Phytoplasma diseases of plants: molecular diagnostics and way forward

Parasitic Plants-Potential Vectors of Phytopathogens

Parasitic plants represent a peculiar group of semi- or fully heterotrophic plants, possessing the ability to extract water, minerals, and organic compounds from other plants. All parasitic plants, either root or stem, hemi- or holoparasitic, establish a vascular connection with their host plants through a highly specialized organ called haustoria. Apart from being the organ responsible for nutrient extraction, the haustorial connection is also a highway for various macromolecules, including DNA, proteins, and, apparently, phytopathogens. At least some parasitic plants are considered significant agricultural pests, contributing to enormous yield losses worldwide. Their negative effect is mainly direct, by the exhaustion of host plant fitness and decreasing growth and seed/fruit formation. However, they may pose an additional threat to agriculture by promoting the trans-species dispersion of various pathogens. The current review aims to summarize the available information and to raise awareness of this less-explored problem. We further explore the suitability of certain phytopathogens to serve as specific and efficient methods of control of parasitic plants, as well as methods for control of the phytopathogens.

Use of the 23S rRNA gene as a target template in the universal loop-mediated isothermal amplification (LAMP) of genomic DNA from phytoplasmas

Plant-pathogenic bacteria cause numerous diseases in host plants and can result in serious damage. Timely and accurate diagnostic techniques are, therefore, crucial. While advances in molecular techniques have led to diagnostic systems able to distinguish known plant pathogens at the species or strain level, systems covering larger categories are mostly lacking. In this study, a specific and universal LAMP-based diagnostic system was developed for phytoplasmas, a large group of insect-borne plant-pathogenic bacteria that cause significant agricultural losses worldwide. Targeting the 23S rRNA gene of phytoplasma, the newly designed primer set CaPU23S-4 detected 31 'Candidatus Phytoplasma' tested within 30 min. This primer set also showed high specificity, without false-positive results for other bacteria (including close relatives of phytoplasmas) or healthy plants. The detection sensitivity was ~10,000 times higher than that of PCR methods for phytoplasma detection. A simple, rapid method of DNA extraction, by boiling phytoplasma-infected tissues, was developed as well. When used together with the universal LAMP assay, it enabled the prompt and accurate detection of phytoplasmas from plants and insects. The results demonstrate the potential of the 23S rRNA gene as a versatile target for the LAMP-based universal detection of bacteria at the genus level and provide a novel avenue for exploring this gene as molecular marker for phytoplasma presence detection.IMPORTANCEPhytoplasmas are associated with economically important diseases in crops worldwide, including lethal yellowing of coconut palm, "flavescence dorée" and "bois noir" of grapevine, X-disease in stone fruits, and white leaf and grassy shoot in sugarcane. Numerous LAMP-based diagnostic assays, mostly targeting the 16S rRNA gene, have been reported for phytoplasmas. However, these assays can only detect a limited number of 'Candidatus Phytoplasma' species, whereas the genus includes at least 50 of these species. In this study, a universal, specific, and rapid diagnostic system was developed that can detect all provisionally classified phytoplasmas within 1 h by combining the LAMP technique targeting the 23S rRNA gene with a simple method for DNA extraction. This diagnostic system will facilitate the on-site detection of phytoplasmas and may aid in the discovery of new phytoplasma-associated diseases and putative insect vectors, irrespective of the availability of infrastructure and experimental resources.

Phytoplasma: A plant pathogen that cannot be ignored in agricultural production-Research progress and outlook

Phytoplasmas are phloem-restricted plant-pathogenic bacteria transmitted by insects. They cause diseases in a wide range of host plants, resulting in significant economic and ecological losses worldwide. Research on phytoplasmas has a long history, with significant progress being made in the past 30 years. Notably, with the rapid development of phytoplasma research, scientists have identified the primary agents involved in phytoplasma transmission, established classification and detection systems for phytoplasmas, and 243 genomes have been sequenced and assembled completely or to draft quality. Multiple possible phytoplasma effectors have been investigated, elucidating the molecular mechanisms by which phytoplasmas manipulate their hosts. This review summarizes recent advances in phytoplasma research, including identification techniques, host range studies, whole- or draft-genome sequencing, effector pathogenesis and disease control methods. Additionally, future research directions in the field of phytoplasma research are discussed.

Current and emerging trends in techniques for plant pathogen detection

Plant pathogenic microorganisms cause substantial yield losses in several economically important crops, resulting in economic and social adversity. The spread of such plant pathogens and the emergence of new diseases is facilitated by human practices such as monoculture farming and global trade. Therefore, the early detection and identification of pathogens is of utmost importance to reduce the associated agricultural losses. In this review, techniques that are currently available to detect plant pathogens are discussed, including culture-based, PCR-based, sequencing-based, and immunology-based techniques. Their working principles are explained, followed by an overview of the main advantages and disadvantages, and examples of their use in plant pathogen detection. In addition to the more conventional and commonly used techniques, we also point to some recent evolutions in the field of plant pathogen detection. The potential use of point-of-care devices, including biosensors, have gained in popularity. These devices can provide fast analysis, are easy to use, and most importantly can be used for on-site diagnosis, allowing the farmers to take rapid disease management decisions.

One-step Reverse Transcription-LoopMediated Isothermal Amplification (RT-LAMP) for closed-tube colorimetric detection of banana bract mosaic virus in Banana (Musa spp.)

The banana bract mosaic virus (BBrMV) is a major virus affecting bananas and plantains. Banana being propagated vegetatively, there arises a high risk of virus transmission through planting materials. Available molecular detection technique like the Reverse Transcriptase Polymerase Chain Reaction needs post-amplification sample handling, predisposing to sample cross contamination. A one-step Reverse Transcription-LoopMediated Isothermal Amplification (RT-LAMP) assay coupled with colorimetric detection was optimised for easy and quick detection of BBrMV in banana. The viral coat protein gene was amplified under isothermal conditions at 65 ºC. The RT-LAMP assay was optimised with respect to concentrations of MgSO4, dNTP, Bst polymerase enzyme and HNB dye. The total RNA purified from symptomatic samples was directly amplified under isothermal conditions by including 100 U M-MLV reverse transcriptase and 20 U RNasin® plus RNase inhibitor in the reaction. With the addition of 120 µM of Hydroxy Naphthol Blue (HNB) dye in the RT-LAMP reaction, the BBrMV-positive samples had a colour change from violet to sky blue after the reaction. The RT-LAMP assay detected BBrMV in 0.1 pg of total RNA isolated from symptomatic plants. Molecular characterisation of RT-LAMP products was done using restriction profiling and sequence analysis. The RT-LAMP assay was validated using field-collected banana leaf samples. The assay successfully detected the virus from symptomatic samples while the healthy samples showed no amplification. Samples sourced from banana plants with symptoms of banana bunchy top virus, banana streak virus and cucumber mosaic virus tested negative in the RT-LAMP assay, thus ensuring the specificity of the assay.

Development of a Specific Nested PCR Assay for the Detection of 16SrI Group Phytoplasmas Associated with Sisal Purple Leafroll Disease in Sisal Plants and Mealybugs

Sisal purple leafroll disease (SPLD) is currently the most destructive disease affecting sisal in China, yet its aetiology remains unclear. In our previous research, it was verified to be associated with phytoplasmas, and nested PCR based on the 16S rRNA gene using universal primers R16mF2/R16mR1 followed by R16F2n/R16R2 was confirmed as the most effective molecular method for the detection of phytoplasmas associated with SPLD (SPLDaP). However, the method has a shortcoming of inaccuracy, for it could produce false positive results. To further manage the disease, accurate detection is needed. In this study, we developed a specific nested PCR assay using universal primers R16F2n/R16R2, followed by a set of primers designed on 16Sr gene sequences amplified from SPLDaP, nontarget bacteria from sisal plants, and other phytoplasma subgroups or groups. This established method is accurate, specific, and effective for detection of 16SrI group phytoplasma in sisal, and its sensitivity is up to 10 fg/μL of total DNA. It also minimized the false positive problem of nested PCR using universal primers R16mF2/R16mR1 followed by R16F2n/R16R2. This method was further used to verify the presence of phytoplasma in Dysmicoccusneobrevipes, and the results showed that D. neobrevipes could be infected by SPLDaP and thus could be a candidate for vector transmission assays.

Detection and Multigene Typing of 'Candidatus Phytoplasma solani'-Related Strains Infecting Tomato and Potato Plants in Different Regions of Turkey

‘Candidatus Phytoplasma solani’ (‘Ca. P. solani’) is a crop pathogen that is a member of the 16SrXII-A ribosomal subgroup. It is also known as stolbur phytoplasma and causes yield losses in several important crops, especially in Solanaceous crops. Different strains of the pathogen are regularly reported all over the world, particularly in the Mediterranean region. In this study, the determination of genetic diversity for the pathogen infecting tomatoes and potatoes was carried out by using multilocus sequence typing analysis for the Tuf, SecY, and Vmp1 genes to gain insight into the epidemiology of ‘Ca. P. solani’ in Turkey. Genetic diversity of the phytoplasmas was investigated by sequence-based phylogenetic analyses and in silico RFLP analysis of related genes. It was determined that all ‘Ca. P. solani’-related strains infecting tomatoes and potatoes were tuf-b, which is linked to field bindweed (Convolvulus arvensis L.). Tomato or potato-infecting ‘Ca. P. solani’-related strains showed similarities with each other; however, the isolates collected from different plants showed genetic differences in terms of the SecY gene. This study indicates that the highest genetic variability of collected samples was found in the Vmp1 gene. RsaI-RFLP analysis of TYPH10F/R amplicons showed that potato-infecting ‘Ca. P. solani’-related strains were found to be similar to some existing V types. However, the V-type of tomato-infecting isolates is not similar to any previously reported V-type. The results indicate that there could be an important genetic diversity of ‘Ca. P. solani’-related phytoplasmas in Turkey. This could indicate various ways in which the pathogen has adapted to the two host plants as a consequence of the various Vmp1 gene rearrangements seen in these two plant hosts. Obtained results also indicate that the epidemiology of ‘Ca. P. solani’-related phytoplasmas in the tomato and potato agroecosystem may be better understood with the use of molecular data on the complex of vmp-types.

Colorimetric detection platform for banana bunchy top virus (BBTV) based on closed-tube loop mediated isothermal amplification (LAMP) assay

A colorimetric closed-tube Loop mediated isothermal amplification (LAMP) assay was developed for rapid and sensitive detection of banana bunchy top virus (BBTV) from leaf and sucker tissues of infected banana plants. Six LAMP primers were designed targeting BBTV coat protein gene. Isothermal amplification was set at 65 °C and end point detection made by including hydroxy naphthol blue dye in the reaction where the positive samples showed colour change from violet to sky blue. Molecular characterization of LAMP amplicon was made with restriction digestion and sequencing. Restriction digestion with Sau3AI having single cut site within the LAMP internal primer flanking region yielded two fragments of expected size. Sequence analysis confirmed that amplification corresponded to BBTV coat protein gene. Comparison of LAMP assay with conventional PCR showed that LAMP assay was 1000 times more sensitive than conventional PCR in BBTV detection with a detection limit of 0.05 ng total DNA per reaction.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13337-022-00784-w.

Volatile Organic Compounds and Physiological Parameters as Markers of Potato (Solanum tuberosum L.) Infection with Phytopathogens

The feasibility of early disease detection in potato seeds storage monitoring of volatile organic compounds (VOCs) and plant physiological markers was evaluated using 10 fungal and bacterial pathogens of potato in laboratory-scale experiments. Data analysis of HS-SPME-GC-MS revealed 130 compounds released from infected potatoes, including sesquiterpenes, dimethyl disulfide, 1,2,4-trimethylbenzene, 2,6,11-trimethyldodecane, benzothiazole, 3-octanol, and 2-butanol, which may have been associated with the activity of Fusarium sambucinum, Alternaria tenuissima and Pectobacterium carotovorum. In turn, acetic acid was detected in all infected samples. The criteria of selection for volatiles for possible use as incipient disease indicators were discussed in terms of potato physiology. The established physiological markers proved to demonstrate a negative effect of phytopathogens infecting seed potatoes not only on the kinetics of stem and root growth and the development of the entire root system, but also on gas exchange, chlorophyll content in leaves, and yield. The negative effect of phytopathogens on plant growth was dependent on the time of planting after infection. The research also showed different usefulness of VOCs and physiological markers as the indicators of the toxic effect of inoculated phytopathogens at different stages of plant development and their individual organs.

Cassane diterpenoids from the aerial parts of Caesalpinia pulcherrima and their antibacterial and anti-glioblastoma activity

Sixteen cassane diterpenoids (CAs), including four undescribed lactam-type, four unreported lactone-type, along with eight known ones, were isolated from the aerial parts of Caesalpinia pulcherrima (L.) Sw. Their structures were characterized by comprehensive spectroscopic analyses (including NMR and HRESIMS). The absolute configuration of pulcherritam A was finally established by single-crystal X-ray diffraction with Cu Kα radiation. Notably, pulcherritam s A-D were elucidated as a group of rare CAs bearing an α, β-unsaturated γ-lactam ring rather than a typical lactone moiety. Almost all compounds were examined for their antibacterial. The results reveal that pulcherritam H exhibited significant antibacterial activities against Bacillus cereus, Staphylococcus aureus, as well as Pseudomonas syringae pv. actinidae (Psa) with the MIC from 6.25 to 12.5 μM, while pulcherritams A and C displayed potent antibacterial activities against methicillin-resistant Staphylococcus aureus (MRSA). Then, all isolates were evaluated for their anti-glioblastoma activities. Pulcherritam A and Pulcherrimin G illustrated moderate inhibitory activity against glioblastoma multiforme (GBM) U87MG cell, and the other compounds did not show obvious inhibitory activity against GBM U87MG cell. Furthermore, the preliminary structure-activity relationship and their biosynthetic pathway were also discussed.