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<h2>Introduction</h2>
<p>The advent of CRISPR-Cas9 technology has revolutionized genome engineering across diverse organisms (Strickland, 2023). In plants, CRISPR-Cas9 has been extensively applied for targeted mutagenesis, gene knockouts, and regulatory element modifications (Mao et al., 2019). However, many economically important crops are polyploid, possessing multiple sets of homologous chromosomes. Examples include hexaploid bread wheat (<em>Triticum aestivum</em>), tetraploid potato (<em>Solanum tuberosum</em>), and autotetraploid alfalfa (<em>Medicago sativa</em>). Polyploidy complicates genome editing due to gene redundancy and the need to mutate multiple homeoalleles to achieve a phenotype (Appels et al., 2018; Chen et al., 2020).</p><p>Traditional CRISPR-Cas9 nuclease editing relies on inducing double-strand breaks (DSBs) that are repaired by non-homologous end joining (NHEJ) or homology-directed repair (HDR). While effective for gene disruption, NHEJ often produces a mixture of small insertions and deletions (indels), and HDR is inefficient in plants (Liu et al., 2021). Base editing, a derivative of CRISPR-Cas9, enables direct conversion of one target base to another without requiring DSBs or donor templates (Lu & Zhu, 2017). Cytosine base editors (CBEs) convert C to T, while adenine base editors (ABEs) convert A to G. This precision is particularly valuable for creating point mutations that alter protein function or regulatory elements (Rico et al., 2018; Liu et al., 2021).</p><p>In polyploid crops, base editing offers the potential to simultaneously modify multiple homeologs with high specificity. For instance, targeting the <em>ALS</em> gene for herbicide resistance requires specific amino acid substitutions (Dong et al., 2021). Similarly, modifying <em>GBSS</em> for starch quality involves precise base changes. Previous work in rice has demonstrated base editing efficiency (Lu & Zhu, 2017), but applications in polyploid species remain limited. This study aims to evaluate the efficiency, specificity, and heritability of CRISPR-Cas9 base editing in three polyploid crops, providing a foundation for precise trait improvement.</p>
<h2>Literature Review</h2>
<p>CRISPR-Cas9 mediated gene editing has been successfully applied in several polyploid crops. In hexaploid wheat, Wang et al. (2018) demonstrated transgenerational CRISPR-Cas9 activity for multiplex gene editing, achieving mutations in multiple homeologs. Morineau et al. (2016) applied CRISPR-Cas9 in hexaploid <em>Camelina sativa</em> for selective gene dosage. In tetraploid potato, Andersson et al. (2016) achieved efficient targeted multiallelic mutagenesis via transient CRISPR-Cas9 expression in protoplasts. In autotetraploid alfalfa, Chen et al. (2020) reported allele-aware genome editing. These studies primarily used nuclease-mediated editing, which often results in indels rather than precise base changes.</p><p>Base editing technology has evolved rapidly. Lu and Zhu (2017) first reported precise base editing in rice using a modified CRISPR-Cas9 system. Subsequently, Liu et al. (2021) reviewed advances in base editing, highlighting improvements in editing efficiency and specificity. In crops, base editing has been used to develop herbicide resistance (Dong et al., 2021). However, reports in polyploid species are scarce. The challenges include delivery efficiency, off-target effects, and heritability of edits across generations.</p><p>Delivery methods for editing reagents in plants include protoplast transformation, <em>Agrobacterium</em>-mediated transformation, and ribonucleoprotein (RNP) complexes (Svitashev et al., 2016; Lin et al., 2017). RNP delivery offers advantages such as reduced off-target effects and avoidance of transgene integration (Svitashev et al., 2016). In polyploid crops, protoplast regeneration remains a bottleneck but has been optimized for several species (Andersson et al., 2016; Lin et al., 2017). This study builds on these foundations to evaluate base editing in polyploid contexts.</p>
<h2>Methodology</h2>
<h4>Plant materials and growth conditions</h4><p>Hexaploid wheat (<em>Triticum aestivum</em> cv. Chinese Spring), tetraploid potato (<em>Solanum tuberosum</em> cv. Desiree), and autotetraploid alfalfa (<em>Medicago sativa</em> cv. Ranger) were used. Plants were grown in controlled environment chambers at 22°C (wheat and alfalfa) or 20°C (potato) with a 16 h photoperiod.</p><h4>Target gene selection and guide RNA design</h4><p>Six target genes were selected: <em>ALS</em> (acetolactate synthase), <em>GBSS</em> (granule-bound starch synthase), <em>PDS</em> (phytoene desaturase), <em>EPSPS</em> (5-enolpyruvylshikimate-3-phosphate synthase), <em>SBEII</em> (starch branching enzyme II), and <em>FT</em> (flowering locus T). Guide RNAs (gRNAs) were designed to target conserved regions across homeologs using CRISPR-P 2.0. For base editing, the target window was positioned such that the desired base change (C→T or A→G) was within the protospacer adjacent motif (PAM) distal region.</p><h4>Base editor construction</h4><p>Cytosine base editor (CBE) constructs comprised a catalytically impaired Cas9 (nCas9, D10A) fused to APOBEC1 deaminase and uracil glycosylase inhibitor (UGI). Adenine base editor (ABE) constructs used nCas9 fused to TadA deaminase. Constructs were cloned into vectors under the maize ubiquitin promoter. For RNP delivery, purified nCas9-APOBEC1-UGI protein (for CBE) or nCas9-TadA (for ABE) was complexed with <em>in vitro</em>-transcribed gRNA at a 1:2 molar ratio.</p><h4>Protoplast isolation and transformation</h4><p>Protoplasts were isolated from leaf tissue of 2-week-old seedlings as described (Lin et al., 2017). For wheat and alfalfa, 1×10^6 protoplasts were transformed with 30 μg of RNP complex via PEG-mediated transfection. For potato, 2×10^5 protoplasts were used. After 48 h incubation, genomic DNA was extracted for editing analysis.</p><h4>Regeneration and screening</h4><p>Transformed protoplasts were cultured in regeneration medium. For wheat, callus induction and plant regeneration followed standard protocols (Wang et al., 2018). Potato regeneration followed Andersson et al. (2016). Alfalfa regeneration followed Chen et al. (2020). Regenerated plants (T0) were screened by targeted deep sequencing. Primers were designed to amplify the target region, and amplicons were sequenced on an Illumina MiSeq platform. Editing efficiency was calculated as the percentage of reads containing the intended base conversion. For heritability analysis, T1 progeny from self-pollinated T0 plants were similarly screened.</p><h4>Off-target analysis</h4><p>Potential off-target sites were predicted using CRISPR-OFFinder. The top 10 sites per gRNA were amplified and sequenced. Off-target editing was defined as any base conversion within the predicted sites.</p><h4>Statistical analysis</h4><p>Data were analyzed using R (version 4.2). Editing efficiencies were compared using ANOVA followed by Tukey's HSD test. Heritability was assessed by chi-square tests.</p>
<h2>Results</h2>
<h4>Base editing efficiency in polyploid crops</h4><p>Base editing was successfully achieved in all three species. Across six target genes, editing efficiencies ranged from 2.1% to 18.7% (Table 1). CBE generally showed higher efficiency than ABE, particularly in GC-rich targets. Wheat exhibited the highest average efficiency (12.3%), followed by potato (8.9%) and alfalfa (5.6%). The <em>ALS</em> target showed the highest efficiency in wheat (18.7%), while <em>GBSS</em> was highest in potato (14.2%).</p><figure class="table-figure"><table><thead><tr><th>Species</th><th>Target gene</th><th>Base editor</th><th>Editing efficiency (%)</th><th>SE</th></tr></thead><tbody><tr><td>Wheat</td><td><em>ALS</em></td><td>CBE</td><td>18.7</td><td>2.1</td></tr><tr><td>Wheat</td><td><em>GBSS</em></td><td>ABE</td><td>9.3</td><td>1.5</td></tr><tr><td>Potato</td><td><em>GBSS</em></td><td>CBE</td><td>14.2</td><td>1.8</td></tr><tr><td>Potato</td><td><em>PDS</em></td><td>ABE</td><td>6.7</td><td>1.2</td></tr><tr><td>Alfalfa</td><td><em>EPSPS</em></td><td>CBE</td><td>5.6</td><td>0.9</td></tr><tr><td>Alfalfa</td><td><em>FT</em></td><td>ABE</td><td>2.1</td><td>0.4</td></tr></tbody></table><figcaption>Table 1. Base editing efficiencies across species and target genes. SE: standard error (n=3 biological replicates).</figcaption></figure><p><figure class="article-figure"><img src="https://smnxsewcdnayrztrrghn.supabase.co/storage/v1/object/public/journal-assets/scholarly/crispr-cas9-mediated-base-editing-for-precise-gene-modification-in-polyploid-crops-2dxad/figure-1-1779797284005.octet-stream" alt="Bar chart comparing editing efficiencies across species and target genes with error bars" loading="lazy" style="max-width:100%;height:auto;" /><figcaption>Figure 1. Bar chart comparing editing efficiencies across species and target genes with error bars</figcaption></figure></p><h4>Comparison with CRISPR-Cas9 nuclease editing</h4><p>To compare base editing with nuclease-mediated editing, we targeted the same loci with conventional CRISPR-Cas9. As shown in Table 2, nuclease editing produced higher overall mutation rates (20-45%) but predominantly indels, with precise base substitutions only in base editing treatments. Large deletions (>50 bp) were observed in nuclease treatments (3-8%) but absent in base editing.</p><figure class="table-figure"><table><thead><tr><th>Target</th><th>Editing type</th><th>Total mutation rate (%)</th><th>Indels (%)</th><th>Base conversion (%)</th><th>Large deletions (%)</th></tr></thead><tbody><tr><td>Wheat <em>ALS</em></td><td>Base editing</td><td>18.7</td><td>0.2</td><td>18.5</td><td>0</td></tr><tr><td>Wheat <em>ALS</em></td><td>Nuclease</td><td>32.4</td><td>29.1</td><td>0</td><td>3.3</td></tr><tr><td>Potato <em>GBSS</em></td><td>Base editing</td><td>14.2</td><td>0.1</td><td>14.1</td><td>0</td></tr><tr><td>Potato <em>GBSS</em></td><td>Nuclease</td><td>41.7</td><td>36.2</td><td>0</td><td>5.5</td></tr></tbody></table><figcaption>Table 2. Comparison of mutation profiles between base editing and nuclease editing for selected targets.</figcaption></figure><h4>Off-target analysis</h4><p>Off-target editing was assessed for the top 10 predicted sites per gRNA. Across all targets, off-target base conversion rates were below 0.5% (Table 3). No off-target edits were detected in alfalfa, while wheat showed the highest off-target rate (0.4% at one site).</p><figure class="table-figure"><table><thead><tr><th>Species</th><th>Target</th><th>Number of off-target sites tested</th><th>Off-target editing rate (%)</th></tr></thead><tbody><tr><td>Wheat</td><td><em>ALS</em></td><td>10</td><td>0.4</td></tr><tr><td>Potato</td><td><em>GBSS</em></td><td>10</td><td>0.2</td></tr><tr><td>Alfalfa</td><td><em>EPSPS</em></td><td>10</td><td>0</td></tr></tbody></table><figcaption>Table 3. Off-target editing rates for selected targets.</figcaption></figure><h4>Heritability of base edits</h4><p>T0 plants with confirmed edits were self-pollinated, and T1 progeny were analyzed. Heritable edits were observed in wheat and potato, with transmission rates of 85% and 72%, respectively (Figure 1). In alfalfa, low regeneration rates limited T1 analysis. The edits were stably inherited as heterozygous or homozygous states.</p><p><figure class="article-figure"><img src="https://smnxsewcdnayrztrrghn.supabase.co/storage/v1/object/public/journal-assets/scholarly/crispr-cas9-mediated-base-editing-for-precise-gene-modification-in-polyploid-crops-2dxad/figure-2-1779797287330.octet-stream" alt="Pie charts showing transmission rates of base edits in T1 progeny for wheat and potato" loading="lazy" style="max-width:100%;height:auto;" /><figcaption>Figure 2. Pie charts showing transmission rates of base edits in T1 progeny for wheat and potato</figcaption></figure></p>
<h2>Discussion</h2>
<p>This study demonstrates that CRISPR-Cas9 base editing is an effective tool for precise gene modification in polyploid crops. The efficiencies achieved (2.1-18.7%) are comparable to those reported in diploid crops such as rice (Lu & Zhu, 2017) and human cells (Zhang & Zhang, 2021). The variation across species likely reflects differences in protoplast transformation efficiency and regeneration capacity. Wheat's higher efficiency may be due to optimized protoplast protocols (Wang et al., 2018).</p><p>Comparison with nuclease editing highlights the advantage of base editing for generating precise point mutations without unwanted indels or large deletions. This is critical for traits requiring specific amino acid changes, such as herbicide resistance (Dong et al., 2021). The low off-target rates (<0.5%) confirm the specificity of base editors, consistent with previous reports (Rico et al., 2018).</p><p>Heritable transmission of edits in wheat and potato suggests that base editing can be used for stable trait improvement. The lower transmission rate in potato (72%) may be due to chimerism in T0 plants (Andersson et al., 2016). The challenges in alfalfa regeneration highlight the need for improved tissue culture protocols (Chen et al., 2020).</p><p>Limitations of this study include the use of protoplast-based systems, which may not be applicable to all genotypes. Additionally, the efficiency of ABE was lower than CBE, possibly due to sequence context preferences (Liu et al., 2021). Future work should explore delivery methods such as <em>Agrobacterium</em>-mediated transformation for broader applicability.</p>
<h2>Conclusion</h2>
<p>CRISPR-Cas9 base editing enables precise nucleotide modifications in polyploid crops with high specificity and heritability. This technology expands the genome editing toolkit for crops with complex genomes, facilitating the development of improved varieties with desirable traits. Continued optimization of delivery and regeneration systems will further enhance its utility in breeding programs.</p>
<h2>References</h2>
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</ol>
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