Utilizing resequencing big data to facilitate Brassica vegetable breeding:tracing introgression pedigree and developing highly specific markers for clubroot resistance

Clubroot caused by Plasmodiophora brassicae is a devastating disease of Cruciferous crops.Developing cultivars with clubroot resistance(CR)is the most effective control measure.For the two major Brassica vegetable species B.rapa and B.oleracea,several commercial cultivars with unclear CR pedigrees have been intensively used as CR donors in breeding.However,the continuous occurrence of CR-breaking makes the CR pedigree underlying these cultivars one of the breeders'most urgent concerns.The complex intraspecific diversity of these two major Brassica vegetables has also limited the applicability of CR markers in different breeding programs.Here we first traced the pedigree underlying two kinds of CR that have been widely applied in breeding by linkage and introgression analyses based on public resequencing data.In B.rapa,a major locus CRzi8 underlying the CR of the commercial CR donor‘DegaoCR117’was identified.CRzi8 was further shown to have been introgressed from turnip(B.rapa ssp.rapifera)and that it carried a potential functional allele of Crr1a.The turnip introgression carried CRb^(c),sharing the same coding sequence with the CRb that was also identified from chromosome C07 of B.oleracea CR cultivars with different morphotypes.Within natural populations,variation analysis of linkage intervals of CRzi8,PbBa8.1,CRb,and CRb^(c)yielded easily resolved InDel markers(>20 bp)for these fundamental CR genes.The specificity of these markers was tested in diverse cultivars panels,and each exhibited high reliability in breeding.Our research demonstrates the value of the practice of applying resequencing big data to solve urgent concerns in breeding programs.

VG,encoding a thylakoid formation protein,regulates the formation of variegated leaves in tomato

Leaf-color mutations have been studied extensively in plants.However,to better understand the complex mechanisms underlying the formation of leaf color,it is essential to continue discover novel genes involved in the process of leaf color development.In this study,we identified a variegated-leaf(vg)mutant in tomato that exhibited defective phenotypes in thylakoids and photosynthesis.To clone the vg locus,an F2population was constructed from the cross between the vg mutant(Solanum lycopersicum)and the wild tomato LA1589(S.pimpinellifolium).Using the map-based cloning approach,the vg locus was mapped on chromosome 7 and narrowed down to a 128 kb region that contained 21 open reading frames(ORFs).The expression levels of ORF9,ORF10,and ORF13 were significantly lower in vg than in the wild-type plants,while the ORF11 transcript level was elevated in vg.We then mutagenized ORF9,ORF10,and ORF13 by the CRISPR/Cas9 system in the wild-type tomato background and found that only the ORF10 mutation reproduced the phenotype of variegated leaves,indicating that ORF10 represents VG and its down-regulated expression was responsible for the variegated leaf phenotype.ORF10 encodes a thylakoid formation protein and its mutant lines showed reduced levels of chlorophyll synthesis and photosynthesis.Taken together,these results suggest that VG is necessary for chloroplast development,chlorophyll synthesis,and photosynthesis in tomato.

SlZF3 regulates tomato plant height by directly repressing SlGA20ox4 in the gibberellic acid biosynthesis pathway

Plant height is an important target trait for crop genetic improvement.Our previous work has identified a salt-tolerant C2H2 zinc finger,SlZF3,and its overexpression lines also showed a semi-dwarf phenotype,but themolecular mechanism remains to be elucidated.Here,we characterized the dwarf phenotype in detail.The dwarfism is caused by a decrease in stem internode cell elongation and deficiency of bioactive gibberellic acids(GAs),and can be rescued by exogenous GA3 treatment.Gene expression assays detected reduced expression of genes in the GA biosynthesis pathway of the overexpression lines,including SlGA20ox4.Several protein-DNA interaction methods confirmed that SlZF3 can directly bind to the SlGA20ox4 promoter and inhibit its expression,and the interaction can also occur for SlKS and SlKO.Overexpression of SlGA20ox4 in the SlZF3-overexpressing line can recover the dwarf phenotype.Therefore,SlZF3 regulates plant height by directly repressing genes in the tomato GA biosynthesis pathway.

A 21-bp InDel in the promoter of STP1 selected during tomato improvement accounts for soluble solid content in fruits

Domestication and improvement are important processes that generate the variation in genome and phonotypes underlying crop improvement.Unfortunately,during selection for certain attributes,other valuable traits may be inadvertently discarded.One example is the decline in fruit soluble solids content(SSC)during tomato breeding.Several genetic loci for SSC have been identified,but few reports on the underlying mechanisms are available.In this study we performed a genome-wide association study(GWAS)for SSC of the red-ripe fruits in a population consisting of 481 tomato accessions with large natural variations and found a new quantitative trait locus,STP1,encoding a sugar transporter protein.The causal variation of STP1,a 21-bp InDel located in the promoter region 1124 bp upstream of the start codon,alters its expression.STP1 Insertion accessions with an 21-bp insertion have higher SSC than STP1Deletion accessions with the 21-bp deletion.Knockout of STP1 in TS-23 with high SSC using CRISPR/Cas9 greatly decreased SSC in fruits.In vivo and in vitro assays demonstrated that ZAT10-LIKE,a zinc finger protein transcription factor(ZFP TF),can specifically bind to the promoter of STP1Insertion to enhance STP1 expression,but not to the promoter of STP1Deletion,leading to lower fruit SSC in modern tomatoes.Diversity analysis revealed that STP1 was selected during tomato improvement.Taking these results together,we identified a naturally occurring causal variation underlying SSC in tomato,and a new role for ZFP TFs in regulating sugar transporters.The findings enrich our understanding of tomato evolution and domestication,and provide a genetic basis for genome design for improving fruit taste.

A CCAAT-binding factor,SlNFYA10,negatively regulates ascorbate accumulation by modulating the D-mannose/L-galactose pathway in tomato

Ascorbic acid(AsA),an important antioxidant and growth regulator,and it is essential for plant development and human health.Specifically,humans have to acquire AsA from dietary sources due to their inability to synthesize it.The AsA biosynthesis pathway in plants has been elucidated,but its regulatory mechanism remains largely unknown.In this report,we biochemically identified a CCAAT-box transcription factor(SlNFYA10)that can bind to the promoter of SlGME1,which encodes GDP-Man-3’,5’-epimerase,a pivotal enzyme in the D-mannose/L-galactose pathway.Importantly,SlNFYA10 simultaneously binds to the promoter of SlGGP1,a downstream gene of SlGME1 in the Dmannose/L-galactose pathway.Binding assays in yeast and functional analyses in plants have confirmed that SlNFYA10 exerts a negative effect on the expression of both SlGME1 and SlGGP1.Transgenic tomato lines overexpressing SlNFYA10 show decreased levels of SlGME1 and SlGGP1 abundance and AsA concentration in their leaves and fruits,accompanied by enhanced sensitivity to oxidative stress.Overall,SlNFYA10 is the first CCAAT-binding factor identified to date to negatively regulate the AsA biosynthetic pathway at multiple sites and modulate plant responses to oxidative stress.

SlBBX20 interacts with the COP9 signalosome subunit SlCSN5-2 to regulate anthocyanin biosynthesis by activating SlDFR expression in tomato

Anthocyanins play vital roles in plant stress tolerance and growth regulation.Previously,we reported that the photomorphogenesis-related transcription factor SlBBX20 regulates anthocyanin accumulation in tomato.However,the underlying mechanism remains unclear.Here,we showed that SlBBX20 promotes anthocyanin biosynthesis by binding the promoter of the anthocyanin biosynthesis gene SlDFR,suggesting that SlBBX20 directly activates anthocyanin biosynthesis genes.Furthermore,we found by yeast two-hybrid screening that SlBBX20 interacts with the COP9 signalosome subunit SlCSN5-2,and the interaction was confirmed by bimolecular fluorescence complementation and coimmunoprecipitation assays.SlCSN5 gene silencing led to anthocyanin hyperaccumulation in the transgenic tomato calli and shoots,and SlCSN5-2 overexpression decreased anthocyanin accumulation,suggesting thSlCSN5-2 enhanced the ubiquitination of SlBBX20 and promoted the degradation of SlBBX20 in vivo.Consistently,silencing the SlCSN5-2 homolog in tobacco significantly increased the accumulation of the SlBBX20 protein.Since SlBBX20 is a vital regulator of photomorphogenesis,the SlBBX20-SlCSN5-2 module may represent a novel regulatory pathway in light-induced anthocyanin biosynthesis.

The loss of function of HEL,which encodes a cellulose synthase interactive protein,causes helical and vine-like growth of tomato

Helical growth is an economical way for plant to obtain resources.The classic microtubule–microfibril alignment model of Arabidopsis helical growth involves restriction of the appropriate orientation of cellulose microfibrils appropriately in the cell walls.However,the molecular mechanism underlying tomato helical growth remains unknown.Here,we identified a spontaneous tomato helical(hel)mutant with right-handed helical cotyledons and petals but left-handed helical stems and true leaves.Genetic analysis revealed that the hel phenotype was controlled by a single recessive gene.Using map-based cloning,we cloned the HEL gene,which encodes a cellulose interacting protein homologous to CSI1 of Arabidopsis.We identified a 27 bp fragment replacement that generated a premature stop codon.Transgenic experiments showed that the helical growth phenotype could be restored by the allele of this gene from wild-type Pyriforme.In contrast,the knockout mutation of HEL in Pyriforme via CRISPR/Cas9 resulted in helical growth.These findings shed light on the molecular control of the helical growth of tomato.

Interactions between ShPP2-1, an F-box family gene, and ACR11A regulate cold toleranee of tomato

There is a critical need to identify germplasm resources and genes that promote cold tolerance of tomato because global tomato production is threatened by cold stress.We found that the expression of an F-box gene family member named ShPP2-1 from Solanum habrochaites is cold inducible and studied its contribution to cold tolerance.Overexpression of ShPP2-1 in cultivated tomato(AC)reduced cold tolerance by intensifying damage to cell membranes.To explore the underlying molecular mechanism,we conducted a yeast two-hybrid library screen and found that a protein containing ACT domain repeats named ACR11A interacts with PP2-1.Overexpression of SIACR11A in AC enhanced the cold tolerance of seedlings and germinating seeds.Cold tolerance decreased in tomato plants that overexpressed both of these genes.Additionally,we performed seed germination experiments in the cold with 177 tomato accessions and identified two alleles of SlACR11A that differ in one single-nucleotide polymorphism.We found that one of these alleles,SlACR11A G,is significantly enriched in cold-tolerant tomato plants.Taken together,our fi ndings indicate that the combination of low expression levels of PP2-1 and high expression levels of ACR11A can promote cold tolerance.These genes may therefore serve as direct targets for both genetic engineering and improvement projects that aim to enhance the cold tolerance of tomato.

SIRCM1,which encodes tomato Lutescent1,is required for chlorophyll synthesis and chloroplast development in fruits

In plants,chloroplasts are the sites at which photosynthesis occurs,and an increased abundance of chloroplasts increases the nutritional quality of plants and the resultant color of fruits.However,the molecular mechanisms underlying chlorophyll synthesis and chloroplast development in tomato fruits remain unknown.In this study,we isolated a chlorophyll-de fi cient mutant,reduced chlorophyll mutant 1(rcml),by ethylmethanesulfonate mutagenesis;this mutant produced yellowish fruits with altered chloroplast development.MutMap revealed that Solyc08g005010 is the causal gene underlying the rcm1 mutant phenotype.A single-nucleotide base substitution in the second exon of SIRCM1 results in premature termination of its translated protein.SIRCM1 encodes a chloroplast-targeted metalloendopeptidase that is orthologous to the BCM1 protein of Arabidopsis and the stay-green G protein of soybean(Glycine max L.Merr.).Notably,the yellowish phenotype of the lutescent1 mutant can be restored with the allele of SlRCM1 from wild-type tomato.In contrast,knockout of SlRCM1 by the CRISPR/Cas9 system in Alisa Craig yielded yellowish fruits at the mature green stage,as was the case for lutescent1.Amino acid sequence alignment and functional complementation assays showed that SlRCM1 is indeed Lutescent1.These fi ndings provide new insights into the regulation of chloroplast development in tomato fruits.

Cyclin gene SlCycB1 alters plant architecture in association with histone H3.2 in tomato

Cell cycle regulation plays a critical role in plant growth and development.In this study,the role of a tomato cell cycle gene SlCycB1 has been characterized.Expression analysis revealed that SlCycB1 was mostly expressed in stem,root,and leaves,with relative lower expression in flower and fruit.Tomato plants overexpressing SlCycB1 exhibited a reduction in cell number and increased cell size leading to the growth retardation.Furthermore,yeast two-hybrid analysis and bimolecular fluorescence complementation revealed that SlCycB1 interacted with histone H3.2,an essential component of the nucleosome.Histone H3.2 was transcriptionally up-regulated in the SlCycB1 overexpressing tomato lines.Furthermore,the overexpression of histone H3.2 in transgenic plants showed similar phenotypes to SlCycB1 overexpressing lines.Based on these findings,we concluded that SlCycB1 overexpression altered tomato architecture in association with histone H3.2.

Genome-wide Analysis of Plant-specific Dof Transcription Factor Family in Tomato

The Dof （DNA binding with One Finger） family encoding single zinc finger proteins has been known as a family of plant-specific transcription factors. These transcription factors are involved in a variety of functions of importance for different biological processes in plants. In the current study, we identified 34 Dof family genes in tomato （Solanum lycopersicum L.）, distributed on 11 chromosomes. A complete overview of SlDof genes in tomato is presented, including the gene structures, chromosome locations, phylogeny, protein motifs and evolution pattern. Phylogenetic analysis of 34 SlDof proteins resulted in four classes constituting six clusters. In addition, a comparative analysis between these genes in tomato, Arabidopsis （Arabidopsis thaliana L.） and rice （Oryza sativa L.） was also performed. The tomato Dof family expansion has been dated to recent duplication events, and segmental duplication is predominant for the SlDof genes. Furthermore, the SlDof genes displayed differential expression either in their transcript abundance or in their expression patterns under normal growth conditions. This is the first step towards genome-wide analyses of the Dof genes in tomato. Our study provides a very useful reference for cloning and functional analysis of the members of this gene family in tomato and other species.

Rapid breeding of pink-fruited tomato hybrids using the CRISPR/Cas9 system

As one of the most important vegetables,tomato (Solanum lycopersicum) is extensively produced and consumed worldwide and substantially contributes to human nutrition and health (The Tomato Genome Consortium,2012).Although red tomatoes are the most common,pink tomatoes are more popular in Asia,particularly in China and Japan,because of their better taste (Ballester et al.,2010;Zhu et al.,2018).Compared with red tomatoes,pink tomatoes fail to accumulate the yellow-colored flavonoid pigment,naringenin chalcone (NarCh),in their peels,resulting in a colorless peel phenotype (Adato et al,2009;Ballester et al.,2010).

Silencing GRAS2 reduces fruit weight in tomato

GRAS family transcription factors are involved in multiple biological processes in plants. Here, we report that GRAS2 plays a vital role in regulating fruit weight in tomato （Solanum lycopersicum）. We establish that the expression of GRAS2 was elevated in ovaries and maintained at a constant level in fertilized ovules. Reduction of GRAS2 expression in transgenic plants reduced fruit weight through modulating ovary growth and cell size. At the metabolic level, downregulation of GRAS2 decreased activities of the gibberellic acid biosyn- thesis and signal transduction pathways, leading to insufficient levels of active gibberellic acid during the initial ovary development of tomato. Moreover, genotypic diversity of GRAS2 was consistent with the molecular basis of fruit weight evolution, suggesting that GRAS2 contributes to the molecular basis of the evolution of fruit weight in tomato. Collectively, these findings enhance our understanding of GRAS2 functions, in fruit development of tomato, and demonstrate a strong association between the GRAS gene family and fruit development.

FUNCTIONAL GAIN OF FRUIT NETTED-CRACKING IN AN INTROGRESSION LINE OF TOMATO WITH HIGHER EXPRESSION OF THE FNC GENE

Fruit cracking is a major disorder that affects the integrity of fruit and reduces the commercial value of tomato and other fleshy fruit.Here,we have found a novel fruit'netted-cracking'(FNC)phenotype in tomato introgression line IL4-4 which is present in neither the donor parent(LA0716)nor the receptor parent(M82).An F2 population was generated by crossing IL4-4 with M82 to genetically characterize the FNC gene and this showed that a single dominant gene determined fruit netted-cracking.Further map-based cloning narrowed down the FNC locus to a 230 kb region on chromosome 4.Sequencing and annotation analysis show that FNC(Solyc04 g082540)was the most likely candidate gene.Functional characterization of FNC by overexpressing FNC^c and FNC1^resulted in the fruit netted-cracking phenotype,suggesting that the FNC transcript level results in the functional gain of fruit netted-cracking.These findings were further confirmed by FNC ortholog in netted-cracking pepper and melon,indicating a common regulatory mechanism in different plant species.Furthermore,cytoplasm and nucleus-localized FNC indicates increased expression of genes involved in suberin,lignin,lipid transport and cell wall metabolism.These findings provide novel genetic insights into fruit netted-cracking and offer a way to promote molecular improvement toward cracking resistant cultivars.