3.1. LAMP-LFT Detection of Plant Viruses
LAMP technique exhibits superior specificity, efficiency, and rapidity in DNA amplification under isothermal conditions, utilizing a DNA polymerase to amplify targeted DNA strands [
8]. Unlike polymerase chain reaction (PCR), which necessitates cycling through various temperature steps, LAMP functions at a constant temperature, typically between 60°C and 65°C. This characteristic obviates the need for a thermal cycler, simplifying equipment requirements and rendering it well-suited for field applications, point-of-care diagnostics, and resource-limited settings [
9,
10].
The LAMP reaction employs four to six primers targeting multiple regions on the DNA, comprising two outer primers (forward and backward), two inner primers (forward inner and backward inner), and optionally, loop primers that expedite the reaction. In the LAMP reaction, the thermophilic enzyme Bst DNA polymerase, with strand displacement activity, synthesizes DNA at a constant temperature through a self-cycling amplification process. In this process, the displaced strand serves as a template for further amplification, leading to the formation of loop structures that contribute to exponential amplification of the target DNA [
9,
10]. The endpoint of the reaction can be visually detected through turbidity, fluorescence, or color change, depending on the chosen detection method.
Over the years, LAMP has undergone further development, integrating with other molecular approaches for plant pathogens diagnosis [
1,
2,
8]. As an illustrative case, cassava brown streak virus (CBSV) and ugandan cassava brown streak virus (UCBSV) are causative agents of cassava brown streak disease in East Africa. These viruses, belonging to the genus
Ipomovirus, family
Potyviridae, possess a positive-sense (+), single-stranded (ss) RNA genome, leading to yield losses and reduced marketability of cassava roots [
11]. Tomlinson et al. devised primers for the rapid detection of these viruses through reverse transcription loop-mediated isothermal amplification (RT-LAMP). To label target sequences, markers such as fluorescein isothiocyanate (FITC), biotin, or digoxigenin (DIG) were incorporated with primers. RT-LAMP achieved amplification within 40 minutes, and the products were detectable using lateral flow devices containing antibodies specific to the incorporated labels [
11].
A similar strategy was employed for the detection of tobacco rattle virus (TRV), another plant virus with a +ssRNA genome. Edgu et al. developed and optimized a mini-LAMP-lateral flow device (LFD) approach for the sensitive and specific detection of TRV in potatoes. This approach offers an economical and efficient platform for disease management in potato breeding and cultivation [
12]. Notably, viral RNA purification was circumvented, and the filtered supernatant of incubation samples was diluted 1:100 with water and directly used for amplification, simplifying sample processing without the need for sophisticated laboratory equipment [
12].
Recently, Lu et al. identified a novel member of the genus
Badnavirus in the family
Caulimoviridae, named Chinaberry tree badnavirus 1 (ChTBV1), which harbors a single molecule of non-covalently closed circular double-stranded (ds) DNA in the Chinaberry tree. They developed a LAMP assay for viral detection and adapted it for rapid visualization of results using a lateral flow dipstick chromatographic detection method [
13].
3.2. RPA-LFT Detection of Plant Viruses
RPA, an additional molecular biology technique utilized for the isothermal amplification of DNA [
14], shares similarities with LAMP in its capacity to facilitate DNA amplification at a constant temperature. This feature positions RPA as well-suited for field applications and point-of-care diagnostics, addressing challenges associated with maintaining precise temperature control [
15,
16].
The RPA process encompasses several pivotal components, including recombinase enzymes such as recombinase A, RecA, or recombinase UvsX. These enzymes play a crucial role in facilitating strand-exchange reactions and promoting the invasion of primers into the target DNA. Additionally, the Single-stranded DNA-binding Protein (SSB) is indispensable for stabilizing single-stranded DNA regions, preventing reannealing. Two primers, each possessing homologous regions to the target DNA, recognize specific sequences and bind to opposite strands. The DNA polymerase, featuring strand displacement activity, extends the primers and synthesizes new DNA strands. Significantly, the reaction is conducted isothermally at a constant temperature, typically ranging between 37°C and 42°C.
In contrast to the LAMP-LFT amplification system, which necessitates 4~6 primers, the RPA-LFT amplification system only requires three primers for amplifying a target gene—forward and reverse primers and probes. The outer primer pair can generate specific and cloned amplification products, ensuring the accuracy of the amplification target. This streamlined primer requirement in the RPA-LFT system reduces the complexity of primer design, enhances the accuracy of detection, and lowers the detection cost compared to the more intricate primer design of the LAMP-LFT system.
RPA finds diverse applications, including molecular diagnostics, environmental monitoring, and field-based pathogen detection [
15,
17]. Its isothermal nature renders it suitable for resource-limited settings, and the relatively short reaction time positions it as a valuable tool for rapid DNA amplification. By incorporating specially modified probe primers into the amplification system and collaborating with LFT, RPA-LFT has emerged as a primary method for diagnosing plant viruses with varied genome types, encompassing +ssRNA, negative sense (-) ssRNA, ambisense RNA (±RNA), dsRNA, ssDNA, dsDNA, and even viroids with naked circle RNA [
1,
6,
18,
19,
20,
21,
22,
23,
24,
25,
26,
27,
28,
29,
30] (
Table 1).
Little cherry virus 2 (LChV2), belonging to the genus
Ampelovirus in the family
Closteroviridae with a +ssRNA genome, causes little cherry disease (LCD) in sweet cherries (
Prunus avium) globally. Early detection of LChV2 is crucial for controlling LCD [
31]. Mekuria et al. devised an effective diagnostic method based on RPA-LFT. They developed a simple, fast, and specific RT-RPA method utilizing LChV2 coat protein-specific primers and probes, exhibiting comparable sensitivity to RT-PCR from crude extracts. The terminally labeled amplicons were detected using a high-affinity lateral flow strip [
31]. A similar approach was applied for the detection of another
Closteroviridae family member, citrus tristeza virus (CTV), in the genus
Closterovirus [
30]. This method proves to be a powerful tool for early-stage virus detection in field samples [
30,
31].
Plum pox virus (PPV), a member of the genus
Potyvirus in the family
Potyviridae with a +ssRNA genome, causes the devastating plum pox or Sharka disease in stone fruit trees. Zhang et al. developed an efficient RPA-LFT method for PPV detection, significantly reducing the diagnostic time to as little as 20 minutes for the entire process from sample preparation to result. This innovation streamlines diagnosis, facilitating both laboratory and field applications [
18].
Alfalfa mosaic virus (AMV), a plant virus belonging to the genus
Alfamovirus in the family
Bromoviridae with a +ssRNA genome, affects a wide range of plant species worldwide. Ivanov et al. compared two methods generating labeled RPA amplicons of AMV and found that the RPA-LFT assay based on primer labeling detected 103 copies of RNA in 30 minutes with a half-maximal binding concentration 22 times lower than the probe-dependent RPA-LFT. This indicates the simplicity and efficiency of labeling primers for RPA-LFT in viral diagnosis [
6].
Recently, several other +ssRNA genome viruses have been targeted for detection by RPA-LFT, including cymbidium mosaic virus (CymMV) in the genus
Potexvirus of the family
Alphaflexiviridae, barley yellow dwarf virus (BYDV) in the genus
Polerovirus of the family
Solemoviridae, cowpea mild mottle virus (CPMMV) in the genus
Carlavirus of the family
Betaflexiviridae, actinidia chlorotic ringspot-associated virus (AcCRaV) in the genus
Emaravirus of the family
Fimoviridae, and bean pod mottle virus (BPMV) in the genus
Comovirus of the family
Secoviridae [
21,
22,
23,
26,
28]. The developed RPA-LFT assay for these viruses exhibited 100 times more sensitivity than conventional reverse transcription polymerase chain reaction (RT-PCR), providing a simple, rapid, sensitive, and reliable method for viral diagnosis in the field.
Tomato spotted wilt virus (TSWV) and tomato chlorotic spot virus (TCSV), members of the genus
Orthotospovirus in the family
Tospoviridae with ambisense RNA genomes, cause significant yield loss in ornamental and vegetable crops worldwide. RPA-LFT assays for both viruses have been developed [
27,
32]. Furthermore, in the RPA reaction for TCSV, crude RNAs in the tube are incubated in the palm of the hand to generate sufficient heat for amplification. The detection limit is approximately 6 pg/µL of total RNA from samples, providing an equipment-free, body-heat-mediated RT-RPA-LFA technique [
27]. Correspondingly, methods based on RPA-LFT were developed for the detection of plant viruses with dsRNA, ssDNA, and dsDNA genomes [
19,
20,
24,
25].
Moreover, a multiplex assay based on RPA-LFT to detect two or more kinds of plant viruses has also been developed. Ivanov et al. successfully applied this strategy for the detection of three priority potato RNA viruses: potato virus Y (PVY), potato virus S (PVS), and potato leafroll virus (PLRV). The total assay time is 30 minutes. The multiplex RPA-LFT demonstrated the capability to detect at least 4 ng of PVY per gram of plant leaves, 0.04 ng/g for PVS, and 0.04 ng/g for PLRV [
33]. Multiplex assays capable of simultaneously detecting multiple plant viruses provide a more comprehensive diagnostic approach, especially in regions where multiple viral pathogens may be prevalent.
3.3. RAA-LFT Detection of Plant Viruses
RAA shares a fundamental principle with RPA. The distinguishing factor between these two methods lies in the source of the recombinase. RPA utilizes recombinase from the T4 phage, while RAA employs recombinases from diverse sources such as bacteria and fungi. RAA has been applied to detect several viruses, showcasing its versatility in molecular diagnostics. Maize chlorotic mottle virus (MCMV) has emerged as a significant threat to maize production globally, causing maize lethal necrosis in regions of East Africa, South America, and Asia [
34,
35]. Duan et al. combined RAA with a CRISPR/Cas12a (Clustered Regularly Interspaced Short Palindromic Repeats and CRISPR-associated proteins 12a)-based visual nucleic acid detection system for MCMV, achieving a rapid and sensitive process completed within 45 minutes [
36]. Expanding on this strategy, Wang et al. developed a detection system targeting sorghum mosaic virus and rice stripe mosaic virus [
37]. Recently, Zhao et al. integrated LFT with RAA, creating a visible system for MCMV detection [
38]. Tomato brown rugose fruit virus (ToBRFV), a member of the
Tobamovirus genus, has recently become pandemic in tomato and pepper production areas worldwide. Cao et al. devised an RAA-LFT for field detection of ToBRFV with high sensitivity, demonstrating a detection limit of 2.1 × 101 copies/50-μL reaction [
39]. Subsequently, Zhao et al. combined RAA and CRISPR/Cas12a with LFT, enabling simultaneous detection of four tobamoviruses—pepper mild mottle virus (PMMoV), ToBRFV, tomato mosaic virus (ToMV), and tomato mottle mosaic virus (ToMMV) [
40].
3.4. CRISPR-CAS System-Integrated LFT Detection of Plant Viruses
The CRISPR-CAS technology, renowned for its proficiency in genome editing, has found application in the INAA-LFT for plant virus detection, augmenting both specificity and sensitivity. In this context, CRISPR-CAS systems function to recognize and bind to specific amplified sequences generated by PCR, LAMP, or RPA/RAA. The CRISPR RNA (guide RNA) is meticulously designed to complement the target sequence, and the CAS protein (such as Cas12 or Cas13) undergoes activation upon binding, leading to the cleavage of the target sequence. The activation is concomitant with a detectable signal, often manifesting as a fluorescence signal. The presence of the target sequence can be identified by interpreting this signal. The integration of CRISPR-Cas with RPA confers a robust tool for nucleic acid detection, enhancing specificity through the highly precise binding of the CRISPR-Cas system to target sequences.
Marques et al. harnessed CRISPR-Cas12a and CRISPR-Cas13a/d systems for detecting viral DNA amplicons generated by PCR or isothermal amplification, focusing on three RNA viruses: tobacco mosaic virus (TMV), tobacco etch virus (TEV), and potato virus X (PVX). They innovatively adapted the detection system to circumvent the costly RNA purification step and achieve a visible readout through lateral flow strips, enabling rapid viral diagnostics within a timeframe of half an hour [
41]. Addressing key viruses in rice, rice stripe virus (RSV) and rice black-streaked dwarf virus (RBSDV), Zhu et al. devised a CRISPR/Cas12a-assisted LAMP-LFT system to detect these viruses along with the bacterial pathogen
Xanthomonas oryzae pv.
oryzae (Xoo). The heightened sensitivity of this system reached as low as 9 or 3 copies [
42]. In the LAMP-LFT assay for MCMV detection, assisted by CRISPR-Cas12a, the detection limit achieved an impressive low of 2.5 copies of the coat protein (CP) gene of MCMV [
38].
Results above indicate that that the assistance of CRISPR-CAS in the detection system improve the detection limit to a very low level. Meanwhile, it should be noted that the specific implementation of CRISPR-CAS in detection may vary based on the target application and the desired detection method. It requires continued exploration and optimization of these technologies for a variety of diagnostic and research purposes.