Bud dormancy is an adaptive process that allows trees to survive the hard environmental conditions that they experience during the winter of temperate climates. Dormancy is characterized by the ...reduction in meristematic activity and the absence of visible growth. A prolonged exposure to cold temperatures is required to allow the bud resuming growth in response to warm temperatures. In fruit tree species, the dormancy cycle is believed to be regulated by a group of genes encoding MADS-box transcription factors. These genes are called
(
) and are phylogenetically related to the
floral regulators
(
) and
. The interest in
and other orthologs of
(
-like) genes has notably increased due to the publication of several reports suggesting their role in the control of bud dormancy in numerous fruit species, including apple, pear, peach, Japanese apricot, and kiwifruit among others. In this review, we briefly describe the physiological bases of the dormancy cycle and how it is genetically regulated, with a particular emphasis on
and
-like genes. We also provide a detailed report of the most recent advances about the transcriptional regulation of these genes by seasonal cues, epigenetics and plant hormones. From this information, we propose a tentative classification of
and
-like genes based on their seasonal pattern of expression. Furthermore, we discuss the potential biological role of
and
-like genes in bud dormancy in antagonizing the function of
-like genes. Finally, we draw a global picture of the possible role of
and
-like genes in the bud dormancy cycle and propose a model that integrates these genes in a molecular network of dormancy cycle regulation in temperate fruit trees.
Summary
In many perennial fruit trees, flowering in the year following a year with heavy fruit load can be quite limited. This biennial cycle of fruiting, termed alternate bearing, was described ...170 years ago in apple (Malus domestica). Apple inflorescences are mainly found on short branches (spurs). Bourse shoots (BS) develop from the leaf axils of the spur. BS apices may terminate ~100 days after flowering, with formation of next year's inflorescences. We sought to determine how developing fruit on the spur prevents the adjacent BS apex from forming an inflorescence. The presence of adjacent fruit correlated with reaccumulation of transcript encoding a potential flowering inhibitor, MdTFL1‐2, in BS apices prior to inflorescence initiation. BS apices without adjacent fruit that did not flower due to late fruitlet removal, neighbouring fruit on the tree, or leaf removal, also reaccumulated the MdTFL1‐2 transcript. Fruit load and gibberellin (GA) application had similar effects on the expression of MdTFL1‐2 and genes involved in GA biosynthesis and metabolism. Some apple cultivars are less prone to alternate bearing. We show that the response of a BS apex to different numbers of adjacent fruit differs among cultivars in both MdTFL1‐2 accumulation and return flowering. These results provide a working model for the further study of alternate bearing, and help clarify the need for cultivar‐specific approaches to reach stable fruit production.
Significance Statement
Biennial fruiting, also called alternate bearing, can affect fruit quality in heavy‐bearing years and reduce yield in the subsequent sparce‐bearing year. Here we use apple as a model to dissect the molecular events underlying alternate bearing; we show that the presence of adjacent fruit leads to reaccumulation of a flowering repressor in buds developing for the subsequent year. We further show that cultivars vary for this response, suggesting that cultivar‐specific manipulations can improve annual fruit production.
Summary
A group of MADS transcription factors (TFs) are believed to control temperature‐mediated bud dormancy. These TFs, called DORMANCY‐ASSOCIATED MADS‐BOX (DAM), are encoded by genes similar to ...SHORT VEGETATIVE PHASE (SVP) from Arabidopsis. MADS proteins form transcriptional complexes whose combinatory composition defines their molecular function. However, how MADS multimeric complexes control the dormancy cycle in trees is unclear.
Apple MdDAM and other dormancy‐related MADS proteins form complexes with MdSVPa, which is essential for the ability of transcriptional complexes to bind to DNA. Sequential DNA‐affinity purification sequencing (seq‐DAP‐seq) was performed to identify the genome‐wide binding sites of apple MADS TF complexes. Target genes associated with the binding sites were identified by combining seq‐DAP‐seq data with transcriptomics datasets obtained using a glucocorticoid receptor fusion system, and RNA‐seq data related to apple dormancy.
We describe a gene regulatory network (GRN) formed by MdSVPa‐containing complexes, which regulate the dormancy cycle in response to environmental cues and hormonal signaling pathways. Additionally, novel molecular evidence regarding the evolutionary functional segregation between DAM and SVP proteins in the Rosaceae is presented.
MdSVPa sequentially forms complexes with the MADS TFs that predominate at each dormancy phase, altering its DNA‐binding specificity and, therefore, the transcriptional regulation of its target genes.
Phenotypic characterisation of germplasm collections is a decisive step towards association mapping analyses, but it is particularly expensive and tedious for woody perennial plant species. ...Characterisation could be more efficient if focused on a reasonably sized subset of accessions, or so-called core collection (CC), reflecting the geographic origin and variability of the germplasm. The questions that arise concern the sample size to use and genetic parameters that should be optimized in a core collection to make it suitable for association mapping. Here we investigated these questions in olive (Olea europaea L.), a perennial fruit species. By testing different sampling methods and sizes in a worldwide olive germplasm bank (OWGB Marrakech, Morocco) containing 502 unique genotypes characterized by nuclear and plastid loci, a two-step sampling method was proposed. The Shannon-Weaver diversity index was found to be the best criterion to be maximized in the first step using the Core Hunter program. A primary core collection of 50 entries (CC50) was defined that captured more than 80% of the diversity. This latter was subsequently used as a kernel with the Mstrat program to capture the remaining diversity. 200 core collections of 94 entries (CC94) were thus built for flexibility in the choice of varieties to be studied. Most entries of both core collections (CC50 and CC94) were revealed to be unrelated due to the low kinship coefficient, whereas a genetic structure spanning the eastern and western/central Mediterranean regions was noted. Linkage disequilibrium was observed in CC94 which was mainly explained by a genetic structure effect as noted for OWGB Marrakech. Since they reflect the geographic origin and diversity of olive germplasm and are of reasonable size, both core collections will be of major interest to develop long-term association studies and thus enhance genomic selection in olive species.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Genetic control of biennial bearing in apple Guitton, Baptiste; Kelner, Jean-Jacques; Velasco, Riccardo ...
Journal of experimental botany,
01/2012, Letnik:
63, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Although flowering in mature fruit trees is recurrent, floral induction can be strongly inhibited by concurrent fruiting, leading to a pattern of irregular fruiting across consecutive years referred ...to as biennial bearing. The genetic determinants of biennial bearing in apple were investigated using the 114 flowering individuals from an F₁ population of 122 genotypes, from a ‘Starkrimson’ (strong biennial bearer)בGranny Smith’ (regular bearer) cross. The number of inflorescences, and the number and the mass of harvested fruit were recorded over 6 years and used to calculate 26 variables and indices quantifying yield, precocity of production, and biennial bearing. Inflorescence traits exhibited the highest genotypic effect, and three quantitative trait loci (QTLs) on linkage group (LG) 4, LG8, and LG10 explained 50% of the phenotypic variability for biennial bearing. Apple orthologues of flowering and hormone-related genes were retrieved from the whole-genome assembly of ‘Golden Delicious’ and their position was compared with QTLs. Four main genomic regions that contain floral integrator genes, meristem identity genes, and gibberellin oxidase genes co-located with QTLs. The results indicated that flowering genes are less likely to be responsible for biennial bearing than hormone-related genes. New hypotheses for the control of biennial bearing emerged from QTL and candidate gene co-locations and suggest the involvement of different physiological processes such as the regulation of flowering genes by hormones. The correlation between tree architecture and biennial bearing is also discussed.
Auxin is an important phytohormone for fleshy fruit development, having been shown to be involved in the initial signal for fertilisation, fruit size through the control of cell division and cell ...expansion, and ripening related events. There is considerable knowledge of auxin-related genes, mostly from work in model species. With the apple genome now available, it is possible to carry out genomics studies on auxin-related genes to identify genes that may play roles in specific stages of apple fruit development.
High amounts of auxin in the seed compared with the fruit cortex were observed in 'Royal Gala' apples, with amounts increasing through fruit development. Injection of exogenous auxin into developing apples at the start of cell expansion caused an increase in cell size. An expression analysis screen of auxin-related genes involved in auxin reception, homeostasis, and transcriptional regulation showed complex patterns of expression in each class of gene. Two mapping populations were phenotyped for fruit size over multiple seasons, and multiple quantitative trait loci (QTLs) were observed. One QTL mapped to a region containing an Auxin Response Factor (ARF106). This gene is expressed during cell division and cell expansion stages, consistent with a potential role in the control of fruit size.
The application of exogenous auxin to apples increased cell expansion, suggesting that endogenous auxin concentrations are at least one of the limiting factors controlling fruit size. The expression analysis of ARF106 linked to a strong QTL for fruit weight suggests that the auxin signal regulating fruit size could partially be modulated through the function of this gene. One class of gene (GH3) removes free auxin by conjugation to amino acids. The lower expression of these GH3 genes during rapid fruit expansion is consistent with the apple maximising auxin concentrations at this point.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The present study aimed to dissect tree architectural plasticity into genetic, ontogenetic and environmental effects over the first 4 yr of growth of an apple (Malus x domestica) F1 progeny by means ...of mixed linear modelling of repeated data. Traits related to both growth and branching processes were annually assessed on different axes of the trees planted in a staggered-start design. Both spatial repetitions, (i.e. different axis types) and temporal repetitions (i.e. successive ages of trees) were considered in a mixed linear model of repeated data. A significant genotype effect was found for most studied traits and interactions between genotype and year and/or age were also detected. The analysis of repeated temporal measures highlighted that the magnitude of the decrease in primary growth is mainly determined by the first year of growth, and the decrease in bottom diameter increment is concomitant with the first fruiting occurrence. This approach allowed us to distinguish among the traits that were under genetic control, those for which this control is exerted differentially throughout tree life or depending on climatic conditions or an axis type. Mapping quantitative trait loci (QTL) that are specific to these different effects will constitute the next step in the research.
Crop-to-wild gene flow is a mechanism process widely documented, both in plants and animals. This can have positive or negative impacts on the evolution of admixed populations in natural ...environments, yet the phenomenon is still misunderstood in long-lived woody species, contrary to short-lived crops. Wild olive Olea europaea L. occurs in the same eco-geographical range as domesticated olive, i.e. the Mediterranean Basin (MB). Moreover, it is an allogamous and anemophilous species whose seeds are disseminated by birds, i.e. factors that drive gene flow between crops and their wild relatives. Here we investigated the genetic structure of western MB wild olive populations in natural environments assuming a homogenous gene pool with limited impact of cultivated alleles, as previously suggested. We used a target sequencing method based on annotated genes from the Farga reference genome to analyze 27 western MB olive tree populations sampled in natural environments in France, Spain and Morocco. We also target sequenced cultivated olive tree accessions from the Worldwide Olive Germplasm Bank of Marrakech and Porquerolles and from an eastern MB wild olive tree population. We combined PCA, sNMF, pairwise FST and TreeMix and clearly identified genuine wild olive trees throughout their natural distribution range along a north-south gradient including, for the first time, in southern France. However, contrary to our assumption, we highlighted more admixed than genuine wild olive trees. Our results raise questions regarding the admixed population evolution pattern in this environment, which might be facilitated by crop-to-wild gene flow.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
In most temperate fruit trees, fruits are located on one-year old shoots. In Prunus species, flowers and fruits are born in axillary position along those shoots. The axillary bud fate and branching ...patterns are thus key components of the cultivar potential fruit production. The objective of this study was to analyze the branching and bearing behaviors of 1-year-old shoots of apricot cultivars and clones genetically closely related. Shoot structures were analyzed in terms of axillary bud fates using hidden semi-Markov chains and compared depending on the genotype, year and shoot length. The shoots were composed of three successive zones containing latent buds (basal zone), central flower buds (median zone) and vegetative buds (distal zone), respectively. The last two zones contained few associated flower buds. The zones length (in number of metamers) and occurrence strongly depended on shoot development in the two successive years. With decrease in the number of metamers per shoot, the last two zones become shorter or may not develop. While the number of metamers of the basal and distal zones and the number of associated flower buds correlated to the number of metamers of the shoot, the number of metamers of the median zone and the transition probability from the median to the distal zone were cultivar specific.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
In temperate trees, growth resumption in spring time results from chilling and heat requirements, and is an adaptive trait under global warming. Here, the genetic determinism of budbreak and ...flowering time was deciphered using five related full-sib apple families. Both traits were observed over 3 years and two sites and expressed in calendar and degree-days. Best linear unbiased predictors of genotypic effect or interaction with climatic year were extracted from mixed linear models and used for quantitative trait locus (QTL) mapping, performed with an integrated genetic map containing 6849 single nucleotide polymorphisms (SNPs), grouped into haplotypes, and with a Bayesian pedigree-based analysis. Four major regions, on linkage group (LG) 7, LG10, LG12, and LG9, the latter being the most stable across families, sites, and years, explained 5.6–21.3% of trait variance. Co-localizations for traits in calendar days or growing degree hours (GDH) suggested common genetic determinism for chilling and heating requirements. Homologs of two major flowering genes, AGL24 and FT, were predicted close to LG9 and LG12 QTLs, respectively, whereas Dormancy Associated MADs-box (DAM) genes were near additional QTLs on LG8 and LG15. This suggests that chilling perception mechanisms could be common among perennial and annual plants. Progenitors with favorable alleles depending on trait and LG were identified and could benefit new breeding strategies for apple adaptation to temperature increase.
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