DNA-Based Technologies for Ensuring Seed Purity

Seed purity is the core index to measure seed quality, which directly affects crop yield stability and quality uniformity, and is also the key link to maintain seed market order and ensure food security. Traditional purity identification relies on phenotypic observation and field planting, which is greatly disturbed by the environment and has a long period, so it is difficult to meet the needs of the modern seed industry for efficient detection. The molecular detection technology based on DNA breaks through the phenotypic limitation and realizes accurate and rapid identification of seed purity by analyzing the polymorphism at the genome level.

From SSR markers amplified by PCR to Qualcomm SNP chips, from targeted sequencing to whole genome scanning, DNA technology has become the mainstream means of seed purity detection because of its high specificity, strong stability, and Qualcomm. It can not only identify 0.1% or even lower proportions of miscellaneous plants, but also distinguish varieties with very close genetic backgrounds, which provides a scientific basis for seed production, market supervision, and international trade, and promotes the quality control of the seed industry from empirical judgment to the molecular precision era.

The article discusses DNA-based technologies such as SSR, SNP, DNA barcoding, PCR-based methods, and NGS in ensuring seed purity, including their principles, applications, and cost-benefit analysis.

Role of Molecular Markers in Seed Identification

Molecular marker technology provides an accurate genetic basis for seed variety identification by detecting polymorphism at the genome level. Among them, simple sequence repeat (SSR) and single-nucleotide polymorphism (SNP) are the two most widely used molecular markers, which have their emphasis on technical principles and application scenarios, and together constitute the core tools of seed genetic testing.

SSR Marker

SSR markers, also known as microsatellite markers, are tandem repeats consisting of 1-6 bases, which are widely distributed in eukaryotic genomes. Its polymorphism comes from the difference in the number of repeat units, which can be detected by PCR amplification and electrophoresis separation. The technological process of SSR markers includes: genomic DNA extraction, specific primer design, PCR amplification, electrophoretic separation, and band analysis.

The core advantages of this technology are reflected in three aspects:

• First, the polymorphism is high, and more than 10 alleles can be detected by a single SSR locus, which can effectively distinguish varieties with similar genetic backgrounds
• Second, co-dominant inheritance can accurately identify homozygotes and heterozygotes, which is very important for hybrid purity detection
• Thirdly, it has good repeatability and strong primer specificity, and the consistency of test results in different laboratories can reach more than 95%

However, SSR markers also have limitations:

• Primer development depends on known genome sequences, which is costly for non-model crops
• The detection flux is low, and single-round PCR can only analyze 1-2 loci, which is difficult to meet the needs of large-scale sample screening

Correlation between genetic distance based on SSRs and SNPs (Hamblin et al., 2007)

SNP Marker

SNP marker refers to the variation of a single nucleotide (A/T/C/G substitution) at the genome level, and its density can reach 1 per 100-300 bases, forming a large number of polymorphic sites in the crop genome. SNP detection is mainly realized by gene chip or high-throughput sequencing technology, which can analyze thousands to millions of loci at one time, and has the remarkable characteristics of high quantity and automation.

The technical advantages of SNP markers include:

• First, high density can provide a more detailed genetic map, for example, there are about 5 million SNP loci in maize genome, which can accurately distinguish inbred lines from hybrids
• Second, the stability is strong, the variation of a single base is not affected by the environment, and the repeatability of the detection results is close to 100%
• Thirdly, it is suitable for automatic analysis, and can realize batch processing of samples by combining bioinformatics tools

However, SNP markers also have some shortcomings:

• Two alleles are dominant, and the information content is relatively low compared with SSR
• The initial investment of detection equipment is high, and the cost of a gene chip is about 200-500 yuan per sample, which limits its application in primary laboratories

DNA Barcoding for Seed Authentication

DNA barcode technology can quickly identify species or varieties by analyzing a conserved standard sequence in the genome, and its principle is similar to the unique identification function. This technology provides a standardized and Qualcomm solution for seed identification, especially suitable for cross-laboratory and cross-regional collaborative detection.

Technical Principle and Core Gene Selection

The core of the DNA barcode is to select gene fragments with interspecific specificity and conservation within species. In plants, commonly used barcode regions include rbcL (ribulose-1,5-diphosphate carboxylase subunit gene) of the chloroplast genome, matK (mature enzyme K gene), and ITS (internal transcribed spacer) of the nuclear genome. These fragments are of moderate length (500-800bp), which is convenient for PCR amplification and sequencing, and the evolution rate is moderate, which can effectively distinguish different species and varieties.

For crop variety identification, researchers usually choose a multi-gene combination strategy. For example, the combination of rbcL+matK+ITS in rice identification can improve the variety resolution to over 95%. In maize, the intron sequence of the mitochondrial gene nad1 was added to distinguish cytoplasmic male sterile lines from maintainer lines.

Number of publications per biennium, in the last 20 years, focusing on rice authentication through DNA-based methods (Vieira et al., 2022)

Identification Process and Database Construction

The identification process of DNA barcode includes: sample DNA extraction, PCR amplification of the barcode region, Sanger sequencing, sequence comparison, and species matching. The key link is to establish a perfect reference sequence database. For example, the GenBank database of the National Biotechnology Information Center (NCBI) of the United States has included more than 1 million crop barcode sequences.

In practical application, the sequence of the sample to be tested is compared with the sequence of the known variety in the database. When the matching degree exceeds 98% and the genetic distance is less than 0.02, it can be judged as the same variety. This standardized process reduces human error and makes the test results of different laboratories comparable.

Case for Seed Identification

In the identification of rice seeds, DNA barcode technology shows significant advantages. For the identification of indica rice and japonica rice subspecies, an accuracy rate of 99% can be achieved by analyzing three SNP loci of the matK gene. For F1 hybrid rice seeds, false hybrids can be quickly detected by using parent-specific barcode sequences, and the purity identification efficiency is 20 times higher than that of traditional planting methods.

In maize seed detection, a base variation (C→T) of the rbcL gene can be used as a distinguishing marker between sweet maize and common maize. Combined with the polymorphism of the nad1 gene, the variety authenticity and cytoplasm type can be identified at the same time, which provides a rapid detection method for seed quarantine.

The homology analysis for 11 ITS genes from 6 Gleditsia species (Zhao et al., 2024)

PCR-Based Methods in Seed Identification

Polymerase chain reaction (PCR) technology is the basis of molecular marker detection, and its derived quantitative PCR (qPCR) and multiplex PCR technology play an irreplaceable role in seed impurity detection and target sequence amplification, which significantly improves the sensitivity and efficiency of seed detection.

• A. Application of qPCR in impurity detection
  • a) qCPR can realize the quantitative analysis of the target sequence by monitoring the fluorescence signal intensity in the qPCR amplification process in real time, and its detection limit can reach 0.01%, which can accurately identify the trace impurities in the seed batch. In the purity detection of hybrids, qPCR can quantitatively analyze the relative content of parental genomes in samples by designing parental-specific primers, so as to calculate the proportion of hybrid plants.
  • b) In the purity detection of cotton hybrids, TaqMan probes were designed for SNP loci unique to the parents. qPCR detection can complete the analysis of 96 samples within 2 hours, and the sensitivity of impurity detection reaches 0.1%. The consistency between the results and field planting identification is 98.5%. In addition, qPCR can also be used to detect transgenic seeds. By amplifying the ratio of exogenous genes (such as the Bt toxin gene) to internal reference genes (such as the actin gene), the content of transgenic components can be determined to meet the requirements of transgenic thresholds in different countries.
• B. Efficient amplification strategy of multiplex PCR
  • a) Multiplex PCR can achieve simultaneous amplification of multiple target sequences by adding multiple pairs of primers in the same reaction system, which greatly improves the detection efficiency. The key to this technology is primer design, which needs to avoid complementary pairing and nonspecific amplification between primers. Usually, 4-10 SSR or SNP loci can be amplified at the same time.
  • b) In wheat variety identification, the multiplex PCR system containing 8 pairs of SSR primers can obtain genotype data of 8 loci in one reaction, which is 8 times more efficient than the traditional single PCR and reduces the cost by 60%. In the purity detection of rape seeds, the multiplex PCR system with 6 specific SSR loci can complete the analysis of 192 samples within 4 hours, and successfully distinguish them from the other 10 similar varieties.
  • c) Multiplex PCR is especially suitable for rapid screening of seed authenticity. By selecting the unique molecular marker combination of varieties, multiple non-target varieties can be excluded at one time, which lays the foundation for subsequent accurate identification.

Principal component analysis on SSR data (Bacilieri et al., 2013)

NGS in Seed Identification Analysis

The next-generation sequencing technology (NGS) broke the flux limitation of traditional molecular markers, and provided unprecedented resolution for seed detection through large-scale sequencing of the whole genome or genome region, especially suitable for seed batch detection with high purity requirements and analysis of complex genome crops.

• A. High-resolution detection of whole genome sequencing
  • a) Whole genome sequencing can cover all bases of the crop genome, and can identify structural variations (such as insertion, deletion, and inversion) other than SSR and SNP, providing the most comprehensive genetic information for variety identification. In the purity detection of maize inbred lines, genome-wide sequencing can detect 0.05% heterozygous sites, which is equivalent to identifying one heterozygous plant in 2000 plants, and this resolution is far beyond traditional molecular marker technology.
  • b) For crops with complex genomes such as rice and wheat, a variety-specific "genetic fingerprint" can be constructed by combining whole genome sequencing with bioinformatics analysis. For example, by sequencing the whole genome of rice, 1200 SNP loci unique to rice varieties were identified, and the fingerprints formed can be completely distinguished from those of the other 100 rice varieties, which provides ironclad evidence for the protection of variety rights.
• B. Precision and economic balance of targeted sequencing
  • a) Targeted sequencing is more suitable for the routine detection of large-scale seed samples by capturing specific regions in the genome (such as SSR enrichment regions and known polymorphic sites), which can ensure the detection accuracy and reduce the cost. In soybean seed detection, the targeted sequencing system for 2000 core SNP loci costs about 150 yuan per sample, and 384 samples can be analyzed within 3 days with a resolution of 0.1%, which meets the detection requirements of national soybean seed quality standards (purity ≥98%).
  • b) In addition, NGS technology can also be used for seed health detection, and simultaneously identify pathogenic bacteria (such as fungi and viruses) and genetic information of varieties carried by seeds through metagenome sequencing, so as to realize "one test for multiple purposes" and provide an efficient solution for seed quarantine.

Principal coordinate analysis (PCA) of the 243 wild and cultivated samples representedFigure 3by two axes using a covariance matrix of 20 SSR loci (Zdunić et al., 2020)

Cost-Benefit Analysis of Molecular Methods

The popularization and application of molecular marker technology depend not only on technical performance but also on cost, equipment requirements, and operation difficulty. There are significant differences in the demand and accessibility of molecular marker technology among application subjects of different scales (small farmers, seed enterprises, and testing institutions), so it is necessary to choose the appropriate technical scheme according to the actual scene:

• For small farmers and grass-roots laboratories with limited resources, the technology with low cost and simple operation is more feasible. Although the flux of SSR markers is low, the equipment investment is low (the basic PCR and electrophoresis equipment is about 50 thousand yuan), the cost of a single sample is low, and the technology is mature and easy to master, which is suitable for small-scale seed purity detection.
• Large-scale seed enterprises have huge seed trading volume, which requires high detection efficiency and flux, and prefer to adopt SNP chip and NGS technology. Although the initial investment in the SNP chip platform is high, the cost of a single sample decreases with the detection scale. When the detection amount exceeds 100,000 samples in that year, the cost of a single sample can be reduced, and the degree of automation is high. The 96-channel liquid workstation can process 1,000 samples every day.

Conclusion

Molecular marker technology has become an indispensable core means of seed detection. The accuracy of SSR and SNP markers, the standardization of DNA barcodes, the flexibility of PCR derivative technology, and the scalability of NGS have jointly constructed a multi-level seed genetic detection system. From the simple screening of grass-roots small farmers to the large-scale detection of large enterprises, different molecular marker technologies have formed a complementary application pattern according to the cost-benefit ratio.

The future development trend will focus on the portability, intelligence, and low cost of technology: the development of micro-sequencing equipment is expected to realize real-time genome analysis in the field; The integration of artificial intelligence algorithms can automatically analyze the detection data and generate the appraisal report; CRISPR-based detection technology can further reduce the cost of a single sample. These advances will further promote the application of molecular marker technology in seed quality control, variety rights protection, and food security, and provide solid technical support for the high-quality development of the modern seed industry.

References

1. Hamblin MT, Warburton ML, Buckler ES. "Empirical comparison of Simple Sequence Repeats and single nucleotide polymorphisms in assessment of maize diversity and relatedness." PLoS One. 2007 2(12): e1367.
2. Vieira MB, Faustino MV, Lourenço TF, Oliveira MM. "DNA-Based Tools to Certify Authenticity of Rice Varieties-An Overview." Foods. 2022 11(3): 258.
3. Zhao G, Li L, Shen X, Zhong R, Zhong Q, Lei H. "DNA Barcoding Unveils Novel Discoveries in Authenticating High-Value Snow Lotus Seed Food Products." Foods. 2024 13(16): 2580.
4. Bacilieri R, Lacombe T, Le Cunff L, et al. "Genetic structure in cultivated graevines is linked to geography and human selectionp." BMC Plant Biol. 2013 13: 25.
5. Zdunić G, Lukšić K, Nagy ZA, et al. "Genetic Structure and Relationships among Wild and Cultivated Grapevines from Central Europe and Part of the Western Balkan Peninsula." Genes (Basel). 2020 11(9): 962.