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RSAGBIP, the primer of R.solani AG-1&AG-3 ITS (ITS1 & 5.8SrRNA) sequence

ITS (ITS1 and 5.8 s rRNA) sequence is a conservative sequence in genomic sequence of Rhizoctonia solani. The primer is designed for LAMP amplification of ITS sequence of R.solani AG-1 and AG-3. With this primer and other primers, LAMP reaction can rapidly amplify ITS sequence of R.solani. The successfully amplified product can be detected by LAMP-LFD detection strip, indicating that the corresponding sample contains the pathogen R. solani, thus achieving a rapid, convenient and sensitive detection of rice sheath blight.

Sequence and Features

LAMP system

LAMP Primers

We use the following primers to amplify ITS sequences of R.solani though LAMP reaction (Table 1).

Optimization of Mg(2+) concentration

The effect of LAMP amplification reaction was affected by Mg(2+) concentration. We set a Mg(2+) concentration gradient (5 mM, 6 mM, 7 mM, 8 mM, 9 mM, 10 mM), selected R.solani AG-3 genome DNA as templates, and used primers (AG-3: RSAGF3, RSAGB3, RSAG-3FIP, RSAGBIP) for LAMP reaction. The amplification was carried out at 65℃ for 60 min. Each amplification product was running in 1.3% agarose gel (Fig. 2). The bands were brightest at a Mg(2+) concentration of 7 mM (Fig. 2B). DNA extracted by each method was successfully amplified to present a ladder-like band within this concentration of Mg(2+). Therefore, LAMP Reactions were performed with 7 mM Mg(2+) in subsequent experiments.

Detection limit and sensitivity of LAMP protocol

We used R.solani AG-3 and AG-1 DNA extracted by different methods as templates, each template was serially diluted in sterile water to amplify its ITS sequence. For PCR reaction, we used primers (AG-3: ITS6 and ITS4; AG-1:ITS1 and ITS4) as well as corresponding reaction temperatures. For LAMP reaction, We used primers (AG-3: RSAGF3, RSAGB3, RSAG-3FIP, RSAGBIP; AG-1:RSAGF3, RSAGB3, RSAG-1FIP, RSAGBIP) with Mg(2+) concentration of 7mM and amplified for 60 mins at 65 ℃.

DNA extracted by easy method III, easy method IV, CTAB protocol and TPS protocol were amplified by ordinary PCR and then detected by electrophoresis (Fig. 3-6). The detection limits of them were 1.1619ng, 165.54pg, 676.67fg, and 4.8918fg, respectively (Fig. 3). Our PCR system had the highest sensitivity to DNA extracted by TPS protocol (Fig. 3D). After LAMP amplification, Ladder-like bands can be observed even in samples diluted to the lowest concentration (Fig. 5). In addition, the LAMP reaction was more sensitive than the PCR reaction.

Implementation of LAMP in LFD for the detection of R. solani

DNA crude extraction of isolated rice leaves infected with R.solani

Before LAMP-LFD detection, we collected 10 groups of isolated experimental rice leaves infected with R.solani, extracted DNA from each group of leaves by crude extraction method (Fig. 7), weighed the leaves and measured the extracted DNA concentration (Table 3).

LAMP-LFD test

We used crude extraction of DNA from isolated rice leaves infected with R.solani, amplified DNA templates with biotin-labeled primers(each amplification product was running in 1.3% agarose gel), incubated the amplification products with probes labeled with fluorescein amidite, then loaded final products onto LFD (Fig. 8). For the isolated diseased rice leaves extracted in this experiment, ladders-like bands were found in experimental group CE1 and CE2. No ladders-like bands were found in other experimental groups, which may show rice DNA and primer dimers, and no ladders-like bands were found in the control group (Trichoderma DNA extract) (Fig. 8A). However, on lateral flow devices (LFD), all the experimental group showed positive results, and even a control group showed a false positive result (Fig. 8B).

It is speculated that cross-contamination of laboratory samples has led to false positives. This can be avoided if researchers bring equipment to the field for testing. If LAMP-LFD test is performed in the laboratory, researchers should pay attention to conducting the experiment in different zones in order to avoid contamination among DNA samples. Final results should be analyzed combined with the control group, gel imaging, and LFD imaging. For instance, for the DNA of isolated rice leaves extracted this time, both CE1 and CE2 experimental groups showed ladders-like bands, and the LFD showed positive results, indicating that the corresponding rice leaves had been infected with R.solani.

References

Jaimin S. Patel, Mary S. Brennan, Aftab Khan & Gul Shad Ali (2015) Implementation of loop-mediated isothermal amplification methods in lateral flow devices for the detection of Rhizoctonia solani, Canadian Journal of Plant Pathology, 37:1, 118-129, DOI: 10.1080/07060661.2014.996610