Insights into the dwarfing mechanism of pear(Pyrus betulaefolia) based on anatomical and structural analysis using X-ray scanning

The lack of a suitable rootstock to control scion growth has limited the development of high-density plantations in pear production, which is partly attributed to poor understanding of the dwarfing mechanism. In the present study, the rootstock of the dwarf-type pear (Pyrus betulaefolia)PY-9’ was identified and used as the material for anatomical analysis.PY-9’ grew to half the tree height of the normal cultivar Zhengdu’, along with fewer internodes and shorter length. Significant differences in growth rate betweenPY-9’ andZhengdu’ were detected at approximately 30 days after full bloom, which corresponded with the time of the greatest difference in water potential between the dwarf and normal cultivar.PY-9’ showed a higher photosynthetic rate thanZhengdu’. Anatomical analysis showed thatPY-9’ had higher area ratios of both phloem and xylem and more developed vascular tissues thanZhengdu’. The three-dimensional reconstructed skeleton of the xylem from X-ray computed tomography scanning revealed greater intervessel connectivity inZhengdu’ than inPY-9’, which could contribute to the more vigorous growth ofZhengdu’. This study thus provides the first comparison of the microstructural properties of xylem elements between a dwarfing-type and vigorous-type pear rootstock, providing new insights into the dwarfing mechanism in pear and facilitating breeding of dwarf pear rootstocks to increase crop productivity.

Effects of Different Dwarfing Rootstocks on Growth,Yield and Fruit Quality of‘Tianhong 2’Apple Trees

[Objectives]The paper was to provide a reference for screening dwarfing rootstock suitable for main spur-type Fuji cultivars in central and southern Hebei Province.[Methods]With spur-type Fuji‘Tianhong 2’as the material,the vegetative growth,yield and fruit quality of 8 different rootstock-scion combinations were compared.[Results]‘Tianhong 2’/SH6 as self-rooted rootstock had large average single fruit weight(256.33 g),large number of fruits per plant(188.68),the highest yield[(3250.08±23.42)kg/667 m ^(2)]and the highest colored area(93.5%),and the soluble solid content reached the requirement of high quality fruit(15.78%).[Conclusions]In central and southern Hebei Province,‘Tianhong 2’grafted on SH6 self-rooted rootstock has moderate growth,high yield and good fruit quality,so it can be considered as the preferred rootstock-scion combination in the local area.

Studies of Multi-Allelic Polymorphism of Dominant Dwarfing Genes in Wheat

Dwarfing breeding of wheat in the world is confined to the exploitation of recessive dwarfing sources. None of the dominant dwarfing sources discovered in common wheat (Triticum aestivum L.) has found wide exploitation in wheat breeding due to the extreme dwarfness of their plants (2055 cm). We found in our work that some stable mutant lines with their plant height enhanced to different extents could be obtained in large populations derived from the stock seeds of the dominant dwarfing sources Aibian1 carrying Rht10 on 4DS and being 2055 cm tall and Aisu2 carrying Rht3 on 4BS and being 55 cm tall, or from their descendants of induced mutation treatments, or from the segregating descendants of their crosses with mid- or tall-statured genotypes. Subsequently, we studied these mutation-derived lines differing in plant height with near isogenic lines and observed that the character of their enhanced plant height bred true, each carrying a semi-dominant dwarfing gene for a definite height and that as the plant height of the mutation-derived lines increased, the yield-contributing characters of their near isogenic lines were significantly improved. When test crosses with marker genes and physiological and biochemical genetic marker tests were performed to re-localize the semi-dominant dwarfing genes carried by the mutation-derived lines, it was confirmed that they shared common loci with Rht10 and Rht3 and that they were all mutation-derived multiple alleles. It is thus speculated that dominant dwarfing genes are of 'multi-allelic polymorphism'. In other words, dominant dwarfing genes, which are ultra-dwarfing, are liable to develop by mutation into a group of multiple alleles with plant height enhanced to different extents and some may have a height close to the ideal plant height for wheat breeding. Therefore, these results offer a fundamentally new approach for the exploitation of dominant dwarfing sources in wheat breeding.

Comparative Study on the Dwarfing Pre-selected Indices of Pear Germplasms with Different Growth Potentials

[ Objective] The aim of this study was to provide a theoretical basis for breeding selection, matching parents and the identification of traits during early period. [ Method ] With Shanli （ Pyrus ussuriensis Maxim） , S2 × Shanli （vigorous）, S2 x ShanU （dwarfing）, S2, super-dwarfing germplasm as the matedais, the dwarfing traits of each germplasm were identified by indices including leaf stomata density, branch-cortex ratio, leaf thickness, palisade tissue thickness, paisade-spongy ratio and vessel density. [Result] Among five kinds of pear germplasms, Shanli with strong growth potential had the smallest branch-cortex ratio, leaf thickness, palisade tissue thickness and palisade-spengy ratio, but the largest stomata density and vessel density. On the contrary, super-dwarfing germplasm with weak growth potential had the largest branch-cortex ratio, leaf thickness, palisade tissue thickness and palisade-spongy ratio, but the smallest stomata density and vessel density. There was a difference in stomata density, branch-cortex ratio, leaf thickness, palisade tissue thickness, palisade-spongy ratio and vessel density for every germplasm. [ Conclusion] Stomata density, branch-cortex ratio, leaf thickness, palisade tissue thickness, palisade-spongy ratio and vessel density can be used as indices of identification for pear growth potential in early period.

The Effects of Dwarfing Genes (Rht-B1b, Rht-D1b, and Rht8) with Different Sensitivity to GA_3 on the Coleoptile Length and Plant Height of Wheat

Understanding the effects of wheat dwarfing genes on the coleoptile length and plant height is crucial for the proper utilization of dwarfing genes in the improvement of wheat yield. Molecular marker analysis combined with pedigree information were used to classify wheat cultivars widely planted in major wheat growing regions in China into different categories based on the dwarfing genes they carried. The effects of the dwarfing genes with different sensitivity to gibberellins （GA3） on the coleoptile length and plant height were analyzed. Screening of 129 cultivars by molecular marker analysis revealed that 58 genotypes of wheat contained the dwarfing gene Rht-B1b, 24 genotypes of wheat contained Rht-D1b gene and 73 genotypes of wheat possessed Rht8 gene. In addition, among these 129 cultivars, 35 genotypes of wheat cultivars contained both Rht-B1b and Rht8 genes and 16 genotypes of wheat cultivars contained both Rht-D1b and Rht8 genes. Wheat cultivars with the dwarfing genes Rht-B1b or Rht-D1b were insensitive to GA3, while the cultivars with the dwarfing gene Rht8 were sensitive to GA3. Most of the wheat genotypes containing combination of Rht8 gene with either Rht-B1b or Rht-D1b gene were insensitive to GA3. The plant height was reduced by 24.6, 30.4, 28.2, and 32.2%, respectively, for the wheat cultivars containing Rht-B1b, Rht-D1b, Rht-B1b ＋ Rht8, and Rht-D1b ＋ Rht8 genes. The plant height was reduced by 14.3% for the wheat cultivar containing GA3-sensitive gene Rht8. The coleoptile length was shortened by 25.4, 31.3, 28.4 and 31.3%, respectively, in the wheat cultivars containing Rht-B1b, Rht-D1b, Rht-B1b ＋Rht8 and Rht-D1b ＋ Rht8 genes, while the coleoptile length was shortened only by 6.2% for the wheat cultivar containing Rht8 gene. We conclude that GA3-insensitive dwarfing genes （Rht-B1b and Rht-D1b） are not suitable for the wheat improvement in dryland because these two genes have effect on reducing both plant height and coleoptile length. In contrast, GA3- sensitive dwarfing gene （Rht8） is a relatively ideal candidate for the wheat improvement since it significantly reduces the plant height of wheat, but has less effect on the coleoptile length.

The Value of Different GA Insensitive Rht Dwarfing Genes in Winter Wheat Breeding

The value of different dwarfing genes in winter wheat breeding was studied using 6 near-isogenic lines carrying different Rht dwarfing genes over three years experiment.Results showed that both the Rht1 and Rht2 semi-dwarfing genes had significantlypositive effects on kernel number and grain weight per spike, and had significantlynegative effects on 1000-grain weight comparing to the tall line(rht) and the Rht3 line.The Rht3 dwarfing gene had a significantly negative effect on kernel number per spike,and had positive effect on 1000-grain weight. The combination of the Rht2 and Rht3 geneshowed significantly negative effect on yield components. All of these 5 dwarfing orsemidwarfing genotypes mentioned above had a significantly negative effect on plantheight and no significant effect on the area of flag leaf, spikelets per spike and spikelength.

Analysis on the Dwarfing Genes in Zhepi 1 and Aizao 3:Two Dwarfing Gene Donors in Barley Breeding in China

Inheritance of plant height was studied in the two dwarfing gene donors, Zhepi 1 and Aizao 3, in barley breeding in China. Using direct cross and para-back-cross methods, allelic tests were carried out not only between the dwarfing genes in Zhepi 1 and Aizao 3, but also with br, uzu, denso, or sdwl and the six novel dwarfing genes that have been recently identified in barley in China. The results showed that the plant height was attributed to Mendelian inheritance of a recessive dwarfing gene both in Zhepi 1 and Aizao 3. The dwarfing genes carried by the two cultivars were the same, but different from the three known and the six novel dwarfing genes. On the basis of the present study, only two dwarfing genes have been used in barley breeding in China since 1950.

Identification of SSR Marker Linked to a Major Dwarfing Gene in Common Wheat

A segregating population with 410 F2 individuals from the cross MERCIA （Rht-Bla）×Dwarf 123 was made to identify a new major dwarfing gene carrying by novel wheat germplasm Dwarf 123. Combination of bulk segerant analysis method was used. A total of 145 SSR markers were tested for polymorphisms among parental lines and DNA bulks of F2 population. Out of 145 primer pairs only three markers revealed corresponding polymorphism among parental lines and F2 DNA bulks. The marker Barc20 was close to the dwarfing gene with a genetic distance of 1.8 cM, and markers Gwm513 and Gwm495 were linked to the gene with genetic distance of 6.7 and 13 cM, respectively. Linkage analysis mapped the dwarfing gene to the long arm of chromosome 4B with the order of Barc20-dwarfing gene-Gwm513-Gwm495. The Comparision between the new gene and the known Rht-B1 alleles showed that dwarfing gene Rht-Ai123 was different from the others. The identification of the new dwarfing gene and its linked markers will greatly facilitate its utilization in wheat high yield breeding for reducing plant height.

Genetic analysis and fine-mapping of a dwarfing with withered leaf-tip mutant in rice

A dwarf mutant of rice （Oryza sativa L.） by mutagenesis of ethylene methylsulfonate （EMS） treatment from Nipponbare was identified. The mutant exhibited phenotypes of dwarfism and withered leaf tip （dwll）. Based on the intemode length of dwll, this mutant belongs to the dm type of dwarfing. Analysis of elongation of the second sheath and m-amylase activity in endosperm showed that the phenotype caused by dwll was insensitive to gibberellin acid treatment. Using a large F2 population derived from a cross between the dwll and an indica rice variety, TN1, the DWL1 gene was mapped to the terminal region of the long arm of chromosome 3. Fine-mapping delimited it into a 46 kb physical distance between two STS markers, HL921 and HL944, where 6 open reading frames were predicted. Cloning of DWL1 will contribute to dissecting molecular mechanism that regulates plant height in rice, which will be beneficial to molecular assisted selection of this important trait.

Two quantitative trait loci, Dw1 and Dw2, are primarily responsible for rootstock-induced dwarfing in apple

The apple dwarfing rootstock‘Malling9’(‘M9’)has been used worldwide both to reduce scion vigour and as a genetic source for breeding new rootstocks.Progeny of‘M9’segregate for rootstock-induced dwarfing of the scion,indicating that this trait is controlled by one or more genetic factors.A quantitative trait locus(QTL)analysis of a rootstock population derived from the cross between‘M9’בRobusta5’(non-dwarfing)and grafted with‘Braeburn’scions identified a major QTL(Dw1)on linkage group(LG)5,which exhibits a significant influence on dwarfing of the scion.A smaller-effect QTL affecting dwarfing(Dw2)was identified on LG11,and four minor-effect QTLs were found on LG6,LG9,LG10 and LG12.Phenotypic analysis indicates that the combination of Dw1 and Dw2 has the strongest influence on rootstock-induced dwarfing,and that Dw1 has a stronger effect than Dw2.Genetic markers linked to Dw1 and Dw2 were screened over 41 rootstock accessions that confer a range of effects on scion growth.The majority of the dwarfing and semi-dwarfing rootstock accessions screened carried marker alleles linked to Dw1 and Dw2.This suggests that most apple dwarfing rootstocks have been derived from the same genetic source.

Phenotype and mechanism analysis of plant dwarfing in pear regulated by abscisic acid

Close planting of dwarf varieties is currently the main cultivation direction for pear trees,and the screening of excellent dwarf varieties is an important goal for breeders.In this study,the dwarfing pear variety‘601D’and its vigorous mutant‘601T’were used to show their biological characteristics and further explore the dwarfing mechanism in‘601D’.The biological characteristics showed that‘601D’had a shorter internode length,a shorter and more compact tree body,thicker and broader leaves,lower stomata density,larger stomata size(dimension),and higher photosynthetic capacity.The biological characteristics of‘601T’showed notable contrasts.The results of endogenous hormone tests indicated that the contents of abscisic acid(ABA),ABA-glucosyl ester,and GA_(4) were higher in‘601D’,but the trans-zeatin content was lower.By transcriptomic analysis,significant differences were found in the biosynthetic and metabolic pathways of ABA.Related transcription factors such as bHLH,WRKY,and homeobox also participated in the regulation of plant dwarfing.We therefore examined three hormones with obvious differences with‘601T’,and found that only ABA could induce‘601T’to return to a dwarfing plant phenotype.Therefore,we conclude that the dwarfing of‘601D’is caused by an excessive accumulation of ABA.This study provides a new theoretical basis for breeding dwarf varieties.

Apple dwarfing rootstocks exhibit an imbalance in carbohydrate allocation and reduced cell growth and metabolism

Apple dwarfing rootstocks cause earlier shoot termination and reduced root and shoot mass.To identify physiological factors responsible for rootstock-induced growth restriction,we compared vascular-enriched gene expression between two dwarfing rootstocks(‘M27’and‘M9’)and the vigorous rootstock‘M793’using RNA sequencing and quantitative reverse transcriptase PCR.Differentially expressed genes common to both dwarfing rootstocks belonged to five main biological processes:(1)primary metabolism,(2)cell wall synthesis and modification,(3)secondary metabolism,(4)hormone signalling and response and(5)redox homeostasis.Genes promoting the biosynthesis of amino acids,lipids and cell walls were downregulated in dwarfing rootstocks,whereas genes promoting the breakdown of these compounds were upregulated.The only exception to this trend was the upregulation of starch synthesis genes in dwarfing rootstocks.Non-structural carbohydrate analysis demonstrated that starch concentrations in‘M9’roots,stems and grafted‘Royal Gala’(‘RG’)scions were double that of equivalent tissues from‘RG’homo-grafted trees(‘RG’/‘RG’).Fructose and glucose concentrations were much lower in all three tissues of the‘RG’/‘M9’trees.Together,these data indicate that dwarfing rootstocks are in a state of sugar depletion and reduced cellular activity despite having large starch reserves.Another significant finding was the over-accumulation of flavonoids and the downregulation of auxin influx transporters MdAUX1 and MdLAX2 in dwarfing rootstocks.We propose that both factors reduce polar auxin transport.The results of this study contribute novel information about the physiological state of dwarfing rootstocks.

Comparative transcriptome analysis provides insights into the mechanism of pear dwarfing

Dwarfism is an important trait which is closely related to the efficiency of fruit orchard management and production.However,dwarfing cannot be widely applied in the cultivation of pears,especially Asian pears.Developing varieties with dwarf characteristics is a goal of paramount importance in pear breeding.In the present study,dwarf phenotype pears(DPPs)and arborescent phenotype pears(APPs)were obtained from the offspring of a cross between‘Aiyuxiang’and‘Cuiguan’pear cultivars,which exhibited dwarfed and arborescent statures,respectively.When compared with APPs,the heights of DPPs showed a 62.8%reduction,and the internode lengths were significantly shorter.Crossgrafting between DPPs and APPs demonstrated that the dwarfed phenotype of DPPs was primarily induced by the aerial portions of the plant,and independent of the root system.Observations of stem tissue sections showed that DPP cells were arranged chaotically with irregular shapes,and the average length was larger than that of the APP cells.A total of 1401 differently expressed genes(DEGs)in shoot apices between DPPs and APPs were identified by RNA-sequencing(RNA-Seq),and these DEGs were mainly enriched in the‘phytohormone-related pathways,cell wall metabolism and cell division’categories.Moreover,101 DEGs were identified as transcription factors(TFs).In DPPs,several brassinosteroids(BR)signaling and cell cycle-related genes were significantly down-regulated,while genes involved in BR and GA degradation were up-regulated.Comprehensive analysis of RNA-Seq data and stem tissue sections suggested that the dwarfed phenotype of DPPs could be primarily attributed to deficiencies in cell division.Previous work using simple sequence repeat(SSR)markers narrowed the location of the gene responsible for the dwarf phenotype of‘Le Nain Vert’.Through combined analysis of our transcriptomic data with the SSR results,we identified four genes as promising candidates for the dwarf phenotype,among which,a DELLA gene could be the most promising.The results presented in this study provide a sound foundation for further exploration into the genetic and molecular mechanisms underlying pear dwarfing.

Studies on the POD and IOD Activities of the Dwarfing Stocks and the Red Fuji Apple Grafted on Corresponding Interstocks

The seasonal changes of POD and IOD enzyme activities in the leaves of 4 dwarfing stocks and Red Fuji apple grafted on 5 interstocks and the POD and IOD activities of different positions of Red Fuji Apple on various interstocks were studied. The results showed that the seasonal changes of POD and IOD activities in the leaves of 4 dwarfing stocks were similar to that of the Red Fuji Apple. The enzyme activities of stocks were closely correlated with those of the Red Fuji Apple grafted in corresponding interstocks and the POD and IOD activities of semi-dwarfing stocks were lower than those of the dwarfing and very-dwarfing stocks. There were many more differences of POD activities in phloem and activities in leaves among different interstocks. There was a significant negative correlation between the POD and IOD activities and the growth potential of the Red Fuji Apple grafted on interstocks in the last ten days of both May and September. The best time to use POD and IOD activities to forecast the dwarfing ability of stocks was in the last ten days of both May and September.

An insight into dwarfing mechanism:contribution of scion-rootstock interactions toward fruit crop improvement

Grafting has been commonly practiced for many centuries in the cultivation of horticultural crops.The use of dwarfing rootstocks has enabled a high-density plantation to produce maximum yield.Rootstock regulates scion phenotype,including precocity,fruit size,yield,quality characteristics,and tolerance to various environmental stresses.This review summarizes the existing information on the influence of rootstocks on scion growth and dwarfing mechanisms induced by multiple factors,including hormone signaling,photosynthesis,mineral transport,water relations,anatomical characteristics,and genetic markers.It has been shown that the complex interactions between scion and rootstock can regulate plant development and its structure.This information will provide interesting insights for future research related to rootstock-mediated dwarfing mechanisms and accelerate the breeding progress of dwarfing rootstocks.

OsbZIP01 Affects Plant Growth and Development by Regulating OsSD1 in Rice

As the‘Green Revolution’gene,SD1(encoding GA20ox2),has been widely applied to improve yield in rice breeding.However,research on its transcriptional regulation is limited.Here,we identified a transcription factor OsbZIP01,which can suppress the expression of SD1 and regulate gibberellin(GA)biosynthesis in rice.Knockout mutants of OsbZIP01 exhibited increased plant height,while the overexpression lines showed a semi-dwarf phenotype and diminished germination rate.Furthermore,the semi-dwarf phenotype of OE-bZIP01,was caused by the reduced internode length,which was accompanied by a thin stem width.The predominant expression of OsbZIP01 was observed in leaves and sheaths.OsbZIP01 protein was localized in the nucleus and showed transcriptional repression activity.In addition,OsbZIP01 could directly bind to the promoter of the OsSD1 gene,and inhibit its transcription.The semi-dwarf phenotype of OE-bZIP01 could be rescued by exogenous GA_(3).Meanwhile,the bzip01 sd1 double mutant showed a shorter shoot length compared with the wild type,indicating that OsbZIP01 regulated plant growth mainly through the GA biosynthesis pathway.Collectively,OsbZIP01 negatively regulates GA biosynthesis by restraining SD1 transcription,thereby affecting plant growth and development.

N-terminal truncated RHT-1 proteins generated by translational reinitiation cause semi-dwarfing of wheat Green Revolution alleles

The unprecedented wheat yield increases during the Green Revolution were achieved through the introduc-tion of the Reduced height(Rht)-B1b and Rht-D1b semi-dwarfing alleles.These Rht-1 alleles encode growth-repressing DELLA genes containing a stop codon within their open reading frame that confers gibberellin(GA)-insensitive semi-dwarfism.In this study,we successfully took the hurdle of detecting wild-type RHT-1 proteins in different wheat organs and confirmed their degradation in response to GAs.We further demonstrated that Rht-B1b and Rht-D1b produce N-terminal truncated proteins through trans-lational reinitiation.Expression of these N-terminal truncated proteins in transgenic lines and in Rht-D1c,an allele containing multiple Rht-D1b copies,demonstrated their ability to cause strong dwarfism,resulting from their insensitivity to GA-mediated degradation.N-terminal truncated proteins were detected in spikes and nodes,but not in the aleurone layers.Since Rht-B 1b and Rht-D1b alleles cause dwarfism but have wild-type dormancy,this finding suggests that tissue-specific differences in translational reinitiation may explain why the Rht-1 alleles reduce plant height without affecting dormancy.Taken together,our findings not only reveal the molecular mechanism underlying the Green Revolution but also demonstrate that trans-lational reinitiation in the main open reading frame occurs in plants.

Effects of dwarfing genes on water use efficiency of bread wheat

Climate change has increased the risk of drought, which significantly limits plant productivity.Various ways of increasing water availability and sustaining growth of crop plants in drought-prone environments are available. Genetic advances in grain yields under rainfed conditions have been achieved with the introduction of dwarfing genes. A thorough understanding of the effects of different dwarfing genes on root growth, coleoptile length, grain yields and water using efficiency(WUE) will provide opportunities to select appropriate Rht genes for breeding high WUE and grain yield cultivars. This review focuses on the mechanism involved in Rht genes that reduce plant height and affect root and coleoptile length, their consequent effects on grain yields and WUE, and suggests that for rainfed and irrigation-limited environments, combining GAR and GAI dwarfing genes in breeding may help boost WUE and yields, and more materials from different parental sources should be collected to assess opportunities for potential comprehensive application of specific Rht genes.

BREEDING OF DWARFING-STERILE WHEAT AND ITS POTENTIAL VALUES IN WHEAT BREEDING

A male sterile form of common wheat(Triticum aestivum) was found in China and it has been shown that the sterility is controlled by a single, dominant gene. The dominant Ms2 gene for male sterility (originally Tal assigned by Deng and Gao, 1982)is located on the short ann of chromosome 4D. This Ms2 gene has been extensively used as a