots coordinate communication with the environment across barriers with single-cell precision. Increased understanding of root physiology at a high-resolution is intriguing, as it is likely to provide us with new tools to improve overall plant health. With this in mind, we here provide a brief overview of passage cells, their presence across plant species, as well as a molecular update and future directions for passage cell-related research.

Coumarin accumulation and trafficking in Arabidopsis thaliana: a complex and dynamic process

Iron (Fe) is a major micronutrient and is required for plant growth and development. Nongrass species have evolved a reduction-based strategy to solubilize and take up Fe. The secretion of Fe-mobilizing coumarins (e.g. fraxetin, esculetin and sideretin) by plant roots plays an important role in this process. Although the biochemical mechanisms leading to their biosynthesis have been well described, very little is known about their cellular and subcellular localization or their mobility within plant tissues. Spectral imaging was used to monitor, in Arabidopsis thaliana, the in planta localization of Fe-mobilizing coumarins and scopolin. Molecular, genetic and biochemical approaches were also used to investigate the dynamics of coumarin accumulation in roots. These approaches showed that root hairs play a major role in scopoletin secretion, whereas fraxetin and esculetin secretion occurs through all epidermis cells. The findings of this study also showed that the transport of coumarins from the cortex to the rhizosphere relies on the PDR9 transporter under Fe-deficient conditions. Additional experiments support the idea that coumarins move throughout the plant body via the xylem sap and that several plant species can take up coumarins present in the surrounding media. Altogether, the data presented here demonstrate that coumarin storage and accumulation in roots is a highly complex and dynamic process.

Elucidation and Identification of an Antifungal Compound from Pseudomonas aeruginosa DA3.1 Isolated from Soil in Vietnam

Background: Fusarium sp. and Rhizoctonia sp. fungi have been always threats to short-term crops. In Vietnam, corn and soybean suffer serious losses annually. Therefore, it is necessary to utilize an environmentally friendly antifungal compound that is highly effective against phytopathogenic fungi. Pseudomonas sp. is a popular soil bacterial strain and well known for its high antifungal activity. Objectives: This study was carried out to evaluate and assess the antifungal activity of a local bacterial strain namely DA3.1 that was later identified as Pseudomonas aeruginosa. This would be strong scientific evidence to develop an environmentally friendly biocide from a local microorganism strain for commercial use. Methods: The antifungal compound was purified from ethyl acetate extraction of deproteinized cell culture broth by a silica gel column (CH2Cl2/MeOH (0% - 10% MeOH)). The purity of the isolated compound was determined by HPLC, and its molecular structure was elucidated using spectroscopic experiments including one-dimensional (1D) (1H NMR, 13C NMR, DEPT) and two-dimensional (2D) (HMBC and HSQC) spectra. The activity of the purified compound against Fusarium sp. and Rhizoctonia sp. fungi was measured using the PDA-disk diffusion method, and its growth-promoting ability was evaluated using the seed germination test of corn and soybean. Results: The results showed that the antifungal compound produced by Pseudomonas aeruginosa DA3.1 had a retention factor (Rf) of 0.86 on thin layer chromatography (TLC). Based on the evidence of spectral data including proton nuclear magnetic resonance (1H NMR), carbon nuclear magnetic resonance (13C NMR), distortionless enhancement by polarization transfer (DEPT), heteronuclear multiple bond correlation (HMBC), and heteronuclear single quantum coherence (HSQC), the chemical structure was elucidated as phenazine-1-carboxylic. The purified compound showed inhibitory activity against F. oxysporum and R. solani and exhibited the ability of the germination of corn and soybean seeds. The results revealed the benefit of native P. aeruginosa DA3.1 and phenazine-1-carboxylic acid for use as a biocontrol agent, as well as a plant growth promoter. Conclusions: The antifungal compound isolated from local Pseudomonas DA3.1 was identified as phenazine-1-carboxylic acid that posed high antifungal activity and was a plant germination booster.

The Leafless Vanilla Species-Complex from the South-West Indian Ocean Region: A Taxonomic Puzzle and a Model for Orchid Evolution and Conservation Research

The pantropical genus Vanilla is a member of the Orchidaceae family, one of the largest and most diverse families of angiosperms in the world. It originated in America and differentiated in America, Africa, and Asia. About 132 species of Vanilla have now been accepted since the discovery of the genus. Within the Vanilla genus, leaflessness has appeared at least three independent times, in Africa, Asia, and America as an adaptation to drought conditions. The South-West Indian Ocean (SWIO) region, a biodiversity hotspot for orchids, is home to seven leafless species. Chloroplast markers failed to differentiate species within this recent SWIO clade. Morphological traits used for description are very similar between species and do not provide reliable identification. Moreover, some of the species have overlapping flowering periods and geographic distribution in Madagascar, increasing the probability of finding some sympatric species and, therefore, hybrids. Leafless Vanilla species from the SWIO islands are thus an excellent model for understanding the evolution of orchids. For their conservation, and to confirm the identity of these leafless species, an integrated approach with classical taxonomy using a large number of samples, intense fieldwork on biology and ecology, and molecular studies using variable markers is necessary.

Regulation of phenylpropanoid biosynthesis by MdMYB88 and MdMYB124 contributes to pathogen and drought resistance in apple

MdMYB88 and MdMYB124 have been demonstrated to be responsible for lignin accumulation in apple under drought stress. In this study, using a metabolomic approach, we identified differentially accumulated phenylpropanoid and flavonoid metabolites in MdMYB88/124 transgenic RNAi plants under control and long-term drought stress conditions in apple roots. We confirmed the regulation of phenylalanine by MdMYB88 and MdMYB124 via UPLC-MS in apple roots under both control and drought conditions. Using Electrophoretic Mobility Shift Assay (EMSA) and ChIP-quantitative PCR (qPCR) analyses, we found that MdMYB88 positively regulates the MdCM2 gene, which is responsible for phenylalanine biosynthesis, through binding to its promoter region. Under long-term drought conditions, MdMYB88/124 RNAi plants consistently accumulated increased amounts of H2O2 and MDA, while MdMYB88 and MdMYB124 overexpression plants accumulated decreased amounts of H2O2 and MDA. We also examined the accumulation of metabolites in the phenylpropanoid biosynthesis pathway in the leaves of MdMYB88 and MdMYB124 transgenic apple plants after long-term drought stress. We found that metabolites responsible for plant defense, including phenylpropanoids and flavonoids, accumulated less in the RNAi plants but more in the overexpression plants under both control and drought conditions. We further demonstrated that MdMYB88/124 RNAi plants were more sensitive to Alternaria alternata f. sp. mali and Valsa mali, two pathogens that currently severely threaten apple production. In contrast, MdMYB88 and MdMYB124 overexpression plants were more tolerant to these pathogens. The cumulative results of this study provided evidence for secondary metabolite regulation by MdMYB88 and MdMYB124, further explained the molecular roles of MdMYB88 and MdMYB124 in drought resistance, and provided information concerning molecular aspects of their roles in disease resistance.

An Imaging Approach to Identify Mechanisms of Resistance to Pineapple Fruitlet Core Rot

Fruitlet core rot is one of the major postharvest disease of pineapple (Ananas comosus var. comosus). In the past, control strategies were designed to eliminate symptoms without addressing their causes or mechanisms, thus achieving only moderate success. In this study, (i) we focused on the anatomy of the fruitlets in the resistant "MD-2" and susceptible "Queen" pineapple cultivars; (ii) we identified the key role of the carpel margin in the infection process; (iii) we identified the key role of the sinuous layer of thick-walled cells in the inhibition of Fusarium ananatum colonization; and (iv) we linked the anatomy of the fruitlets with the phenolic content of cell walls. The fruitlet anatomy of the two cultivars was studied using X-ray, fluorescence, and multiphoton microscopy. Sepals and bracts were not perfectly fused with each other, allowing the pathogen to penetrate the fruit even after flowering. In fact, the fungi were found in the blossom cups of both cultivars but only became pathogenic in the flesh of the "Queen" pineapple fruit under natural conditions. The outer layer of the "MD-2" cavity was continuous with thick cell walls composed of ferulic and coumaric acids. The cell walls of the "Queen" blossom cup were less lignified at the extremities, and the outer layer was interspersed with cracks. The carpel margins were fused broadly in the "MD-2" pineapple, in contrast to the "Queen" pineapple. This blemish allows the fungus to penetrate deeper into the susceptible cultivar. In pineapple fruitlets, the hyphae of F. ananatum mainly progressed directly between cell walls into the parenchyma but never reached the vascular region. A layer of thick-walled cells, in the case of the resistant cultivar, stopped the colonization, which were probably the infralocular septal nectaries. Anatomical and histochemical observations coupled with spectral analysis of the hypodermis suggested the role of lignin deposition in the resistance to F. ananatum. The major phenolics bound to the cell walls were coumaric and ferulic acids and were found in higher amounts in the resistant cultivar postinoculation. The combination of fruitlet anatomy and lignification plays a role in the mechanism of host resistance to fruitlet core rot.

Fusarium oxysporum and the Fusarium Wilt Syndrome

The Fusarium oxysporum species complex (FOSC) comprises a multitude of strains that cause vascular wilt diseases of economically important crops throughout the world. Although sexual reproduction is unknown in the FOSC, horizontal gene transfer may contribute to the observed diversity in pathogenic strains. Development of disease in a susceptible crop requires F. oxysporum to advance through a series of transitions, beginning with spore germination and culminating with establishment of a systemic infection. In principle, each transition presents an opportunity to influence the risk of disease. This includes modifications of the microbial community in soil, which can affect the ability of pathogen propagules to survive, germinate, and infect plant roots. In addition, many host attributes, including the composition of root exudates, the structure of the root cortex, and the capacity to recognize and respond quickly to invasive growth of a pathogen, can impede development of F. oxysporum.