Identification of QTLs controlling seed dormancy in peach (Prunus persica)

Dormancy is a condition that delays or inhibits growth in seed, vegetative buds, and floral buds. In peach, seed germination occurs when seed accumulate sufficient stratification and growing degree hours to break dormancy and begin growing. Correlations have been reported between mean seed stratification requirements and mean bud chilling requirements among Prunus families, but an individual seed’s germination date and subsequent vegetative and floral bud break date are not correlated. Prior to this study, the genetic factors involved in regulating seed dormancy and their location on the peach genomic map were unknown. Segregating F₂ seed were collected from a high?×?low chill F₁ peach hybrid in 2005, 2006, and 2008. Germination date and growth habit was measured after the stratification requirement of the 2005 seed was fully met. The seed collected in 2006 and 2008 received varying amounts of stratification, which enabled data on stratification requirement, heat requirement, and growth habit to be collected. Genomic DNA was extracted from seedling leaf tissue and screened with SSR markers selected from the Prunus reference map at an average resolution of 20 cM. Seed dormancy quantitative trait loci (QTLs) were detected on G1, G4, G6/8, and G7. The QTLs detected on G6/8 and G7 were discovered in the same region as QTLs associated with floral bud chilling requirement and bloom time in peach.     

Identification and Characterization of PpLFL, a Homolog of FLORICAULA/LEAFY in Peach (Prunus persica)

A LEAFY/FLORICAULA (LFY/FLO) homolog PpLFL (P runus p ersica L EAFY/ F LORICAULA L ike) gene was isolated from axillary buds of peach (Prunus persica (L.) Batsch. cv. Bayuecui) during flower induction period. The open reading frame of PpLFL spanned 1,248 bp, encoding a putative protein of 415 amino acid residues, which was with high similarity (50.48 %–84.69 %) to other FLO/LFY inferred proteins from different species. The spatial expression patterns of PpLFL were detected in axillary buds during the periods of flower induction by using immunohistolocalisation. The results showed that PpLFL gene was mainly expressed during flower induction time, and also in leaf and petal promordia at the SAM. For further functional analysis, the PpLFL was constitutively expressed in the Arabidopsis lfy mutant background, and the results showed that overexpression of PpLFL under the control of CaMV 35S promoter can accelerate flowering and give rise to normal flower organs. Our results suggest that PpLFL might play an important role in flower induction, and could act as a functional flower meristem identity gene in peach.     

<Emphasis Type="Italic">PpVIN2</Emphasis>, an acid invertase gene family member,is sensitive to chilling temperature and affects sucrose metabolism in postharvest peach fruit

We previously reported that expression and activity of acid invertases (AI) are increased in peach fruit under chilling stress. In order to determine which AI genes respond to chilling stress, seven AI genes, two vacuolar invertases (VINs) and five cell wall-bound invertases (CWINs), were identified and cloned. The predicted amino acid sequences of the genes contain conserved sites characteristic of plant AIs such as NDPNG/A, the sucrose-binding site, and MWECV/P, a cysteine catalytic motif. Using gene-specific primers, the expression of each gene was measured in ‘Baifeng’ and ‘Yulu’ peach fruits stored at 0, 5, 10 and 20 °C. Of the seven genes, expression of PpVIN2 was the most affected by chilling stress; the largest increases were in fruit stored at 5 °C, up to 17-fold in ‘Baifeng’ fruit, and up to 280-fold in ‘Yulu’ fruit. Overall, VIN activity was much higher than CWIN activity in stored peach fruit. In both cultivars reducing sugar content increased significantly and sucrose content decreased gradually during storage at 5 °C relative to other temperatures, and was accompanied by severe chilling injury symptoms. Thus, PpVIN2 appears to be induced by chilling and may play an important role in sucrose metabolism in peach fruit subjected to cold storage.     

Primigenic and positional dominance among reproductive buds in shoots of two apple (Malus × domestica (Borkh.)) cultivars in a warmer and cooler site

Key message

In two apple cultivars, fruit set was due to primigenic dominance within the annual shoot in areas with insufficient winter chilling while positional dominance took precedence when chilling was sufficient.

Abstract

The purpose of our study was to use fruit set and inflorescence size to characterize the positional (position along the shoot) and/or temporal (relative time of budburst and flowering) influences on competition between reproductive laterals within an annual shoot. The relative time of budburst and flowering, and the relative position within the shoot of reproductive buds were recorded on 2-year-old shoots of ‘Granny Smith’ and ‘Golden Delicious’ apple (Malus × domestica (Borkh.)) trees. The trees were grown at two locations in South Africa, a cool area, Koue Bokkeveld, and a warm area, Warm Bokkeveld, with sufficient and insufficient winter chilling, respectively. Inflorescence size (leaf number, leaf area, and flower number) did not differ temporally or with position. For both cultivars, fruit set in the cool area was acrotonic and independent of relative flowering time, while it was more influenced by temporal (primigenic) dominance in the warm area. Therefore, there is a clear positional advantage within the shoot to fruit set in cool areas (i.e., better local climate conditions for the growing fruit), while there is a clear temporal advantage (first bud to burst sets a fruit), or a “first come, first serve” approach to fruit set, in warm areas, which have limited and delayed budbreak. Inflorescence size and fruit set indicate a separation of environmental (degree of winter chilling) and innate factors in competition among reproductive buds along the 2-year-old annual shoot.     

ABA-responsive ABRE-BINDING FACTOR3 activates DAM3 expression to promote bud dormancy in Asian pear

Bud dormancy is indispensable for the survival of perennial plants in cold winters. Abscisic acid (ABA) has essential functions influencing the endo-dormancy status. Dormancy-associated MADS-box/SHORT VEGETATIVE PHASE-like genes function downstream of the ABA signalling pathway to regulate bud dormancy. However, the regulation of DAM/SVP expression remains largely uncharacterized. In this study, we confirmed that endo-dormancy maintenance and PpyDAM3 expression are controlled by the ABA content in pear (Pyrus pyrifolia) buds. The expression of pear ABRE-BINDING FACTOR3 (PpyABF3) was positively correlated with PpyDAM3 expression. Furthermore, PpyABF3 directly bound to the second ABRE in the PpyDAM3 promoter to activate its expression. Interestingly, both PpyABF3 and PpyDAM3 repressed the cell division and growth of transgenic pear calli. Another ABA-induced ABF protein, PpyABF2, physically interacted with PpyABF3 and disrupted the activation of the PpyDAM3 promoter by PpyABF3, indicating DAM expression was precisely controlled. Additionally, our results suggested that the differences in the PpyDAM3 promoter in two pear cultivars might be responsible for the diversity in the chilling requirements. In summary, our data clarify the finely tuned regulatory mechanism underlying the effect of ABA on DAM gene expression and provide new insights into ABA-related bud dormancy regulation.     

Phylogenetic analysis and molecular evolution of the dormancy associated MADS-box genes from peach

Background  

Dormancy associated MADS-box (DAM) genes are candidates for the regulation of growth cessation and terminal bud formation in peach. These genes are not expressed in the peach mutant evergrowing, which fails to cease growth and enter dormancy under dormancy-inducing conditions. We analyzed the phylogenetic relationships among and the rates and patterns of molecular evolution within DAM genes in the phylogenetic context of the MADS-box gene family.     

Effects of endogenous hormones on variation of shoot branching in a variety of non-heading Chinese cabbage and related gene expression

Shoot branching (tillering) primarily determines plant shoot architecture and has been studied in many plants. Shoot branching is an important trait in non-heading Chinese cabbage (Brassica rapa ssp. chinensis Makino). The B. rapa ssp. chinensis var. multiceps exhibits unique and multiple shoot branching characteristics. Here, we analyzed the variation in shoot branching between ‘Maertou,’ with multiple shoot branching, and ‘Suzhouqing,’ a common variety. The levels of endogenous indole-3-acetic acid (IAA), zeatin riboside and active gibberellins in the shoot meristem tissues of the two cultivars were quantified by enzyme-linked immunosorbent assay during the vegetative growth stage. High levels of IAA maintained axillary bud dormancy and repressed axillary bud outgrowth allowing shoot branching to form in the vegetative stage in ‘Suzhouqing.’ In contrast, low levels of IAA did not inhibit axillary buds in ‘Maertou,’ while a high level of cytokinin promoted axillary bud growth and branch shoot development. Exogenous hormone (rac-GR24 and 6-benzylaminopurine) treatment showed that ‘Maertou’ was relatively sensitive to cytokinin, because the fold changes of cytokinin-responsive genes in ‘Maertou’ were significantly more frequent than those in ‘Suzhouqing’. Cytokinin was the direct regulator for axillary bud growth of ‘Maertou’. Compared with ‘Suzhouqing’, ‘Maertou’ was sensitive to cytokinin and this weakened the strigolactone–cytokinin branching pathway.     

High-density linkage maps constructed in sweet cherry (<Emphasis Type="Italic">Prunus avium</Emphasis> L.) using cross- and self-pollination populations reveal chromosomal homozygosity in inbred families and non-syntenic regions with the peach genome

The landrace sweet cherry (Prunus avium L.) cultivar ‘Cristobalina’ is a useful resource for sweet cherry breeding due to several important traits, including low chilling requirement, early maturity date, and self-compatibility. In this work, three families (N?=?325), derived from ‘Cristobalina’, were used to develop high-density genetic maps using the RosBREED 6K Illumina Infinium® cherry SNP array. Two of the families were derived from self-pollination, which allowed construction of the first F₂ genetic maps in the species. The other map developed was from an interspecific cross of cultivars ‘Vic’?×?‘Cristobalina’. The maps developed include 511 to 816 mapped SNPs covering 622.4 to 726.0 cM. Mapped SNP marker order and position were compared to the sweet cherry and peach genome sequences, and a high degree of synteny was observed. However, inverted and small translocated regions between peach and sweet cherry genomes were observed with the most noticeable inversion at the top of LG5. The progeny resulting from self-pollination also revealed a high level of homozygosity, as large presumably homozygous regions as well as entire homozygous LGs were observed. These maps will be used for genetic analysis of relevant traits in sweet cherry breeding by QTL analysis, and self-pollination populations will be useful for investigating inbreeding depression in a naturally outbreeding species.     

Expressional regulation of PpDAM5 and PpDAM6, peach (Prunus persica) dormancy-associated MADS-box genes, by low temperature and dormancy-breaking reagent treatment

The present study investigated the expressional regulation of PpDAM5 and PpDAM6, two of the six peach (Prunus persica) dormancy-associated MADS-box genes, in relation to lateral bud endodormancy. PpDAM5 and PpDAM6 were originally identified as homologues of Arabidopsis SHORT VEGETATIVE PHASE/AGAMOUS-LIKE 24 identified in the EVERGROWING locus of peach. Furthermore, PpDAM5 and PpDAM6 have recently been suggested to be involved in terminal bud dormancy. In this study, seasonal expression analyses using leaves, stems, and lateral buds of high-chill and low-chill peaches in field conditions indicated that both genes were up-regulated during the endodormancy period and down-regulated with endodormancy release. Controlled environment experiments showed that the expression of both PpDAM5 and PpDAM6 were up-regulated by ambient cool temperatures in autumn, while they were down-regulated by the prolonged period of cold temperatures in winter. A negative correlation between expression levels of PpDAM5 and PpDAM6 and bud burst percentage was found in the prolonged cold temperature treatment. Application of the dormancy-breaking reagent cyanamide to endo/ecodormant lateral buds induced early bud break and down-regulation of PpDAM5 and PpDAM6 expression at the same time. These results collectively suggest that PpDAM5 and PpDAM6 may function in the chilling requirement of peach lateral buds through growth-inhibiting functions for bud break.     

Developmental function of Nm23/awd: a mediator of endocytosis

The metastasis suppressor gene Nm23 is highly conserved from yeast to human, implicating a critical developmental function. Studies in cultured mammalian cells have identified several potential functions, but many have not been directly verified in vivo. Here, we summarize the studies on the Drosophila homolog of the Nm23 gene, named a bnormal w ing d iscs (awd), which shares 78% amino acid identity with the human Nm23-H1 and H2 isoforms. These studies confirmed that awd gene encodes a nucleoside diphosphate kinase, and provided strong evidence of a role for awd in regulating cell differentiation and motility via regulation of growth factor receptor signaling. The latter function is mainly mediated by control of endocytosis. This review provides a historical account of the discovery and subsequent analyses of the awd gene. We will also discuss the possible molecular function of the Awd protein that underlies the endocytic function.     

ParSOC1, a MADS-box gene closely related to Arabidopsis AGL20/SOC1, is expressed in apricot leaves in a diurnal manner and is linked with chilling requirements for dormancy break

Dormancy break is a physiological phenomenon associated with the ability of plants to cope with changing environmental conditions and adjust their growth habits accordingly. In order to understand the potential role of genes in the control of dormancy break ParSOC1, a distinct apricot MADS-box gene which is most closely related to the AGL20/SOC1 MADS-box family was studied in several apricot cultivars that differ in their chilling requirements. The ParSOC1 gene is expressed in a diurnal manner and is highly polymorphic among apricot cultivars in the transcribed region upstream to the putative ATG translation initiation site. Genotyping of 48 different apricot cultivars revealed 13 different ParSOC1 alleles. By associating the chilling requirements of the apricot cultivars with their ParSOC1 genotype, it was possible to demonstrate a significant correlation between the presence of specific ParSOC1 alleles and chilling requirements. The data provided suggest that ParSOC1 or a gene in its close proximity could be involved in the regulation of dormancy break of vegetative shoots in apricot.     

Comparative transcript profiling of a peach and its nectarine mutant at harvest reveals differences in gene expression related to storability

Gene expression at harvest was compared for two stone fruit cultivars, a peach and its near-isogenic nectarine mutant, using two microarray platforms, μPEACH1.0 and ChillPeach. Together, both platforms covered over 6,000 genes out of which 417 were differentially expressed between the fruits of the two cultivars at a p value of 0.05. A total of 47 genes in nectarine and 60 genes in peach were at least twofold higher relative to each other. Nectarine had much better storage characteristics than peach and could be stored for over 5 weeks at 5 °C without storage disorders. In an attempt to determine whether gene expression at harvest could give an indication of storage potential, the expression analysis of the two cultivars was compared to that of two genotypes with different sensitivities to chilling injury. Principal component analysis of gene expression results across four fruit types differing in chilling sensitivity resulted in 41 genes whose expression levels separated the fruits according to sensitivity to storage disorders, suggesting that the genes have a role in cold response adaptation.     

Chilling and heat requirements of sweet cherry cultivars and the relationship between altitude and the probability of satisfying the chill requirements

Sweet cherry cultivars have different chilling and heat requirements for breaking rest and flowering. The knowledge of these requirements may be valuable in the selection of the appropriate cultivars for producers and to avoid losses caused by an inadequate cultivar selection in a particular area. Determination of chilling and heat requirements is also important within a breeding programme, when choosing parents to obtain early flowering cultivars. Chilling requirements of seven cherry cultivars growing in south-eastern Spain were calculated using different methods (hours below 7 °C, Utah and Dynamic model), which were compared. Recording hourly average temperatures at several locations during 2 years, the Utah and Dynamic models performed better than hours below 7 °C. Different chilling requirements and slight differences in heat requirements were observed in the studied cultivars. ‘Cristobalina’ and ‘Brooks’, the earlier-flowering cultivars, were those with the lowest chilling requirements. ‘Burlat’, ‘New Star’ and ‘Somerset’ had medium chilling and heat requirements for flowering, and ‘Marvin’ showed the highest values and also the latest blooming date. All the studied cultivars may have their chilling requirements satisfied in the region of Murcia, if grown at least 650 m above sea level. Some cultivars, such as ‘Cristobalina’ and ‘Brooks’, could successfully break dormancy already when grown at an altitude above 325 m.     

Influence of different temperatures on bud break and 1-aminocyclopropane-1-carboxylic acid content of young peach trees

Effects of different temperatures on bud break and 1-aminocyclopropane-1-carboxylic acid (ACC) content were determined by using potted two-year-old ‘Akatsuki’ peach trees. One group of trees were subjected to 1°C for four weeks and then transferred to a growth chamber at 24°C, while the other was kept at 24 °C throughout the experiment. After four-week temperature treatments floral and vegetative bud break were evaluated weekly and bud break percentage was calculated. Bud break was greater under 1 °C than 24 °C in both November and December. The time required to release buds from dormancy was shorter in December than November. In November ACC content in peach buds increased after one and two weeks, then decreased in the forth week under both treatments. However, in December ACC content after two and four weeks showed a similar trend under 1 °C and a reverse trend under 24 °C. It was higher under low temperature treatment. These data indicate that chilling requirements for bud break of peach seems to be associated with the promotion of ethylene biosynthesis caused by low temperature stress.