Recent advances in plant-based bioproduction.

Unable to move on their own, plants have acquired the ability to produce a wide variety of low molecular weight compounds to survive against various stresses. It is estimated that there are as many as one million different kinds. Plants also have the ability to accumulate high levels of proteins. Although plant-based bioproduction has traditionally relied on classical tissue culture methods, the attraction of bioproduction by plants is increasing with the development of omics and bioinformatics and other various technologies, as well as synthetic biology. This review describes the current status and prospects of these plant-based bioproduction from five advanced research topics, (i) de novo production of plant-derived high value terpenoids in engineered yeast, (ii) biotransformation of plant-based materials, (iii) genome editing technology for plant-based bioproduction, (iv) environmental effect of metabolite production in plant factory, and (v) molecular pharming.

Characterization of unique EDTA-insensitive methylthioalkylmalate synthase from Eutrema japonicum and its potential application in synthetic biology.

6-(Methylsulfinyl)hexyl isothiocyanate (6-MSITC), a derivative of glucosinolate with a six-carbon chain, is a compound found in wasabi and has diverse health-promoting properties. The biosynthesis of glucosinolates from methionine depends on a crucial step catalyzed methylthioalkylmalate synthases (MAMs), which are responsible for the generation of glucosinolates with varying chain lengths. In this study, our primary focus was the characterization of two methylthioalkyl malate synthases, MAM1-1 and MAM1-2, derived from Eutrema japonicum, commonly referred to as Japanese wasabi. Eutremajaponicum MAMs (EjMAMs) were expressed in an Escherichiacoli expression system, subsequently purified, and in vitro enzymatic activity was assayed. We explored the kinetic properties, optimal pH conditions, and cofactor preferences of EjMAMs and compared them with those of previously documented MAMs. Surprisingly, EjMAM1-2, categorized as a metallolyase family enzyme, displayed 20% of its maximum activity even in the absence of divalent metal cofactors or under high concentrations of EDTA. Additionally, we utilized AlphaFold2 to generate structural homology models of EjMAMs, and used in silico analysis and mutagenesis studies to investigate the key residues participating in catalytic activity. Moreover, we examined in vivo biosynthesis in E. coli containing Arabidopsis thaliana branched-chain amino acid transferase 3 (AtBCAT3) along with AtMAMs or EjMAMs and demonstrated that EjMAM1-2 exhibited the highest conversion rate among those MAMs, converting l-methionine to 2-(2-methylthio) ethyl malate (2-(2-MT)EM). EjMAM1-2 shows a unique property in vitro and highest activity on converting l-methionine to 2-(2-MT)EM in vivo which displays high potential for isothiocyanate biosynthesis in E. coli platform.

Foreign DNA detection in genome-edited potatoes by high-throughput sequencing.

Genome editing is a powerful breeding technique that introduces mutations into specific gene sequences in genomes. For genome editing in higher plants, nucleotides for artificial nuclease (e.g. TALEN or CRISPR-Cas9) are transiently or stably introduced into the plant cells. After the introduction of mutations by artificial nucleases, it is necessary to select lines that do not contain the foreign nucleotides to overcome GMO regulation; however, there is still no widely legally authorized and approved method for detecting foreign genes in genome-edited crops. Recently, k-mer analysis based on next-generation sequencing (NGS) was proposed as a new method for detecting foreign DNA in genome-edited agricultural products. Compared to conventional methods, such as PCR and Southern hybridization, in principle, this method can detect short DNA fragments with high accuracy. However, this method has not yet been applied to genome-edited potatoes. In this study, we evaluated the feasibility of k-mer analysis in tetraploid potatoes by computer simulation, and also evaluated whether the k-mer method can detect foreign genes with high accuracy by analyzing samples of genome-edited potatoes. We show that when NGS data (at a depth of × 30 the genome size) are used, the k-mer method can correctly detect foreign genes in the potato genome even with the insertion of DNA fragments of 20 nt in length. Based on these findings, we expect that k-mer analysis will be one of the main methods for detecting foreign genes in genome-edited potatoes.

Class I and II NADPH-cytochrome P450 reductases exhibit different roles in triterpenoid biosynthesis in Lotus japonicus.

Cytochrome P450 monooxygenases (CYPs) are enzymes that play critical roles in the structural diversification of triterpenoids. To perform site-specific oxidations of the triterpene scaffold, CYPs require electrons transferred by NADPH-cytochrome P450 reductase (CPR), which is classified into two main classes, class I and class II, based on their structural difference. Lotus japonicus is a triterpenoids-producing model legume with one CPR class I gene (LjCPR1) and a minimum of two CPR class II genes (LjCPR2-1 and LjCPR2-2). CPR classes I and II from different plants have been reported to be involved in different metabolic pathways. By performing gene expression analyses of L. japonicus hairy root culture treated with methyl jasmonate (MeJA), this study revealed that LjCPR1, CYP716A51, and LUS were down-regulated which resulted in no change in betulinic acid and lupeol content. In contrast, LjCPR2s, bAS, CYP93E1, and CYP72A61 were significantly upregulated by MeJA treatment, followed by a significant increase of the precursors for soyasaponins, i.e. ß-amyrin, 24-OH ß-amyrin, and sophoradiol content. Triterpenoids profile analysis of LORE1 insertion and hairy root mutants showed that the loss of the Ljcpr2-1 gene significantly reduced soyasaponins precursors but not in Ljcpr1 mutants. However, Ljcpr1 and Ljcpr2-1 mutants showed a significant reduction in lupeol and oleanolic, ursolic, and betulinic acid contents. Furthermore, LjCPR1, but not LjCPR2, was crucial for seed development, supporting the previous notion that CPR class I might support plant basal metabolism. This study suggests that CPR classes I and II play different roles in L. japonicus triterpenoid biosynthesis.

Integrated gene-free potato genome editing using transient transcription activator-like effector nucleases and regeneration-promoting gene expression by Agrobacterium infection.

Genome editing is highly useful for crop improvement. The method of expressing genome-editing enzymes using a transient expression system in Agrobacterium, called agrobacterial mutagenesis, is a shortcut used in genome-editing technology to improve elite varieties of vegetatively propagated crops, including potato. However, with this method, edited individuals cannot be selected. The transient expression of regeneration-promoting genes can result in shoot regeneration from plantlets, while the constitutive expression of most regeneration-promoting genes does not result in normally regenerated shoots. Here, we report that we could obtain genome-edited potatoes by positive selection. These regenerated shoots were obtained via a method that combined a regeneration-promoting gene with the transient expression of a genome-editing enzyme gene. Moreover, we confirmed that the genome-edited potatoes obtained using this method did not contain the sequence of the binary vector used in Agrobacterium. Our data have been submitted to the Japanese regulatory authority, the Ministry of Education, Culture, Sports, Science and Technology (MEXT), and we are in the process of conducting field tests for further research on these potatoes. Our work presents a powerful method for regarding regeneration and acquisition of genome-edited crops through transient expression of regeneration-promoting gene.

Identification of key amino acid residues toward improving the catalytic activity and substrate specificity of plant-derived cytochrome P450 monooxygenases CYP716A subfamily enzyme for triterpenoid production in Saccharomyces cerevisiae.

Triterpenoids constitute a group of specialized plant metabolites with wide structural diversity and high therapeutic value for human health. Cytochrome P450 monooxygenases (CYP) are a family of enzymes important for generating the structural diversity of triterpenoids by catalyzing the site-specific oxidization of the triterpene backbone. The CYP716 enzyme family has been isolated from various plant families as triterpenoid oxidases; however, their experimental crystal structures are not yet available and the detailed catalytic mechanism remains elusive. Here, we address this challenge by integrating bioinformatics approaches with data from other CYP families. Medicago truncatula CYP716A12, the first functionally characterized CYP716A subfamily enzyme, was chosen as the model for this study. We performed homology modeling, structural alignment, in silico site-directed mutagenesis, and molecular docking analysis to search and screen key amino acid residues relevant to the catalytic activity and substrate specificity of the CYP716A subfamily enzyme in triterpenoid biosynthesis. An in vivo functional analysis using engineered yeast that endogenously produced plant-derived triterpenes was performed to elucidate the results. When the amino acids in the signature region and substrate recognition sites (SRSs) were substituted, the product profile of CYP716A12 was modified. We identified amino acid residues that control the substrate contraction of the enzyme (D292) and engineered the enzyme to improve its catalytic activity and substrate specificity (D122, I212, and Q358) for triterpenoid biosynthesis. In addition, we demonstrated the versatility of this strategy by changing the properties of key residues in SRSs to improve the catalytic activity of Arabidopsis thaliana CYP716A1 (S356) and CYP716A2 (M206, F210) at C-28 on the triterpene backbone. This research has the potential to help in the production of desired triterpenoids in engineered yeast by increasing the catalytic activity and substrate specificity of plant CYP716A subfamily enzymes.

High-yield bioactive triterpenoid production by heterologous expression in Nicotiana benthamiana using the Tsukuba system.

Oleanolic acid is a pentacyclic triterpenoid found in numerous plant species and is a precursor to several bioactive triterpenoids with commercial potential. However, oleanolic acid accumulates at low levels in plants, and its chemical synthesis is challenging. Here, we established a method for producing oleanolic acid in substantial quantities via heterologous expression of pathway enzymes in Nicotiana benthamiana. The "Tsukuba system" is one of the most efficient agroinfiltration-based transient protein expression systems using the vector pBYR2HS, which contains geminiviral replication machinery and a double terminator for boosting expression. Additionally, the pBYR2HS vector contains an expression cassette for the gene-silencing suppressor p19 protein from tomato bushy stunt virus, which can also contribute to enhancing the expression of target proteins. In this study, we evaluated the applicability of this system to heterologous triterpenoid production in N. benthamiana. Medicago truncatula cytochrome P450 monooxygenase (CYP) 716A12 is the first enzyme to be functionally characterized as ß-amyrin C-28 oxidase producing oleanolic acid. A mutant CYP716A12 (D122Q) with improved catalytic activity engineered in our previous study was co-expressed with other enzymes in N. benthamiana leaves. Using pBYR2HS, oleanolic acid yield was increased 13.1-fold compared with that using the conventional binary vector, indicating the advantage of the Tsukuba system. We also demonstrated the efficacy of co-expressing a mutant Arabidopsis thaliana HMGR1 catalytic domain, additional NADPH-cytochrome P450 reductase (CPR) transferring electrons to heterologous CYPs, and application of ascorbic acid for preventing leaf necrosis after agroinfiltration, to improve product yield. As a result, the product yields of both simple (ß-amyrin) and oxidized (oleanolic acid and maslinic acid) triterpenoids were significantly improved compared with the previously reported yield in heterologous triterpenoid production in N. benthamiana leaves.

Comparative Analysis of NADPH-Cytochrome P450 Reductases From Legumes for Heterologous Production of Triterpenoids in Transgenic Saccharomyces cerevisiae.

Triterpenoids are plant specialized metabolites with various pharmacological activities. They are widely distributed in higher plants, such as legumes. Because of their low accumulation in plants, there is a need for improving triterpenoid production. Cytochrome P450 monooxygenases (CYPs) play critical roles in the structural diversification of triterpenoids. To perform site-specific oxidations, CYPs require the electrons that are transferred by NADPH-cytochrome P450 reductase (CPR). Plants possess two main CPR classes, class I and class II. CPR classes I and II have been reported to be responsible for primary and specialized (secondary) metabolism, respectively. In this study, we first analyzed the CPR expression level of three legumes species, Medicago truncatula, Lotus japonicus, and Glycyrrhiza uralensis, showing that the expression level of CPR class I was lower and more stable, while that of CPR class II was higher in almost all the samples. We then co-expressed different combinations of CYP716As and CYP72As with different CPR classes from these three legumes in transgenic yeast. We found that CYP716As worked better with CPR-I from the same species, while CYP72As worked better with any CPR-IIs. Using engineered yeast strains, CYP88D6 paired with class II GuCPR produced the highest level of 11-oxo-ß-amyrin, the important precursor of high-value metabolites glycyrrhizin. This study provides insight into co-expressing genes from legumes for heterologous production of triterpenoids in yeast.

Substitution of Thr572 to Ala in mouse c-Myb attenuates progression of early erythroid differentiation.

The expression level of transcription factor c-Myb oscillates during hematopoiesis. Fbw7 promotes ubiquitin-mediated degradation of c-Myb, which is dependent on phosphorylation of Thr572. To investigate the physiological relevance of Fbw7-mediated c-Myb degradation, we generated mutant mice carrying c-Myb-T572A (TA). Homozygous mutant (TA/TA) mice exhibited a reduction in the number of peripheral red blood cells and diminished erythroblasts in bone marrow, presumably as a result of failure during erythroblast differentiation. We found that c-Myb high-expressing cells converged in the Lin-CD71+ fraction, and the expression of c-Myb was higher in TA/TA mice than in wild-type mice. Moreover, TA/TA mice had an increased proportion of the CD71+ subset in Lin- cells. The c-Myb level in the Lin-CD71+ subset showed three peaks, and the individual c-Myb level was positively correlated with that of c-Kit, a marker of undifferentiated cells. Ultimately, the proportion of c-Mybhi subgroup was significantly increased in TA/TA mice compared with wild-type mice. These results indicate that a delay in reduction of c-Myb protein during an early stage of erythroid differentiation creates its obstacle in TA/TA mice. In this study, we showed the T572-dependent downregulation of c-Myb protein is required for proper differentiation in early-stage erythroblasts, suggesting the in vivo significance of Fbw7-mediated c-Myb degradation.

Targeted genome editing in tetraploid potato through transient TALEN expression by Agrobacterium infection.

Genome editing using site-specific nucleases, such as transcription activator-like effector nucleases (TALENs) and clustered regularly interspaced short palindromic repeat-CRISPR-associated protein 9 (CRISPR-Cas9), is a powerful technology for crop breeding. For plant genome editing, the genome-editing reagents are usually expressed in plant cells from stably integrated transgenes within the genome. This requires crossing processes to remove foreign nucleotides from the genome to generate null segregants. However, in highly heterozygous plants such as potato, the progeny lines have different agronomic traits from the parent cultivar and do not necessarily become elite lines. Agrobacteria can transfer exogenous genes on T-DNA into plant cells. This has been used both to transform plants stably and to express the genes transiently in plant cells. Here, we infected potato, with Agrobacterium tumefaciens harboring TALEN-expression vector targeting sterol side chain reductase 2 (SSR2) gene and regenerated shoots without selection. We obtained regenerated lines with disrupted-SSR2 gene and without transgene of the TALEN gene, revealing that their disruption should be caused by transient gene expression. The strategy using transient gene expression by Agrobacterium that we call Agrobacterial mutagenesis, developed here should accelerate the use of genome-editing technology to modify heterozygous plant genomes.

Production of the bioactive plant-derived triterpenoid morolic acid in engineered Saccharomyces cerevisiae.

Morolic acid is a plant-derived triterpenoid that possesses pharmacological properties such as cytotoxicity, as well as anti-HIV, anti-HSV, anti-inflammatory, and antidiabetic effects. The significant therapeutic properties of morolic acid are desirable in the context of pharmacological and drug development research, but the low accessibility of morolic acid from natural resources limits its applications. In the present study, we developed a microbial system for the production of morolic acid. Using a combinatorial biosynthesis approach, a novel production pathway was constructed in Saccharomycescerevisiae by coexpressing BfOSC2 (germanicol synthase) from Bauhinia forficata and CYP716A49 (triterpene C-28 oxidase) from Beta vulgaris. Moreover, we reconstructed the cellular galactose regulatory network by introducing a chimeric transcriptional activator (fusion of Gal4dbd.ER.VP16) to overdrive the genes under the control of the galactose promoter. We also overexpressed truncated HMG1, encoding feedback-inhibition-resistant form of 3-hydroxy-3-methylglutaryl-coenzyme A reductase 1 and sterol-regulating transcription factor upc2-1, to increase the isoprenoid precursors in the mevalonate pathway. Using this yeast system, we achieved morolic acid production up to 20.7 ± 1.8 mg/L in batch culture. To our knowledge, this is the highest morolic acid titer reported from a heterologous host, indicating a promising approach for obtaining rare natural triterpenoids.

Efficient genome engineering using Platinum TALEN in potato.

Potato (Solanum tuberosum) is one of the most important crops in the world. However, it is generally difficult to breed a new variety of potato crops because they are highly heterozygous tetraploid. Steroidal glycoalkaloids (SGAs) such as α-solanine and α-chaconine found in potato are antinutritional specialized metabolites. Because of their toxicity following intake, controlling the SGA levels in potato varieties is critical in breeding programs. Recently, genome-editing technologies using artificial site-specific nucleases such as TALEN and CRISPR-Cas9 have been developed and used in plant sciences. In the present study, we developed a highly active Platinum TALEN expression vector construction system, and applied to reduce the SGA contents in potato. Using Agrobacterium-mediated transformation, we obtained three independent transgenic potatoes harboring the TALEN expression cassette targeting SSR2 gene, which encodes a key enzyme for SGA biosynthesis. Sequencing analysis of the target sequence indicated that all the transformants could be SSR2-knockout mutants. Reduced SGA phenotype in the mutants was confirmed by metabolic analysis using LC-MS. In vitro grown SSR2-knockout mutants exhibited no differences in morphological phenotype or yields when compared with control plants, indicating that the genome editing of SGA biosynthetic genes such as SSR2 could be a suitable strategy for controlling the levels of toxic metabolites in potato. Our simple and powerful plant genome-editing system, developed in the present study, provides an important step for future study in plant science.

Identification of oxidosqualene cyclases from the medicinal legume tree Bauhinia forficata: a step toward discovering preponderant α-amyrin-producing activity.

Triterpenoids are widely distributed among plants of the legume family. However, most studies have focused on triterpenoids and their biosynthetic enzymes in model legumes. We evaluated the triterpenoid aglycones profile of the medicinal legume tree Bauhinia forficata by gas chromatography-mass spectrometry. Through transcriptome analyses, homology-based cloning, and heterologous expression, we discovered four oxidosqualene cyclases (OSCs) which are responsible for the diversity of triterpenols in B. forficata. We also investigated the effects of the unique motif TLCYCR on α-amyrin synthase activity. B. forficata highly accumulated α-amyrin. We discovered an OSC with a preponderant α-amyrin-producing activity, which accounted for at least 95% of the total triterpenols. We also discovered three other functional OSCs (BfOSC1, BfOSC2, and BfOSC4) that produce ß-amyrin, germanicol, and cycloartenol. Furthermore, by replacing the unique motif TLCYCR from BfOSC3 with the MWCYCR motif, we altered the function of BfOSC3 such that it no longer produced α-amyrin. Our results provide new insights into OSC cyclization, which is responsible for the diversity of triterpenoid metabolites in B. forficata, a non-model legume plant.

Functional Characterization of CYP716 Family P450 Enzymes in Triterpenoid Biosynthesis in Tomato.

Triterpenoids are a group of structurally diverse specialized metabolites that frequently show useful bioactivities. These chemicals are biosynthesized from the common precursor 2,3-oxidosqualene in plants. The carbon skeletons produced by oxidosqualene cyclase (OSC) are usually modified by cytochrome P450 monooxygenases (P450s) and UDP-dependent glycosyltransferases. These biosynthetic enzymes contribute to the structural diversification of plant triterpenoids. Until now, many P450 enzymes have been characterized as triterpenoid oxidases. Among them, the CYP716 family P450 enzymes, which have been isolated from a wide range of plant families, seem to contribute to the triterpenoid structural diversification. Many CYP716 family P450 enzymes have been characterized as the multifunctional triterpene C-28 oxidases, which oxidize α-amyrin and ß-amyrin to the widely distributed triterpenoids ursolic and oleanolic acids, respectively. Tomato (Solanum lycopersicum) is one of the most important solanaceous crops in the world. However, little information is known regarding its triterpenoid biosynthesis. To understand the mechanism of triterpenoid biosynthesis in tomato, we focused on the function of CYP716 family enzymes as triterpenoid oxidases. We isolated all six CYP716 family genes from the Micro-Tom cultivar of tomato, and functionally characterized them in the heterologous yeast expression system. The in vivo enzymatic assays showed that CYP716A44 and CYP716A46 exhibited the ordinary C-28 oxidation activity against α-amyrin and ß-amyrin to produce ursolic and oleanolic acids, respectively. Interestingly, one CYP716E subfamily enzyme, CYP716E26, exhibited the previously unreported C-6ß hydroxylation activity against ß-amyrin to produce a rare bioactive triterpenoid, daturadiol (olean-12-ene-3ß,6ß-diol). To determine the roles of the CYP716 family genes in tomato triterpenoid biosynthesis, we analyzed the gene expression and triterpenoid accumulation patterns in different plant tissues by performing the quantitative real-time polymerase chain reaction (qPCR) and gas chromatography-mass spectrometry (GC-MS) analyses, respectively. High levels of the CYP716A44 gene expression and the accumulation of C-28-oxidized triterpenoids, ursolic acid, and oleanolic acid were observed in the roots, indicating a significant contribution of the CYP716A44 gene in the triterpenoid biosynthesis in tomato. Thus, our study partially elucidated the mechanism of triterpenoid biosynthesis in tomato, and identified CYP716E26 as a novel C-6ß hydroxylase for its subsequent use in the combinatorial biosynthesis of bioactive triterpenoids.

Novel triterpene oxidizing activity of Arabidopsis thaliana CYP716A subfamily enzymes.

Triterpenoids have diverse chemical structures and bioactivities. Cytochrome P450 monooxygenases play a key role in their structural diversification. In higher plants, CYP716A subfamily enzymes are triterpene oxidases. In this study, Arabidopsis thaliana CYP716A1 and CYP716A2 were characterized by heterologously expressing them in simple triterpene-producing yeast strains. In contrast to the C-28 oxidative activity of CYP716A1 shown in several CYP716A subfamily enzymes, remarkably, CYP716A2 displayed 22α-hydroxylation activity against α-amyrin that has not been previously reported, which produces the cytotoxic triterpenoid, 22α-hydroxy-α-amyrin. Our results contribute to the enrichment of the molecular toolbox that allows for the combinatorial biosynthesis of diverse triterpenoids.

Planteose as a storage carbohydrate required for early stage of germination of Orobanche minor and its metabolism as a possible target for selective control.

Root parasitic weeds in Orobanchaceae cause serious damage to worldwide agriculture. Germination of the parasites requires host-derived germination stimulants, such as strigolactones, as indicators of host roots within reach of the parasite's radicles. This unique germination process was focused on to identify metabolic pathways required for germination, and to design a selective control strategy. A metabolomic analysis of germinating seeds of clover broomrape, Orobanche minor, was conducted to identify its distinctive metabolites. Consequently, a galactosyl-sucrose trisaccharide, planteose (α-d-galactopyranosyl-(1→6)-ß-d-fructofuranosyl-(2→1)-α-d-glucopyranoside), was identified as a metabolite that decreased promptly after reception of the germination stimulant. To investigate the importance of planteose metabolism, the effects of several glycosidase inhibitors were examined, and nojirimycin bisulfite (NJ) was found to alter the sugar metabolism and to selectively inhibit the germination of O. minor. Planteose consumption was similar in NJ-treated seeds and non-treated germinating seeds; however, NJ-treated seeds showed lower consumption of sucrose, a possible intermediate of planteose metabolism, resulting in significantly less glucose and fructose. This inhibitory effect was recovered by adding glucose. These results suggest that planteose is a storage carbohydrate required for early stage of germination of O. minor, and NJ inhibits germination by blocking the supply of essential glucose from planteose and sucrose. Additionally, NJ selectively inhibited radicle elongation of germinated seeds of Orobanchaceae plants (Striga hermonthica and Phtheirospermum japonicum). Thus, NJ will be a promising tool to develop specific herbicides to the parasites, especially broomrapes, and to improve our understanding of the molecular mechanisms of this unique germination.

Sterol side chain reductase 2 is a key enzyme in the biosynthesis of cholesterol, the common precursor of toxic steroidal glycoalkaloids in potato.

Potatoes (Solanum tuberosum) contain α-solanine and α-chaconine, two well-known toxic steroidal glycoalkaloids (SGAs). Sprouts and green tubers accumulate especially high levels of SGAs. Although SGAs were proposed to be biosynthesized from cholesterol, the biosynthetic pathway for plant cholesterol is poorly understood. Here, we identify sterol side chain reductase 2 (SSR2) from potato as a key enzyme in the biosynthesis of cholesterol and related SGAs. Using in vitro enzyme activity assays, we determined that potato SSR2 (St SSR2) reduces desmosterol and cycloartenol to cholesterol and cycloartanol, respectively. These reduction steps are branch points in the biosynthetic pathways between C-24 alkylsterols and cholesterol in potato. Similar enzymatic results were also obtained from tomato SSR2. St SSR2-silenced potatoes or St SSR2-disrupted potato generated by targeted genome editing had significantly lower levels of cholesterol and SGAs without affecting plant growth. Our results suggest that St SSR2 is a promising target gene for breeding potatoes with low SGA levels.