Molecular regulatory events of flower and fruit abscission in horticultural plants

Flower and fruit abscission is a highly programmed physiological process,which is closely related to the yield of horticultural plants.The coordination of many regulatory factors associated with metabolic and signaling pathways plays a key role in the flower and fruit shedding.Hormones,peptides,carbohydrates,polyamines or cell wall modifying proteins regulate flower and fruit shedding.This article reviewed the recent studies of flower and fruit abscission,including the molecular regulation mechanism of abscission zone formation,typical structure and location of abscission zones,and other factors affecting flower and fruit abscission,such as stresses,hormones,peptides,carbohydrates,polyamines and cell wall modifying proteins.Overall,the review summarizes the developmental mechanism and the diversity of abscission zones,and the key factors affecting flower and fruit abscission of horticultural plants,aiming to provide guidance for studying the molecular regulatory mechanism of flower and fruit abscission.

Role of plant hormones in flowering and exogenous hormone application in fruit/nut trees:a review of pecans

Pecan is the only native north American tree nut.The USA produces approximately 80%of the world’s pecans.Pecan trees have an extended juvenility,10 years to the first nut crop.With mature bearing they begin alternate bearing;alternating large and small crops.Theoretically,a heavy crop inhibits flower induction in the current year resulting in a low crop the following year.The flowering of perennial trees involves a complex interplay of multiple hormones.The possible molecular mechanisms regulating tree flowering can be revealed by endogenous plant hormone quantification,exogenous hormone application and RNA-sequencing.In this review,we synthesize the investigations of transcriptomic analysis and exogenous hormone treatments on bud break and flowering in fruit/nut trees with a focus on pecan.Knowledge of how hormones regulate flowering suggest they are a potential tool for improving return bloom and mitigating alternate bearing.

Production of marker-free transgenic plants from mature tissues of navel orange using a Cre/loxP site-recombination system

Genetic transformation with mature material as the explants could shorten the transgenic period and avoid seed dependence compared with genetic transformation using the epicotyl seedling stem segments as the receptor. Here, we constructed an Agrobacterium tumefaciensmediated transformation for generation of marker-free transgenic plants from navel orange(Citrus sinensis Osbeck) mature stems using a CreloxP recombination system. To efficiently recover the regenerated buds from mature tissues, five recovery methods were compared: in vitro micrografting of 0.1-0.5(1-2 weeks), > 0.5 cm(3-4 weeks) and > 1 cm long lignified bud and in vitro micrografting of explants with a bud and rooting regenerated bud. The data showed that in vitro micrografting of > 1 cm long regenerated bud with expanded leaves after one month of continuous culture for lignification was the optimal solution for plant recovery from mature tissues. Transgenic plants without selectable marker genes were created from navel orange(Citrus sinensis Osbeck) tissue using a transformation vector PLI-35SPR1aCB containing a Cre/loxP system recombination together with genes encoding the selectable marker isopentenyl transferase(IPT) and an anti-bacterial peptide(PR1aCB).Using IPT positive selection, the transformation efficiency determined by PCR was 0.9%, and in total, 20 transgenic plants were obtained.Southern blotting confirmed further their transgenicity. PCR and sequencing analysis demonstrated that both the Cre and IPT genes had been successfully removed from the transgenic plants(deletion efficiency 100%). Over all, using Cre/loxP system recombination together with the IPT positive selection, marker-free transgenic plants can be recovered efficiently from mature tissues of navel orange(Citrus sinensis Osbeck), which provides a potential method for production of transgenic plants from citrus mature tissue.

Mapping of QTLs and candidate genes associated with multiple phenotypic traits for Huanglongbing tolerance in citrus

Huanglongbing(HLB)is the most devastating disease for citrus worldwide.Candidatus Liberibacter asiaticus(C Las),vectored by Asian citrus psyllid(ACP,Diaphorina citri Kuwayama),is the most common pathogen causing the disease.Commercial citrus varieties are highly susceptible to HLB,whereas trifoliate orange(Poncirus trifoliata)is considered highly tolerant to HLB.An F1 segregating population and their parent trifoliate orange and sweet orange,which had been exposed to intense HLB pressure for three years,was evaluated for disease symptoms,ACP colonization,C Las titer and tree vigor repeatedly for two to three years.Trifoliate orange and sweet orange showed significant differences for most of the phenotypic traits,and the F1 population exhibited a large variation.A high-density SNP-based genetic map with 1402 markers was constructed for trifoliate orange,which exhibited high synteny and high coverage of its reference genome.A total of 26 quantitative trait locus(QTLs)were identified in four linkage groups LG-t6,LG-t7,LG-t8 and LG-t9,of which four QTL clusters exhibit a clear co-localization of QTLs associated with different traits.Through genome-wide analysis of gene expression in response to C Las infection in‘Flying Dragon’and‘Larger-Flower DPI-50-7’trifoliate orange,85 differentially expressed genes were found located within the QTL clusters.Among them,seven genes were classified as defense or immunity protein which exhibited the highest transcriptional change after C Las infection.Our results indicate a quantitative genetic nature of HLB tolerance and identified candidate genes that should be valuable for searching for genetic solutions to HLB through breeding or genetic engineering.

CsPrx25,a class III peroxidase in Citrus sinensis,confers resistance to citrus bacterial canker through the maintenance of ROS homeostasis and cell wall lignification

Citrus bacterial canker(CBC)results from Xanthomonas citri subsp.citri(Xcc)infection and poses a grave threat to citrus production.Class III peroxidases(CIII Prxs)are key proteins to the environmental adaptation of citrus plants to a range of exogenous pathogens,but the role of CIII Prxs during plant resistance to CBC is poorly defined.Herein,we explored the role of CsPrx25 and its contribution to plant defenses in molecular detail.Based on the expression analysis,CsPrx25 was identified as an apoplast-localized protein that is differentially regulated by Xcc infection,salicylic acid,and methyl jasmone acid in the CBC-susceptible variety Wanjincheng(C.sinensis)and the CBC-resistant variety Calamondin(C.madurensis).Transgenic Wanjincheng plants overexpressing CsPrx25 were generated,and these transgenic plants exhibited significantly increased CBC resistance compared with the WT plants.In addition,the CsPrx25-overexpressing plants displayed altered reactive oxygen species(ROS)homeostasis accompanied by enhanced H_(2)O_(2) levels,which led to stronger hypersensitivity responses during Xcc infection.Moreover,the overexpression of CsPrx25 enhanced lignification as an apoplastic barrier for Xcc infection.Taken together,the results highlight how CsPrx25-mediated ROS homeostasis reconstruction and cell wall lignification can enhance the resistance of sweet orange to CBC.

SIZHD17 is involved in the control of chlorophyll and carotenoid metabolism in tomato fruit

Chlorophylls and carotenoids are essential and bene fi cial substances for both plant and human health.Identifying the regulatory network of these pigments is necessary for improving fruit quality.In a previous study,we identi fi ed an R2R3-MYB transcription factor,SlMYB72,that plays an important role in chlorophyll and carotenoid metabolism in tomato fruit.Here,we demonstrated that the SlMYB72-interacting protein SlZHD17,which belongs to the zinc-fi nger homeodomain transcription factor family,also functions in chlorophyll and carotenoid metabolism.Silencing SIZHD 17 in tomato improved multiple bene fi cial agronomic traits,including dwar fi sm,accelerated fl owering,and earlier fruit harvest.More importantly,downregulating SIZHD17 in fruits resulted in larger chloroplasts and a higher chlorophyll content.Dual-luciferase,yeast one-hybrid and electrophoretic mobility shift assays clari fi ed that SlZHD17 regulates the chlorophyll biosynthesis gene SIPOR-B and chloroplast developmental regulator SITKN2 in a direct manner.Chlorophyll degradation and plastid transformation were also retarded after suppression of SIZHD17 in fruits,which was caused by the inhibition of SISGR1,a crucial factor in chlorophyll degradation.On the other hand,the expression of the carotenoid biosynthesis genes SIPSY1 and SIZISO was also suppressed and directly regulated by SlZHD17,which induced uneven pigmentation and decreased the lycopene content in fruits with SIZHD17 suppression at the ripe stage.Furthermore,the protein-protein interactions between SlZHD17 and other pigment regulators,including SlARF4,SlBEL11,and SlTAGL1,were also presented.This study provides new insight into the complex pigment regulatory network and provides new options for breeding strategies aiming to improve fruit quality.

CitGVD:a comprehensive database of citrus genomic variations

Citrus is one of the most important commercial fruit crops worldwide.With the vast genomic data currently available for citrus fruit,genetic relationships,and molecular markers can be assessed for the development of molecular breeding and genomic selection strategies.In this study,to permit the ease of access to these data,a web-based database,the citrus genomic variation database(CitGVD,http://citgvd.cric.cn/home)was developed as the first citrusspecific comprehensive database dedicated to genome-wide variations including single nucleotide polymorphisms(SNPs)and insertions/deletions(INDELs).The current version(V1.0.0)of CitGVD is an open-access resource centered on 1,493,258,964 high-quality genomic variations and 84 phenotypes of 346 organisms curated from in-house projects and public resources.CitGVD integrates closely related information on genomic variation annotations,related gene annotations,and details regarding the organisms,incorporating a variety of built-in tools for data accession and analysis.As an example,CitGWAS can be used for genome-wide association studies(GWASs)with SNPs and phenotypic data,while CitEVOL can be used for genetic structure analysis.These features make CitGVD a comprehensive web portal and bioinformatics platform for citrus-related studies.It also provides a model for analyzing genome-wide variations for a wide range of crop varieties.

Auxin response factor 6A regulates photosynthesis,sugar accumulation,and fruit development in tomato

Auxin response factors(ARFs)are involved in auxin-mediated transcriptional regulation in plants.In this study,we performed functional characterization of SlARF6A in tomato.SlARF6A is located in the nucleus and exhibits transcriptional activator activity.Overexpression of SlARF6A increased chlorophyll contents in the fruits and leaves of tomato plants,whereas downregulation of SlARF6A decreased chlorophyll contents compared with those of wild-type(WT)plants.Analysis of chloroplasts using transmission electron microscopy indicated increased sizes of chloroplasts in SlARF6A-overexpressing plants and decreased numbers of chloroplasts in SlARF6A-downregulated plants.Overexpression of SlARF6A increased the photosynthesis rate and accumulation of starch and soluble sugars,whereas knockdown of SlARF6A resulted in opposite phenotypes in tomato leaves and fruits.RNA-sequence analysis showed that regulation of SlARF6A expression altered the expression of genes involved in chlorophyll metabolism,photosynthesis and sugar metabolism.SlARF6A directly bound to the promoters of SlGLK1,CAB,and RbcS genes and positively regulated the expression of these genes.Overexpression of SlARF6A also inhibited fruit ripening and ethylene production,whereas downregulation of SlARF6A increased fruit ripening and ethylene production.SlARF6A directly bound to the SAMS1 promoter and negatively regulated SAMS1 expression.Taken together,these results expand our understanding of ARFs with regard to photosynthesis,sugar accumulation and fruit development and provide a potential target for genetic engineering to improve fruit nutrition in horticulture crops.

SlMYB72 affects pollen development by regulating autophagy in tomato

The formation and development of pollen are among the most critical processes for reproduction and genetic diversity in the life cycle of f lowering plants.The present study found that SlMYB72 was highly expressed in the pollen and tapetum of tomato f lowers.Downregulation of SlMYB72 led to a decrease in the amounts of seeds due to abnormal pollen development compared with wild-type plants.Downregulation of SlMYB72 delayed tapetum degradation and inhibited autophagy in tomato anther.Overexpression of SlMYB72 led to abnormal pollen development and delayed tapetum degradation.Expression levels of some autophagy-related genes(ATGs)were decreased in SlMYB72 downregulated plants and increased in overexpression plants.SlMYB72 was directly bound to ACCAAC/ACCAAA motif of the SlATG7 promoter and activated its expression.Downregulation of SlATG7 inhibited the autophagy process and tapetum degradation,resulting in abnormal pollen development in tomatoes.These results indicated SlMYB72 affects the tapetum degradation and pollen development by transcriptional activation of SlATG7 and autophagy in tomato anther.The study expands the understanding of the regulation of autophagy by SlMYB72,uncovers the critical role that autophagy plays in pollen development,and provides potential candidate genes for the production of male-sterility in plants.

RIN enhances plant disease resistance via root exudate-mediated assembly of diseasesuppressive rhizosphere microbiota

The RIPENING-INHIBITOR(RIN)transcriptional factor is a key regulator governing fruit ripening.While RIN also affects other physiological processes,its potential roles in triggering interactions with the rhizosphere microbiome and plant health are unknown.Here we show that RIN affects microbiome-mediated disease resistance via root exudation,leading to recruitment of microbiota that suppress the soil-borne,phytopathogenic Ralstonia solanacearum bacterium.Compared with the wild-type(WT)plant,RIN mutants had different root exudate profiles,which were associated with distinct changes in microbiome composition and diversity.Specifically,the relative abundances of antibiosis-associated genes and pathogensuppressing Actinobacteria(Streptomyces)were clearly lower in the rhizosphere of rin mutants.The composition,diversity,and suppressiveness of rin plant microbiomes could be restored by the application of 3-hydroxyflavone and riboflavin,which were exuded in much lower concentrations by the rin mutant.Interestingly,RIN-mediated effects on root exudates,Actinobacteria,and disease suppression were evident from the seedling stage,indicating that RIN plays a dual role in the early assembly of diseasesuppressive microbiota and late fruit development.Collectively,our work suggests that,while plant disease resistance is a complex trait driven by interactions between the plant,rhizosphere microbiome,and the pathogen,it can be indirectly manipulated using"prebiotic"compounds that promote the recruitment of disease-suppressive microbiota.

SlMYB75,an MYB-type transcription factor,promotes anthocyanin accumulation and enhances volatile aroma production in tomato fruits

Genetic manipulation of genes to upregulate specific branches of metabolic pathways is a method that is commonly used to improve fruit quality.However,the use of a single gene to impact several metabolic pathways is difficult.Here,we show that overexpression of the single gene SlMYB75(SlMYB75-OE)is effective at improving multiple fruit quality traits.In these engineered fruits,the anthocyanin content reached 1.86mg g−1 fresh weight at the red-ripe stage,and these SlMYB75-OE tomatoes displayed a series of physiological changes,including delayed ripening and increased ethylene production.In addition to anthocyanin,the total contents of phenolics,flavonoids and soluble solids in SlMYB75-OE fruits were enhanced by 2.6,4,and 1.2 times,respectively,compared to those of wild-type(WT)fruits.Interestingly,a number of aroma volatiles,such as aldehyde,phenylpropanoid-derived and terpene volatiles,were significantly increased in SlMYB75-OE fruits,with some terpene volatiles showing more than 10 times higher levels than those in WT fruits.Consistent with the metabolic assessment,transcriptomic profiling indicated that the genes involved in the ethylene signaling,phenylpropanoid and isoprenoid pathways were greatly upregulated in SlMYB75-OE fruits.Yeast one-hybrid and transactivation assays revealed that SlMYB75 is able to directly bind to the MYBPLANT and MYBPZM cis-regulatory elements and to activate the promoters of the LOXC,AADC2 and TPS genes.The identification of SlMYB75 as a key regulator of fruit quality attributes through the transcriptional regulation of downstream genes involved in several metabolic pathways opens new avenues towards engineering fruits with a higher sensory and nutritional quality.

SlGRAS4 accelerates fruit ripening by regulating ethylene biosynthesis genes and SlMADS1 in tomato

GRAS proteins are plant-specific transcription factors that play crucial roles in plant development and stress responses.However,their involvement in the ripening of economically important fruits and their transcriptional regulatory mechanisms remain largely unclear.Here,we demonstrated that SlGRAS4,encoding a transcription factor of the GRAS family,was induced by the tomato ripening process and regulated by ethylene.Overexpression of SlGRAS4 accelerated fruit ripening,increased the total carotenoid content and increased PSY1 expression in SlGRAS4-OE fruit compared to wild-type fruit.The expression levels of key ethylene biosynthesis genes(SlACS2,SlACS4,SlACO1,and SlACO3)and crucial ripening regulators(RIN and NOR)were increased in SlGRAS4-OE fruit.The negative regulator of tomato fruit ripening,SlMADS1,was repressed in OE fruit.Exogenous ethylene and 1-MCP treatment revealed that more endogenous ethylene was derived in SlGRAS4-OE fruit.More obvious phenotypes were observed in OE seedlings after ACC treatment.Yeast one-hybrid and dual-luciferase assays confirmed that SlGRAS4 can directly bind SlACO1 and SlACO3 promoters to activate their transcription,and SlGRAS4 can also directly repress SlMADS1 expression.Our study identified that SlGRAS4 acts as a new regulator of fruit ripening by regulating ethylene biosynthesis genes in a direct manner.This provides new knowledge of GRAS transcription factors involved in regulating fruit ripening.

Combinatorial analysis of transcription and metabolism reveals the regulatory network associated with antioxidant substances in waxy corn

Maize is an essential source of nutrition for humans and animals and is rich in various metabolites that determine its quality.Different maize varieties show significant differences in metabolite content.Two kinds of waxy maize parental materials,S181 and 49B,created by the Chongqing Academy of Agricultural Sciences,are widely grown in China.S181 shows higher starch and sugar contents than 49B.This study generated metabolic profiles to assess the differences between the two varieties.A total of 674 metabolites that were significantly differentially expressed between the two varieties were identified by gas chromatography and untargeted metabolomics technology.These metabolites were associated with 21 categories,including antioxidant metabolites.Moreover,6415 differentially expressed genes(DEGs)were identified by RNA-seq.Interestingly,these DEGs comprised starch and sugar synthesis pathway genes and 72 different transcription factor families.Among these,six families that were reported to play an essential role in plant antioxidant action accounted for 39.2%of the transcription factor families.Using the Kyoto Encyclopedia of Genes and Genomes(KEGG)classification,the DEGs were mainly involved in amino acid biosynthesis,glycolysis/glucose metabolism,and the synthetic and metabolic pathways of antioxidant active substances.Furthermore,the correlation analysis of transcriptome and metabolomics identified five key transcription factors(ZmbHLH172,ZmNAC44,ZmNAC-like18,ZmS1FA2,ZmERF172),one ubiquitin ligase gene(ZmE25A)and one sucrose synthase gene(ZmSS1).They likely contribute to the quality traits of waxy corn through involvement in the metabolic regulatory network of antioxidant substances.Thus,our results provide new insights into maize quality-related antioxidant metabolite networks and have potential applications for waxy corn breeding.

Role of ethylene response factors(ERFs)in fruit ripening

The ethylene response factors(ERFs)belong to the APETALA2/ethylene response factor(AP2/ERF)superfamily and act downstream of the ethylene signalling pathway to regulate the expression of ethylene responsive genes.In different species,ERFs have been reported to be involved in plant development,flower abscission,fruit ripening,and defense responses.In this review,based on the new progress made by recent studies,we summarize the specific role and mode of action of ERFs in regulating different aspects of ripening in both climacteric and non-climacteric fruits,and provide new insights into the role of ethylene in non-climacteric fruit ripening.

CRISPR/Cas9-mediated gene-editing technology in fruit quality improvement

Fruits are an essential part of a healthy,balanced diet and it is particularly important for fibre,essential vitamins,and trace elements.Improvement in the quality of fruit and elongation of shelf life are crucial goals for researchers.However,traditional techniques have some drawbacks,such as long period,low efficiency,and difficulty in the modification of target genes,which limit the progress of the study.Recently,the clustered regularly interspaced short palindromic repeats(CRISPR)/Cas9 technique was developed and has become the most popular gene-editing technology with high efficiency,simplicity,and low cost.CRISPR/Cas9 technique is widely accepted to analyse gene function and complete genetic modification.This review introduces the latest progress of CRISPR/Cas9 technology in fruit quality improvement.For example,CRISPR/Cas9-mediated targeted mutagenesis of RIPENING INHIBITOR gene(RIN),Lycopene desaturase(PDS),Pectate lyases(PL),SlMYB12,and CLAVATA3(CLV3)can affect fruit ripening,fruit bioactive compounds,fruit texture,fruit colouration,and fruit size.CRISPR/Cas9-mediated mutagenesis has become an efficient method to modify target genes and improve fruit quality.