TCP transcription factor StAST1 represses potato tuberization by regulating tuberigen complex activity.

Potato (Solanum tuberosum L.) is cultivated worldwide for its underground tubers, which provide an important part of human nutrition and serve as a model system for belowground storage organ formation. Similar to flowering, stolon-expressed FLOWERING LOCUS T-like (FT-like) protein SELF-PRUNING 6A (StSP6A) plays an instrumental role in tuberization by binding to the bZIP transcription factors StABI5-like 1 (StABL1) and StFD-like 1 (StFDL1), causing transcriptional reprogramming at the stolon subapical apices. However, the molecular mechanism regulating the widely conserved FT-bZIP interactions remains largely unexplored. Here, we identified a TCP transcription factor StAST1 (StABL1 and StSP6A-associated TCP protein 1) binding to both StSP6A and StABL1. StAST1 is specifically expressed in the vascular tissue of leaves and developing stolons. Silencing of StAST1 leads to accelerated tuberization and a shortened life cycle. Molecular dissection reveals that the interaction of StAST1 with StSP6A and StABL1 attenuates the formation of the alternative tuberigen activation complex (aTAC). We also observed StAST1 directly activates the expression of potato GA 20-oxidase gene (StGA20ox1) to regulate GA responses. These results demonstrate StAST1 functions as a tuberization repressor by regulating plant hormone levels; our findings also suggest a mechanism by which the widely conserved FT-FD genetic module is fine-tuned.

StMLP1, as a Kunitz trypsin inhibitor, enhances potato resistance and specifically expresses in vascular bundles during Ralstonia solanacearum infection.

Miraculin-like proteins (MLPs), members of the Kunitz trypsin inhibitor (KTI) family that are present in various plants, have been discovered to have a role in defending plants against pathogens. In this study, we identified a gene StMLP1 in potato that belongs to the KTI family. We found that the expression of StMLP1 gradually increases during Ralstonia solanacearum (R. solanacearum) infection. We characterized the promoter of StMLP1 as an inducible promoter that can be triggered by R. solanacearum and as a tissue-specific promoter with specificity for vascular bundle expression. Our findings demonstrate that StMLP1 exhibits trypsin inhibitor activity, and that its signal peptide is essential for proper localization and function. Overexpression of StMLP1 in potato can enhance the resistance to R. solanacearum. Inhibiting the expression of StMLP1 during infection accelerated the infection by R. solanacearum to a certain extent. In addition, the RNA-seq results of the overexpression-StMLP1 lines indicated that StMLP1 was involved in potato immunity. All these findings in our study reveal that StMLP1 functions as a positive regulator that is induced and specifically expressed in vascular bundles in response to R. solanacearum infection.

Leaves and stolons transcriptomic analysis provide insight into the role of phytochrome F in potato flowering and tuberization.

Photoperiod plays a critical role in controlling the formation of sexual or vegetative reproductive organs in potato. Although StPHYF-silenced plants overcome day-length limitations to tuberize through a systemic effect on tuberigen StSP6A expression in the stolon, the comprehensive regulatory network of StPHYF remains obscure. Therefore, the present study investigated the transcriptomes of StPHYF-silenced plants and observed that, in addition to known components of the photoperiodic tuberization pathway, florigen StSP3D and other flowering-related genes were activated in StPHYF-silenced plants, exhibiting an early flowering response. Additionally, grafting experiments uncovered the long-distance effect of StPHYF silencing on gene expression in the stolon, including the circadian clock components, flowering-associated MADSs, and tuberization-related regulatory genes. Similar to the AtFT-AtAP1 regulatory module in Arabidopsis, the present study established that the AP1-like StMADS1 functions downstream of the tuberigen activation complex (TAC) and that suppressing StMADS1 inhibits tuberization in vitro and delays tuberization in vivo. Moreover, the expression of StSP6A was downregulated in StMADS1-silenced plants, implying the expression of StSP6A may be feedback-regulated by StMADS1. Overall, these results reveal that the regulatory network of StPHYF controls flowering and tuberization and targets the crucial tuberization factor StMADS1 through TAC, thereby providing a better understanding of StPHYF-mediated day-length perception during potato reproduction.

Suppression of the tonoplast sugar transporter, StTST3.1, affects transitory starch turnover and plant growth in potato.

Transitory starch and vacuolar sugars function as highly dynamic pools of instantly accessible metabolites in plant leaf cells. Their metabolic regulation is critical for plant survival. The tonoplast sugar transporters (TSTs), responsible for sugar uptake into vacuoles, regulate cellular sugar partitioning and vacuolar sugar accumulation. However, whether TSTs are involved in leaf transient starch turnover and plant growth is unclear. Here, we found that suppressing StTST3.1 resulted in growth retardation and pale green leaves in potato plants. StTST3.1-silenced plants displayed abnormal chloroplasts and impaired photosynthetic performance. The subcellular localization assay and the oscillation expression patterns revealed that StTST3.1 encoded a tonoplast-localized protein and responded to photoperiod. Moreover, RNA-seq analyses identified that starch synthase (SS2 and SS6) and glucan water, dikinase (GWD), were downregulated in StTST3.1-silenced lines. Correspondingly, the capacity for starch synthesis and degradation was decreased in StTST3.1-silenced lines. Surprisingly, StTST3.1-silenced leaves accumulated exceptionally high levels of maltose but low levels of sucrose and hexose. Additionally, chlorophyll content was reduced in StTST3.1-silenced leaves. Analysis of chlorophyll metabolic pathways found that Non-Yellow Coloring 1 (NYC1)-like (NOL), encoding a chloroplast-localized key enzyme that catalyzes the initial step of chlorophyll b degradation, was upregulated in StTST3.1-silenced leaves. Transient overexpression of StNOL accelerated chlorophyll b degradation in tobacco leaves. Our results indicated that StTST3.1 is involved in transitory starch turnover and chlorophyll metabolism, thereby playing a critical role in normal potato plant growth.

A Phytophthora infestans RXLR effector AVR8 suppresses plant immunity by targeting a desumoylating isopeptidase DeSI2.

The potato's most devastating disease is late blight, which is caused by Phytophthora infestans. Whereas various resistance (R) genes are known, most are typically defeated by this fast-evolving oomycete pathogen. However, the broad-spectrum and durable R8 is a vital gene resource for potato resistance breeding. To support an educated deployment of R8, we embarked on a study on the corresponding avirulence gene Avr8. We overexpressed Avr8 by transient and stable transformation, and found that Avr8 promotes colonization of P. infestans in Nicotiana benthamiana and potato, respectively. A yeast-two-hybrid (Y2H) screen showed that AVR8 interacts with a desumoylating isopeptidase (StDeSI2) of potato. We overexpressed DeSI2 and found that DeSI2 positively regulates resistance to P. infestans, while silencing StDeSI2 downregulated the expression of a set of defense-related genes. By using a specific proteasome inhibitor, we found that AVR8 destabilized StDeSI2 through the 26S proteasome and attenuated early PTI responses. Altogether, these results indicate that AVR8 manipulates desumoylation, which is a new strategy that adds to the plethora of mechanisms that Phytophthora exploits to modulate host immunity, and StDeSI2 provides a new target for durable resistance breeding against P. infestans in potato.

Molecular Insights into the Accelerated Sprouting of and Apical Dominance Release in Potato Tubers Subjected to Post-Harvest Heat Stress.

Climate change-induced heat stress (HS) increasingly threatens potato (Solanum tuberosum L.) production by impacting tuberization and causing the premature sprouting of tubers grown during the hot season. However, the effects of post-harvest HS on tuber sprouting have yet to be explored. This study aims to investigate the effects of post-harvest HS on tuber sprouting and to explore the underlying transcriptomic changes in apical bud meristems. The results show that post-harvest HS facilitates potato tuber sprouting and negates apical dominance. A meticulous transcriptomic profiling of apical bud meristems unearthed a spectrum of differentially expressed genes (DEGs) activated in response to HS. During the heightened sprouting activity that occurred at 15-18 days of HS, the pathways associated with starch metabolism, photomorphogenesis, and circadian rhythm were predominantly suppressed, while those governing chromosome organization, steroid biosynthesis, and transcription factors were markedly enhanced. The critical DEGs encompassed the enzymes pivotal for starch metabolism, the genes central to gibberellin and brassinosteroid biosynthesis, and influential developmental transcription factors, such as SHORT VEGETATIVE PHASE, ASYMMETRIC LEAVES 1, SHOOT MERISTEMLESS, and MONOPTEROS. These findings suggest that HS orchestrates tuber sprouting through nuanced alterations in gene expression within the meristematic tissues, specifically influencing chromatin organization, hormonal biosynthesis pathways, and the transcription factors presiding over meristem fate determination. The present study provides novel insights into the intricate molecular mechanisms whereby post-harvest HS influences tuber sprouting. The findings have important implications for developing strategies to mitigate HS-induced tuber sprouting in the context of climate change.

Modulation of JA signalling reveals the influence of StJAZ1-like on tuber initiation and tuber bulking in potato.

Phytohormones and their interactions play critical roles in Solanum tuberosum (potato) tuberization. The stimulatory role of jasmonic acid (JA) in tuber development is well established because of its significant promotion of tuber initiation and tuber bulking. However, the dynamics and potential function of JA signalling in potato tuberization remain largely unknown. The present study investigated the role of the JAZ1 subtype, a suppressor of JA signalling, in potato tuberization. Using 35S:StJAZ1-like-GUS as a reporter, we showed that JA signalling was attenuated from the bud end to the stem end shortly after tuber initiation. Overexpression of StJAZ1-like suppressed tuber initiation by restricting the competence for tuber formation in stolon tips, as demonstrated by grafting an untransformed potato cultivar to the stock of StJAZ1-like-overexpressing transgenic potato plants (StJAZ1-like ox). In addition, transcriptional profiling analysis revealed that StJAZ1-like modulates the expression of genes associated with transcriptional regulators, cell cycle, cytoskeleton and phytohormones. Furthermore, we showed that StJAZ1-like is destabilised upon treatment with abcisic acid (ABA), and the attenuated tuberization phenotype in StJAZ1-like ox plants can be partially rescued by ABA treatment. Altogether, these results revealed that StJAZ1-like-mediated JA signalling plays an essential role in potato tuberization.

Transcription factor StABI5-like 1 binding to the FLOWERING LOCUS T homologs promotes early maturity in potato.

Potato (Solanum tuberosum L.) maturity involves several important traits, including the onset of tuberization, flowering, leaf senescence, and the length of the plant life cycle. The timing of flowering and tuberization in potato is mediated by seasonal fluctuations in photoperiod and is thought to be separately controlled by the FLOWERING LOCUS T-like (FT-like) genes SELF-PRUNING 3D (StSP3D) and SELF-PRUNING 6A (StSP6A). However, the biological relationship between these morphological transitions that occur almost synchronously remains unknown. Here, we show that StABI5-like 1 (StABL1), a transcription factor central to abscisic acid (ABA) signaling, is a binding partner of StSP3D and StSP6A, forming an alternative florigen activation complex and alternative tuberigen activation complex in a 14-3-3-dependent manner. Overexpression of StABL1 results in the early initiation of flowering and tuberization as well as a short life cycle. Using genome-wide chromatin immunoprecipitation sequencing and RNA-sequencing, we demonstrate that AGAMOUS-like and GA 2-oxidase 1 genes are regulated by StABL1. Phytohormone profiling indicates an altered gibberellic acid (GA) metabolism and that StABL1-overexpressing plants are insensitive to the inhibitory effect of GA with respect to tuberization. Collectively, our results suggest that StABL1 functions with FT-like genes to promote flowering and tuberization and consequently life cycle length in potato, providing insight into the pleiotropic functioning of the FT gene.

ScAREB4 promotes potato constitutive and acclimated freezing tolerance associated with enhancing trehalose synthesis and oxidative stress tolerance.

Cold is a major environmental factor that restrains potato production. Abscisic acid (ABA) can enhance freezing tolerance in many plant species, but powerful evidence of the ABA-mediated signalling pathway related to freezing tolerance is still in deficiency. In the present study, cold acclimation capacity of the potato genotypes was enhanced alongside with improved endogenous content of ABA. Further exogenous application of ABA and its inhibitor (NDGA) could enhance and reduce potato freezing tolerance, respectively. Moreover, expression pattern of downstream genes in ABA signalling pathway was analysed and only ScAREB4 was identified with specifically upregulate in S. commersonii (CMM5) after cold and ABA treatments. Transgenic assay with overexpression of ScAREB4 showed that ScAREB4 promoted freezing tolerance. Global transcriptome profiling indicated that overexpression of ScAREB4 induced expression of TPS9 (trehalose-6-phosphate synthase) and GSTU8 (glutathione transferase), in accordance with improved TPS activity, trehalose content, higher GST activity and accumulated dramatically less H2 O2 in the ScAREB4 overexpressed transgenic lines. Taken together, the current results indicate that increased endogenous content of ABA is related to freezing tolerance in potato. Moreover, ScAREB4 functions as a downstream transcription factor of ABA signalling to promote cold tolerance, which is associated with increased trehalose content and antioxidant capacity.

StHAB1, a negative regulatory factor in abscisic acid signaling, plays crucial roles in potato drought tolerance and shoot branching.

Abscisic acid (ABA) is critical in drought tolerance and plant growth. Group A protein type 2C phosphatases (PP2Cs) are negative regulators of ABA signaling and plant adaptation to stress. Knowledge about the functions of potato group A PP2Cs is limited. Here, we report that the potato group A PP2C StHAB1 is broadly expressed in potato plants and strongly induced by ABA and drought. Suppression of StHAB1 enhanced potato ABA sensitivity and drought tolerance, whereas overexpression of the dominant mutant StHAB1G276D compromised ABA sensitivity and drought tolerance. StHAB1 interacts with almost all ABA receptors and the Snf1-Related Kinase OST1. Suppressing StHAB1 and overexpressing StHAB1G276D alter potato growth morphology; notably, overexpression of StHAB1G276D causes excessive shoot branching. RNA-sequencing analyses identified that the auxin efflux carrier genes StPIN3, StPIN5, and StPIN8 were up-regulated in StHAB1G276D-overexpressing axillary buds. Correspondingly, the auxin concentration was reduced in StHAB1G276D-overexpressing axillary buds, consistent with the role of auxin in repressing lateral branch outgrowth. The expression of BRANCHED1s (StBRC1a and StBRC1b) was unchanged in StHAB1G276D-overexpressing axillary buds, suggesting that StHAB1G276D overexpression does not cause axillary bud outgrowth via regulation of BRC1 expression. Our findings demonstrate that StHAB1 is vital in potato drought tolerance and shoot branching.

Protein phosphatase StTOPP6 negatively regulates potato bacterial wilt resistance by modulating MAPK signaling.

Potato (Solanum tuberosum) is an important crop globally and is grown across many regions in China, where it ranks fourth in the list of staple foods. However, its production and quality are severely affected by bacterial wilt caused by Ralstonia solanacearum. In this study, we identified StTOPP6, which belongs to the type one protein phosphatase (TOPP) family, and found that transient knock down of StTOPP6 in potato increased resistance against R. solanacearum. RNA-seq analysis showed that knock down of StTOPP6 activated immune responses, and this defense activation partly depended on the mitogen-activated protein kinase (MAPK) signal pathway. StTOPP6 inhibited the expression of StMAPK3, while overexpression of StMAPK3 enhanced resistance to R. solanacearum, supporting the negative role of StTOPP6 in plant immunity. Consistent with the results of knock down of StTOPP6, overexpressing the phosphatase-dead mutation StTOPP6m also attenuated infection and up-regulated MAPK3, showing that StTOPP6 activity is required for disease. Furthermore, we found that StTOPP6 affected the StMAPK3-mediated downstream defense pathway, eventually suppressing the accumulation of reactive oxygen species (ROS). Consistent with these findings, plants with knock down of StTOPP6, overexpression of StTOPP6m, and overexpression of StMAPK3 all displayed ROS accumulation and enhanced resistance to R. solanacearum. Taken together, the findings of our study demonstrate that StTOPP6 negatively regulates resistance to bacterial wilt by affecting the MAPK3-mediated pathway.

Genome sequence analysis provides insights into the mode of 2n egg formation in Solanum malmeanum.

KEY MESSAGE: Our genomic investigation confirms the mechanism of 2n eggs formation in S. malmeanum and aid in optimizing the use of wild germplasm. Wild potatoes are a valuable source of agronomic traits. However, substantial reproductive barriers limit gene flow into cultivated species. 2n gametes are instrumental in preventing endosperm abortion caused by genetic imbalances in the endosperm. However, little is known about the molecular mechanisms underlying the formation of 2n gametes. Here, the wild species Solanum malmeanum Bitter (2x, 1EBN, endosperm balance number) was used in inter- and intrapoloid crosses with other Solanum species, with viable seeds being produced only when S. malmeanum was used as the female parent to cross the 2EBN Solanum genus and with the likely involvement of 2n gametes. Subsequently, we substantiated the formation of 2n eggs in S. malmeanum using fluorescence in situ hybridization (FISH) and genomic sequencing technology. Additionally, the transmission rate of maternal heterozygous polymorphism sites was assessed from a genomic perspective to analyze the mode of 2n egg formation in S. malmeanum × S. tuberosum and S. malmeanum × S. chacoense crosses; each cross acquired an average of 31.12% and 22.79% maternal sites, respectively. This confirmed that 2n egg formation in S. malmeanum attributed to second-division restitution (SDR) coupled with the occurrence of exchange events. The high-throughput sequencing technology used in this study has strong advantages over traditional cytological analyses. Furthermore, S. malmeanum, which has a variety of excellent traits not available from present cultivated potato genepool, has received little research attention and has successfully achieved gene flow in cultivated species in the current study. These findings will facilitate the understanding and optimization of wild germplasm utilization in potatoes.

Comparative transcriptomics analysis reveals a calcineurin B-like gene to positively regulate constitutive and acclimated freezing tolerance in potato.

Freezing stress is a major limiting factor in crop production. To increase frost-hardiness of crops via breeding, deciphering the genes conferring freezing-tolerance is vital. Potato cultivars (Solanum tuberosum) are generally freezing-sensitive, but some potato wild species are freezing-tolerant, including Solanum commersonii, Solanum malmeanum and Solanum acaule. However, the underlying molecular mechanisms conferring the freezing-tolerance to the wild species remain to be deciphered. In this study, five representative genotypes of the above-mentioned species with distinct freezing-tolerance were investigated. Comparative transcriptomics analysis showed that SaCBL1-like (calcineurin B-like protein) was upregulated substantially in all of the freezing-tolerant genotypes. Transgenic overexpression and known-down lines of SaCBL1-like were examined. SaCBL1-like was shown to confer freezing-tolerance without significantly impacting main agricultural traits. A functional mechanism analysis showed that SaCBL1-like increases the expression of the C-repeat binding factor-regulon as well as causes a prolonged higher expression of CBF1 after exposure to cold conditions. Furthermore, SaCBL1-like was found to only interact with SaCIPK3-1 (CBL-interacting protein kinase) among all apparent cold-responsive SaCIPKs. Our study identifies SaCBL1-like to play a vital role in conferring freezing tolerance in potato, which may provide a basis for a targeted potato breeding for frost-hardiness.

The transcription factor StTINY3 enhances cold-induced sweetening resistance by coordinating starch resynthesis and sucrose hydrolysis in potato.

The accumulation of reducing sugars in cold-stored tubers, known as cold-induced sweetening (CIS), negatively affects potato processing quality. The starch to sugar interconversion pathways that are altered in cold-stored CIS tubers have been elucidated, but the mechanism that regulates them remains largely unknown. This study identified a CBF/DREB transcription factor (StTINY3) that enhances CIS resistance by both activating starch biosynthesis and repressing the hydrolysis of sucrose to reducing sugars in detached cold-stored tubers. Silencing StTINY3 in a CIS-resistant genotype decreased CIS resistance, while overexpressing StTINY3 in a CIS-sensitive genotype increased CIS resistance, and altering StTINY3 expression was associated with expression changes in starch resynthesis-related genes. We showed first that overexpressing StTINY3 inhibited sucrose hydrolysis by enhancing expression of the invertase inhibitor gene StInvInh2, and second that StTINY3 promoted starch resynthesis by up-regulating a large subunit of the ADP-glucose pyrophosphorylase gene StAGPaseL3, and the glucose-6-phosphate transporter gene StG6PT2. Using electrophoretic mobility shift assays, we revealed that StTINY3 is a nuclear-localized transcriptional activator that directly binds to the dehydration-responsive element/CRT cis-element in the promoters of StInvInh2 and StAGPaseL3. Taken together, these findings established that StTINY3 influences CIS resistance in cold-stored tubers by coordinately modulating the starch to sugar interconversion pathways and is a good target for improving potato processing quality.

Profiling of the Candidate Interacting Proteins of SELF-PRUNING 6A (SP6A) in Solanum tuberosum.

SELF-PRUNING 6A (SP6A), a homolog of FLOWERING LOCUS T (FT), has been identified as tuberigen in potato. StSP6A is a mobile signal synthesized in leaves and transmitted to the stolon through phloem, and plays multiple roles in the growth and development of potato. However, the global StSP6A protein interaction network in potato remains poorly understood. In this study, BK-StSP6A was firstly used as the bait to investigate the StSP6A interaction network by screening the yeast two-hybrid (Y2H) library of potato, resulting in the selection of 200 independent positive clones and identification of 77 interacting proteins. Then, the interaction between StSP6A and its interactors was further confirmed by the Y2H and BiFC assays, and three interactors were selected for further expression analysis. Finally, the expression pattern of Flowering Promoting Factor 1.1 (StFPF1.1), No Flowering in Short Days 1 and 2 (StNFL1 and StNFL2) was studied. The three genes were highly expressed in flowers or flower buds. StFPF1.1 exhibited an expression pattern similar to that of StSP6A at the stolon swelling stages. StPHYF-silenced plants showed up-regulated expression of StFPF1.1 and StSP6A, while expression of StNFL1 and StNFL2 was down-regulated in the stolon. The identification of these interacting proteins lays a solid foundation for further functional studies of StSP6A.

Salicylic Acid Improves the Constitutive Freezing Tolerance of Potato as Revealed by Transcriptomics and Metabolomics Analyses.

Freezing severely impacts potato production. Deciphering the pathways and metabolites that regulate the freezing tolerance of potato is useful in cultivation and breeding for hardiness. In the present study, Solanum acaule was identified to be more freezing tolerant than S. tuberosum. Furthermore, the two genotypes before/after exposure to 4 °C for 7 d with additional -1 °C for 12 h were analysed by RNA-seq and metabolomics, and the results were compared with the previous -1 °C for 12 h. The results showed that S. acaule activated numerous genes that differed from those of S. tuberosum. Among the genes, five pathways, such as the hormone signalling pathway, which includes salicylic acid, were enriched. Further metabolomics analysis showed that the content of salicylic acid was improved in S. acaule in response to -1 °C for 12 h. Moreover, exogenous application of 0.1 mM salicylic acid to potato was shown to improve constitutive freezing tolerance and increase the expression of HSFC1. Following transcriptome and metabolome analyses, it was documented that the content of SA that increased in freezing-tolerant S. acaule after exposure to cold condition, associated with the SA signalling pathway, enhanced potato freezing tolerance, probably through HSFC1.

Proteomic Analysis of Potato Responding to the Invasion of Ralstonia solanacearum UW551 and Its Type III Secretion System Mutant.

The infection of potato with Ralstonia solanacearum UW551 gives rise to bacterial wilt disease via colonization of roots. The type III secretion system (T3SS) is a determinant factor for the pathogenicity of R. solanacearum. To fully understand perturbations in potato by R. solanacearum type III effectors(T3Es), we used proteomics to measure differences in potato root protein abundance after inoculation with R. solanacearum UW551 and the T3SS mutant (UW551△HrcV). We identified 21 differentially accumulated proteins. Compared with inoculation with UW551△HrcV, 10 proteins showed significantly lower abundance in potato roots after inoculation with UW551, indicating that those proteins were significantly downregulated by T3Es during the invasion. To identify their functions in immunity, we silenced those genes in Nicotiana benthamiana and tested the resistance of the silenced plants to the pathogen. Results showed that miraculin, HBP2, and TOM20 contribute to immunity to R. solanacearum. In contrast, PP1 contributes to susceptibility. Notably, none of four downregulated proteins (HBP2, PP1, HSP22, and TOM20) were downregulated at the transcriptional level, suggesting that they were significantly downregulated at the posttranscriptional level. We further coexpressed those four proteins with 33 core T3Es. To our surprise, multiple effectors were able to significantly decrease the studied protein abundances. In conclusion, our data showed that T3Es of R. solanacearum could subvert potato root immune-related proteins in a redundant manner.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Ralstonia solanacearum type III effector RipV2 encoding a novel E3 ubiquitin ligase (NEL) is required for full virulence by suppressing plant PAMP-triggered immunity.

Ralstonia solanacearum causes bacterial wilt disease in a broad range of plants, primarily through type Ⅲ secreted effectors. However, the R. solanacearum effectors promoting susceptibility in host plants remain limited. In this study, we determined that the R. solanacearum effector RipV2 functions as a novel E3 ubiquitin ligase (NEL). RipV2 was observed to be locali in the plasma membrane after translocatio into plant cells. Transient expression of RipV2 in Nicotiana benthamiana could induce cell death and suppress the flg22-induced pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) responses, mediating such effects as attenuation of the expression of several PTI-related genes and ROS bursts. Furthermore, we demonstrated that the conserved catalytic residue is highly important for RipV2. Transient expression of the E3 ubiquitin ligase catalytic mutant RipV2 C403A alleviated the PTI suppression ability and cell death induction, indicating that RipV2 requires its E3 ubiquitin ligase activity for its role in plant-microbe interactions. More importantly, mutation of RipV2 in R. solanacearum reduces the virulence of R. solanacearum on potato. In conclusion, we identified a NEL effector that is required for full virulence of R. solanacearum by suppressing plant PTI.

Extreme Resistance to Potato Virus A in Potato Cultivar Barbara is Independently Mediated by Ra and Rysto.

Potato virus A (PVA) and potato virus Y (PVY) are two members of genus Potyvirus infecting potato crops worldwide. Host resistance offers an economical and effective means for the control or management of these viruses. In this study, 20 potato clones were screened for their resistance against PVA and PVY by mechanical or graft inoculation assay, and were explored for the relationship between extreme resistance genes Ra and Ry by the detection of molecular markers linked to Ryadg, Rysto, and Rychc. Six clones, including Barbara, Jizhangshu 8, Longshu 7, Longshu 8, M6, and Solara, were found to be extremely resistant to both PVA and PVY; three clones (AC142, Eshu 3, and Shepody) were deemed to be extremely resistant to PVA but susceptible to PVY. To further reveal the inheritance of the extreme resistance (ER) against PVA, a tetraploid F1 population of Barbara × F58050 (susceptible to both PVY and PVA) and a tetraploid BC1 population of BF145 (a PVA-resistant but PVY-susceptible progeny of Barbara × F58050) × F58050 were obtained. Phenotyping of the F1 and BC1 populations by graft inoculation with PVA showed segregation ratios of 3:1 and 1:1 (resistant:susceptible), respectively. These results suggest that two independent loci control ER against PVA in Barbara: one confers ER to both PVA and PVY and the other confers ER to PVA only. The deduced genotype of Barbara is RyryryryRararara.

Phytochrome F plays critical roles in potato photoperiodic tuberization.

The transition to tuberization contributes greatly to the adaptability of potato to a wide range of environments. Phytochromes are important light receptors for the growth and development of plants, but the detailed functions of phytochromes remain unclear in potato. In this study, we first confirmed that phytochrome F (StPHYF) played essential roles in photoperiodic tuberization in potato. By suppressing the StPHYF gene, the strict short-day potato genotype exhibited normal tuber formation under long-day (LD) conditions, together with the degradation of the CONSTANTS protein StCOL1 and modulation of two FLOWERING LOCUS T (FT) paralogs, as demonstrated by the repression of StSP5G and by the activation of StSP6A during the light period. The function of StPHYF was further confirmed through grafting the scion of StPHYF-silenced lines, which induced the tuberization of untransformed stock under LDs, suggesting that StPHYF was involved in the production of mobile signals for tuberization in potato. We also identified that StPHYF exhibited substantial interaction with StPHYB both in vitro and in vivo. Therefore, our results indicate that StPHYF plays a role in potato photoperiodic tuberization, possibly by forming a heterodimer with StPHYB.