Apple proliferation resistance in apomictic rootstocks and its relationship to phytoplasma concentration and simple sequence repeat genotypes

The antioxidant defense system in Chinese jujube is triggered to cope with phytoplasma invasion

Reactive oxygen species (ROS) in plants increase dramatically under pathogen attack, and the antioxidant defense system is then triggered to protect the plant against the ROS. Jujube witches' broom disease (JWB), caused by phytoplasma, is a destructive disease of Chinese jujube. The results of fluorescence-based measurement revealed that ROS were overproduced within jujube leaves after phytoplasma invasion. Furthermore, analysis based on mRNA and metabolite levels revealed that ascorbic acid (AsA) metabolism was strengthened under phytoplasma stress. The high expression of genes involved in the AsA/glutathione (GSH) cycle and thioredoxin (Trx) synthesis in diseased leaves indicated that GSH and Trx actively respond to phytoplasma infection. Moreover, higher activities of enzymatic antioxidants and the upregulated expression of related genes were confirmed in diseased tissues. Both nonenzymatic and enzymatic antioxidants in the host jujube were strongly stimulated to cope with ROS caused by phytoplasma stress. Compared with that in the susceptible variety, the activities of glutathione S-transferase and peroxidase in the resistant variety at the earlier infection stage were higher, indicating that enzymes might be involved in the resistance to phytoplasma. These results highlight the roles of the antioxidant defense system of the host plant in the tolerance to phytoplasma invasion.

Duplex TaqMan Real-Time PCR for Rapid Quantitative Analysis of a Phytoplasma in Its Host Plant without External Standard Curves

The chapter describes a simple quantitative approach to assess phytoplasma load in samples obtained from "Candidatus Phytoplasma mali"-infected apple plants without the use of external standard curves. The assay is based on the simultaneous detection of a gene of the pathogen and a gene of the host plant in a duplex single-tube real-time PCR reaction using TaqMan chemistry. The quantity of the phytoplasma, relative to its host plant, is determined as the difference between the C_T values of the two target genes (ΔC_T). A critical data analysis step, affecting the inter-assay reproducibility between different amplification runs, is the setting of the threshold level, which is achieved by the recurrent analysis of a calibrator sample. The relative quantification procedure allows analyzing 45 DNA samples in duplicates on a 96-well reaction plate, in addition to the control and calibrator samples, and thus contributes to a substantial increase of analysis throughput and decrease of reagent/consumable costs per sample.

Nested and TaqMan® probe based quantitative PCR for the diagnosis of Ca. Phytoplasma in coconut palms

The root (wilt) disease caused by phytoplasma (Ca. Phytoplasma) is one of the major and destructive occurs in coconut gardens of Southern India. As this organism could not be cultured in vitro, the early detection in the palm is very much challenging. Hence, proper early diagnosis and inoculum assessment relay mostly on the molecular techniques namely nested and quantitative PCR (qPCR). So, the present study qPCR assay conjugated with TaqMan^® probe was developed which is a rapid, sensitive method to detect the phytoplasma. For the study, samples from different parts of infected coconut palms viz., spindle leaflets, roots and the insect vector-leaf hopper (Proutista moesta) were collected and assessed by targeting 16S rRNA gene. Further, nested PCR has been carried out using p1/p7 and fU5/rU3 primers and resulted in the amplification product size of 890 bp. From this amplified product, specifically a target of 69 bp from the 16S rRNA gene region has been detected through primers conjugated with Taqman probe in a step one instrument. The results indicated that the concentration of phytoplasma was more in spindle leaflets (8.9 × 10⁵ g of tissue) followed by roots (7.4 × 10⁵ g of tissue). Thus, a qPCR approach for detection and quantification of coconut phytoplasma was more advantageous than other PCR methods in terms of sensitivity and also reduced risk of cross contamination in the samples. Early diagnosis and quantification will pave way for the healthy coconut saplings selection and management under field conditions.

Study on Interactions between the Major Apple Valsa Canker Pathogen Valsa mali and Its Biocontrol Agent Saccharothrix yanglingensis Hhs.015 Using RT-qPCR

The mechanism of biocontrol agent Saccharothrix yanglingensis Hhs.015 action against Valsa mali, a major apple Valsa canker pathogen, was examined using a novel, sensitive (minimum detection limit 100 pg/μL) and reliably RT-qPCR technique. Prior to lesion formation, total concentration of V. mali in the bark showed a significant decrease (p<0.05) after 24 h of Hhs.015 treatment. This was more pronounced at 48 and 96 h post treatment. After lesion formation, levels of V. mali remained constant at the boundary between infected and uninfected bark tissues, although the relative expansion rate of the lesion was significantly reduced (p<0.05). Gene expression levels of endo-polygalacturonase, a marker for fungal pathogenicity, were sharply reduced while host induced resistance callose synthase levels increased significantly (p<0.05) at the boundary bark at 9 d after Hhs.015 treatment. The results showed that biocontrol agent Hhs.015 prevented infection of V. mali by inhibiting pathogen growth, down-regulating pathogenicity factor expression and inducing a high level of host resistance.

Vector-Borne Bacterial Plant Pathogens: Interactions with Hemipteran Insects and Plants

Hemipteran insects are devastating pests of crops due to their wide host range, rapid reproduction, and ability to transmit numerous plant-infecting pathogens as vectors. While the field of plant-virus-vector interactions has flourished in recent years, plant-bacteria-vector interactions remain poorly understood. Leafhoppers and psyllids are by far the most important vectors of bacterial pathogens, yet there are still significant gaps in our understanding of their feeding behavior, salivary secretions, and plant responses as compared to important viral vectors, such as whiteflies and aphids. Even with an incomplete understanding of plant-bacteria-vector interactions, some common themes have emerged: (1) all known vector-borne bacteria share the ability to propagate in the plant and insect host; (2) particular hemipteran families appear to be incapable of transmitting vector-borne bacteria; (3) all known vector-borne bacteria have highly reduced genomes and coding capacity, resulting in host-dependence; and (4) vector-borne bacteria encode proteins that are essential for colonization of specific hosts, though only a few types of proteins have been investigated. Here, we review the current knowledge on important vector-borne bacterial pathogens, including Xylella fastidiosa, Spiroplasma spp., Liberibacter spp., and 'Candidatus Phytoplasma spp.'. We then highlight recent approaches used in the study of vector-borne bacteria. Finally, we discuss the application of this knowledge for control and future directions that will need to be addressed in the field of vector-plant-bacteria interactions.

A gene expression analysis of cell wall biosynthetic genes in Malus x domestica infected by 'Candidatus Phytoplasma mali'

Apple proliferation (AP) represents a serious threat to several fruit-growing areas and is responsible for great economic losses. Several studies have highlighted the key role played by the cell wall in response to pathogen attack. The existence of a cell wall integrity signaling pathway which senses perturbations in the cell wall architecture upon abiotic/biotic stresses and activates specific defence responses has been widely demonstrated in plants. More recently a role played by cell wall-related genes has also been reported in plants infected by phytoplasmas. With the aim of shedding light on the cell wall response to AP disease in the economically relevant fruit-tree Malus × domestica Borkh., we investigated the expression of the cellulose (CesA) and callose synthase (CalS) genes in different organs (i.e., leaves, roots and branch phloem) of healthy and infected symptomatic outdoor-grown trees, sampled over the course of two time points (i.e., spring and autumn 2011), as well as in in vitro micropropagated control and infected plantlets. A strong up-regulation in the expression of cell wall biosynthetic genes was recorded in roots from infected trees. Secondary cell wall CesAs showed up-regulation in the phloem tissue from branches of infected plants, while either a down-regulation of some genes or no major changes were observed in the leaves. Micropropagated plantlets also showed an increase in cell wall-related genes and constitute a useful system for a general assessment of gene expression analysis upon phytoplasma infection. Finally, we also report the presence of several 'knot'-like structures along the roots of infected apple trees and discuss the occurrence of this interesting phenotype in relation to the gene expression results and the modalities of phytoplasma diffusion.

Multiple infection of apple trees by distinct strains of 'Candidatus Phytoplasma mali' and its pathological relevance

Forty-eight apple trees infected by 'Candidatus Phytoplasma mali' were examined using single-strand conformation polymorphism (SSCP) and sequence analysis of a variable hflB gene fragment and the immunodominant membrane protein-encoding imp gene. SSCP analysis of polymerase chain reaction-amplified hflB gene fragments revealed diverse profiles, differing in number and position of the bands. The 'Ca. P. mali' content of a single infected apple tree was termed an accession. Cloning of fragments from accessions that yielded fewer bands resulted in clone populations showing uniform or moderately polymorphic SSCP patterns and largely homogenous nucleotide sequences. In contrast, inserts from accessions yielding more bands were heterogeneous and formed two to four distinct groups of profiles. DNA fragments from such accessions were diverse and clustered distantly when subjected to phylogenetic analysis, mostly as two homogenous groups plus one or a few other sequences. Similar results were obtained upon imp gene examination. The collective data indicate that accessions exhibiting more complex patterns were composed of two or three distinct 'Ca. P. mali' strains. There is evidence that multiple infections are of pathological relevance due to strain interactions leading to shifts in the populations. In triply infected trees of one accession, no specific symptoms were induced by the presence of two of the strains. The rare appearance of pronounced symptoms was associated with a separate strain that possessed a unique SSCP profile and a unique hflB sequence. The two mild strains from this apple accession also induced only mild symptoms on periwinkle and tobacco and occurred specifically in one of these plants.