The mannose-binding lectin gene FaMBL1 is involved in the resistance of unripe strawberry fruits to Colletotrichum acutatum

The devastating oomycete phytopathogen Phytophthora cactorum: Insights into its biology and molecular features

Phytophthora cactorum is one of the most economically important soilborne oomycete pathogens in the world. It infects more than 200 plant species spanning 54 families, most of which are herbaceous and woody species. Although traditionally considered to be a generalist, marked differences of P. cactorum isolates occur in degree of pathogenicity to different hosts. As the impact of crop loss caused by this species has increased recently, there has been a tremendous increase in the development of new tools, resources, and management strategies to study and combat this devastating pathogen. This review aims to integrate recent molecular biology analyses of P. cactorum with the current knowledge of the cellular and genetic basis of its growth, development, and host infection. The goal is to provide a framework for further studies of P. cactorum by highlighting important biological and molecular features, shedding light on the functions of pathogenicity factors, and developing effective control measures.

TAXONOMY

P. cactorum (Leb. & Cohn) Schröeter: kingdom Chromista; phylum Oomycota; class Oomycetes; order Peronosporales; family Peronosporaceae; genus Phytophthora.

HOST RANGE

Infects about 200 plant species in 154 genera representing 54 families. Economically important host plants include strawberry, apple, pear, Panax spp., and walnut.

DISEASE SYMPTOMS

The soilborne pathogen often causes root, stem, collar, crown, and fruit rots, as well as foliar infection, stem canker, and seedling damping off.

Functional characterization of MANNOSE-BINDING LECTIN 1, a G-type lectin gene family member, in response to fungal pathogens of strawberry

The mannose-binding lectin gene MANNOSE-BINDING LECTIN 1 (MBL1) is a member of the G-type lectin family and is involved in defense in strawberry (Fragaria × ananassa). Genome-wide identification of the G-type lectin family was carried out in woodland strawberry, F. vesca, and 133 G-lectin genes were found. Their expression profiles were retrieved from available databases and indicated that many are actively expressed during plant development or interaction with pathogens. We selected MBL1 for further investigation and generated stable transgenic FaMBL1-overexpressing plants of F. ×ananassa to examine the role of this gene in defense. Plants were selected and evaluated for their contents of disease-related phytohormones and their reaction to biotic stresses, and this revealed that jasmonic acid decreased in the overexpressing lines compared with the wild-type (WT). Petioles of the overexpressing lines inoculated with Colletotrichum fioriniae had lower disease incidence than the WT, and leaves of these lines challenged by Botrytis cinerea showed significantly smaller lesion diameters than the WT and higher expression of CLASS II CHITINASE 2-1. Our results indicate that FaMBL1 plays important roles in strawberry response to fungal diseases caused by C. fioriniae and B. cinerea.

Comparative Transcriptome Analysis to Identify Candidate Genes for FaRCg1 Conferring Resistance Against Colletotrichum gloeosporioides in Cultivated Strawberry (Fragaria × ananassa)

Colletotrichum crown rot (CCR) caused by Colletotrichum gloeosporioides is a serious threat to the cultivated strawberry (Fragaria × ananassa). Our previous study reported that a major locus, FaRCg1, increases resistance. However, the genomic structure of FaRCg1 and potential candidate genes associated with the resistance remained unknown. Here, we performed comparative transcriptome analyses of resistant 'Florida Elyana' and susceptible 'Strawberry Festival' after infection and identified candidate genes potentially involved in resistance. In 'Florida Elyana', 6,099 genes were differentially expressed in response to C. gloeosporioides. Gene ontology analysis showed that the most upregulated genes were functionally associated with signaling pathways of plant defense responses. Three genes in the genomic region of FaRCg1 were highly upregulated: a von Willebrand Factor A domain-containing protein, a subtilisin-like protease, and a TIFY 11A-like protein. Subgenome-specific markers developed for the candidate genes were tested with a diverse panel of 219 accessions from University of Florida and North Carolina State University breeding programs. Significant and positive associations were found between the high-resolution melting (HRM) marker genotypes and CCR phenotypes. These newly developed subgenome-specific functional markers for FaRCg1 can facilitate development of resistant varieties through marker-assisted selection.

Control of postharvest fungal diseases in fruits using external application of RNAi

Contamination with a variety of filamentous fungi can cause deterioration of food and agricultural products. Fungal contaminations reduce the quality and the shelf life of fresh fruits and are one of the main causes of economic loss in the global fresh fruit industry. Although chemical fungicides are effective and traditionally used to control postharvest fungal diseases, they are harmful to human health. In this context, use of RNA interference (RNAi)-based fungicides is a promising alternative strategy. Spray-induced gene silencing (SIGS) is an innovative RNAi-based approach for silencing target genes in phytopathogens. This review aims to discuss the recent findings on the use of RNAi-based fungicides to control the postharvest spoilage of fresh fruits. PRACTICAL APPLICATION: Control of postharvest fungal diseases is one of the most important strategies to make food available to consumers longer. In this sense, the external application of RNAi seems to be technologically advantageous and efficient as it helps to maintain the characteristics of plant products. In this sense, this review discussed what is possible to find in the literature regarding this new technology.

Transient Gene Expression is an Effective Experimental Tool for the Research into the Fine Mechanisms of Plant Gene Function: Advantages, Limitations, and Solutions

A large data array on plant gene expression accumulated thanks to comparative omic studies directs the efforts of researchers to the specific or fine effects of the target gene functions and, as a consequence, elaboration of relatively simple and concurrently effective approaches allowing for the insight into the physiological role of gene products. Numerous studies have convincingly demonstrated the efficacy of transient expression strategy for characterization of the plant gene functions. The review goals are (i) to consider the advantages and limitations of different plant systems and methods of transient expression used to find out the role of gene products; (ii) to summarize the current data on the use of the transient expression approaches for the insight into fine mechanisms underlying the gene function; and (iii) to outline the accomplishments in efficient transient expression of plant genes. In general, the review discusses the main and critical steps in each of the methods of transient gene expression in plants; areas of their application; main results obtained using plant objects; their contribution to our knowledge about the fine mechanisms of the plant gene functions underlying plant growth and development; and clarification of the mechanisms regulating complex metabolic pathways.

Dual Transcriptome and Metabolic Analysis of Vitis vinifera cv. Pinot Noir Berry and Botrytis cinerea During Quiescence and Egressed Infection

Botrytis cinerea is an important necrotroph in vineyards. Primary infections are mostly initiated by airborne conidia from overwintered sources around bloom, then the fungus remains quiescent from bloom till maturity and egresses at ripeness. We previously described in detail the process of flower infection and quiescence initiation. Here, we complete the characterization studying the cross-talk between the plant and the fungus during pathogen quiescence and egression by an integrated transcriptomic and metabolic analysis of the host and the pathogen. Flowers from fruiting cuttings of the cv. Pinot Noir were inoculated with a GFP-labeled strain of B. cinerea at full cap-off stage, and molecular analyses were carried out at 4 weeks post inoculation (wpi, fungal quiescent state) and at 12 wpi (fungal pre-egression and egression states). The expressed fungal transcriptome highlighted that the fungus remodels its cell wall to evade plant chitinases besides undergoing basal metabolic activities. Berries responded by differentially regulating genes encoding for different PR proteins and genes involved in monolignol, flavonoid, and stilbenoid biosynthesis pathways. At 12 wpi, the transcriptome of B. cinerea in the pre-egressed samples showed that virulence-related genes were expressed, suggesting infection process was initiated. The egressed B. cinerea expressed almost all virulence and growth related genes that enabled the pathogen to colonize the berries. In response to egression, ripe berries reprogrammed different defense responses, though futile. Examples are activation of membrane localized kinases, stilbene synthases, and other PR proteins related to SA and JA-mediated responses. Our results indicated that hard-green berries defense program was capable to hamper B. cinerea growth. However, ripening associated fruit cell wall self-disassembly together with high humidity created the opportunity for the fungus to egress and cause bunch rot.

The Strawberry FaWRKY1 Transcription Factor Negatively Regulates Resistance to Colletotrichum acutatum in Fruit Upon Infection

Strawberry (Fragaria ×ananassa) is a major food crop worldwide, due to the flavor, aroma and health benefits of the fruit, but its productivity and quality are seriously limited by a large variety of phytopathogens, including Colletotrichum spp. So far, key factors regulating strawberry immune response remain unknown. The FaWRKY1 gene has been previously proposed as an important element mediating defense responses in strawberry to Colletotrichum acutatum. To get further insight into the functional role that FaWRKY1 plays in the defense mechanism, Agrobacterium-mediated transient transformation was used both to silence and overexpress the FaWRKY1 gene in strawberry fruits (Fragaria ×ananassa cv. Primoris), which were later analyzed upon C. acutatum inoculation. Susceptibility tests were performed after pathogen infection comparing the severity of disease between the two agroinfiltrated opposite halves of the same fruit, one half bearing a construct either for FaWRKY1 overexpression or RNAi-mediated silencing and the other half bearing the empty vector, as control. The severity of tissue damage was monitored and found to be visibly reduced at five days after pathogen inoculation in the fruit half where FaWRKY1 was transiently silenced compared to that of the opposite control half and statistical analysis corroborated a significant reduction in disease susceptibility. Contrarily, a similar level of susceptibility was found when FaWRKY1 overexpression and control fruit samples, was compared. These results unravel a negative regulatory role of FaWRKY1 in resistance to the phytopathogenic fungus C. acutatum in strawberry fruit and contrast with the previous role described for this gene in Arabidopsis as positive regulator of resistance against the bacteria Pseudomonas syringae. Based on previous results, a tentative working model for WRKY75 like genes after pathogen infection is proposed and the expression pattern of potential downstream FaWRKY1 target genes was also analyzed in strawberry fruit upon C. acutatum infection. Our results highlight that FaWRKY1 might display different function according to species, plant tissue and/or type of pathogen and underline the intricate FaWRKY1 responsive defense regulatory mechanism taking place in strawberry against this important crop pathogen.

Characterization of an Insecticidal Protein from Withania somnifera Against Lepidopteran and Hemipteran Pest

Lectins are carbohydrate-binding proteins with wide array of functions including plant defense against pathogens and insect pests. In the present study, a putative mannose-binding lectin (WsMBP1) of 1124 bp was isolated from leaves of Withania somnifera. The gene was expressed in E. coli, and the recombinant WsMBP1 with a predicted molecular weight of 31 kDa was tested for its insecticidal properties against Hyblaea puera (Lepidoptera: Hyblaeidae) and Probergrothius sanguinolens (Hemiptera: Pyrrhocoridae). Delay in growth and metamorphosis, decreased larval body mass and increased mortality was recorded in recombinant WsMBP1-fed larvae. Histological studies on the midgut of lectin-treated insects showed disrupted and diffused secretory cells surrounding the gut lumen in larvae of H. puera and P. sanguinolens, implicating its role in disruption of the digestive process and nutrient assimilation in the studied insect pests. The present study indicates that WsMBP1 can act as a potential gene resource in future transformation programs for incorporating insect pest tolerance in susceptible plant genotypes.

Plant Lectins and Lectin Receptor-Like Kinases: How Do They Sense the Outside?

Lectins are fundamental to plant life and have important roles in cell-to-cell communication; development and defence strategies. At the cell surface; lectins are present both as soluble proteins (LecPs) and as chimeric proteins: lectins are then the extracellular domains of receptor-like kinases (LecRLKs) and receptor-like proteins (LecRLPs). In this review; we first describe the domain architectures of proteins harbouring G-type; L-type; LysM and malectin carbohydrate-binding domains. We then focus on the functions of LecPs; LecRLKs and LecRLPs referring to the biological processes they are involved in and to the ligands they recognize. Together; LecPs; LecRLKs and LecRLPs constitute versatile recognition systems at the cell surface contributing to the detection of symbionts and pathogens; and/or involved in monitoring of the cell wall structure and cell growth.

Diverse Colletotrichum species cause anthracnose of tea plants (Camellia sinensis (L.) O. Kuntze) in China

Anthracnose caused by Colletotrichum is one of the most severe diseases that can afflict Camellia sinensis. However, research on the diversity and geographical distribution of Colletotrichum in China remain limited. In this study, 106 Colletotrichum isolates were collected from diseased leaves of Ca. sinensis cultivated in the 15 main tea production provinces in China. Multi-locus phylogenetic analysis coupled with morphological identification showed that the collected isolates belonged to 11 species, including 6 known species (C. camelliae, C. cliviae, C. fioriniae, C. fructicola, C. karstii, and C. siamense), 3 new record species (C. aenigma, C. endophytica, and C. truncatum), 1 novel species (C. wuxiense), and 1 indistinguishable strain, herein described as Colletotrichum sp. Of these species, C. camelliae and C. fructicola were the dominant species causing anthracnose in Ca. sinensis. In addition, our study provided further evidence that phylogenetic analysis using a combination of ApMat and GS sequences can be used to effectively resolve the taxonomic relationships within the C. gloeosporioides species complex. Finally, pathogenicity tests suggested that C. camelliae, C. aenigma, and C. endophytica are more invasive than other species after the inoculation of the leaves of Ca. sinensis.

Unfoldome variation upon plant-pathogen interactions: strawberry infection by Colletotrichum acutatum

Intrinsically disordered proteins (IDPs) are proteins that lack secondary and/or tertiary structure under physiological conditions. These proteins are very abundant in eukaryotic proteomes and play crucial roles in all molecular mechanisms underlying the response to environmental challenges. In plants, different IDPs involved in stress response have been identified and characterized. Nevertheless, a comprehensive evaluation of protein disorder in plant proteomes under abiotic or biotic stresses is not available so far. In the present work the transcriptome dataset of strawberry (Fragaria X ananassa) fruits interacting with the fungal pathogen Colletotrichum acutatum was actualized onto the woodland strawberry (Fragaria vesca) genome. The obtained cDNA sequences were translated into protein sequences, which were subsequently subjected to disorder analysis. The results, providing the first estimation of disorder abundance associated to plant infection, showed that the proteome activated in the strawberry red fruit during the active fungal propagation is remarkably depleted in disorder. On the other hand, in the resistant white fruit, no significant disorder reduction is observed in the proteins expressed in response to fungal infection. Four representative proteins, FvSMP, FvPRKRIP, FvPCD-4 and FvFAM32A-like, predicted as mainly disordered and never experimentally characterized before, were isolated, and the absence of structure was validated at the secondary and tertiary level using circular dichroism and differential scanning fluorimetry. Their quaternary structure was also established using light scattering. The results are discussed considering the role of protein disorder in plant defense.

Transient transformation meets gene function discovery: the strawberry fruit case

Beside the well known nutritional and health benefits, strawberry (FragariaXananassa) crop draws increasing attention as plant model system for the Rosaceae family, due to the short generation time, the rapid in vitro regeneration, and to the availability of the genome sequence of F.Xananassa and F. vesca species. In the last years, the use of high-throughput sequence technologies provided large amounts of molecular information on the genes possibly related to several biological processes of this crop. Nevertheless, the function of most genes or gene products is still poorly understood and needs investigation. Transient transformation technology provides a powerful tool to study gene function in vivo, avoiding difficult drawbacks that typically affect the stable transformation protocols, such as transformation efficiency, transformants selection, and regeneration. In this review we provide an overview of the use of transient expression in the investigation of the function of genes important for strawberry fruit development, defense and nutritional properties. The technical aspects related to an efficient use of this technique are described, and the possible impact and application in strawberry crop improvement are discussed.

Fusarium oxysporum triggers tissue-specific transcriptional reprogramming in Arabidopsis thaliana

Some of the most devastating agricultural diseases are caused by root-infecting pathogens, yet the majority of studies on these interactions to date have focused on the host responses of aerial tissues rather than those belowground. Fusarium oxysporum is a root-infecting pathogen that causes wilt disease on several plant species including Arabidopsis thaliana. To investigate and compare transcriptional changes triggered by F. oxysporum in different Arabidopsis tissues, we infected soil-grown plants with F. oxysporum and subjected root and leaf tissue harvested at early and late timepoints to RNA-seq analyses. At least half of the genes induced or repressed by F. oxysporum showed tissue-specific regulation. Regulators of auxin and ABA signalling, mannose binding lectins and peroxidases showed strong differential expression in root tissue. We demonstrate that ARF2 and PRX33, two genes regulated in the roots, promote susceptibility to F. oxysporum. In the leaves, defensins and genes associated with the response to auxin, cold and senescence were strongly regulated while jasmonate biosynthesis and signalling genes were induced throughout the plant.