Molecular mechanisms regulating the flowering process have been extensively studied in model annual plants; in perennials, however, understanding of the molecular mechanisms controlling flowering has ...just started to emerge. Here we review the current state of flowering research in perennial plants of the rose family (Rosaceae), which is one of the most economically important families of horticultural plants. Strawberry (Fragaria spp.), raspberry (Rubus spp.), rose (Rosa spp.), and apple (Malus spp.) are used to illustrate how photoperiod and temperature control seasonal flowering in rosaceous crops. We highlight recent molecular studies which have revealed homologues of TERMINAL FLOWER1 (TFL1) to be major regulators of both the juvenile to adult, and the vegetative to reproductive transitions in various rosaceous species. Additionally, recent advances in understanding of the regulation of TFL1 are discussed.
•We review recent literature on the genetic regulation of flower bud formation in apple.•The main factors contributing to flowering induction are endogenous developmental cues including age ...(juvenility), hormones, and sugars.•MdTFL1 and MdFT genes act as the switches for the transition from the vegetative to reproductive phase.•High cytokinin-to-auxin ratios increase floral bud production and regulate expression of MdTFL1.
Apple (Malus×domestica Borkh.) is a perennial tree adapted to temperate climate. It has a long juvenile phase, taking approximately eight years to reach the adult phase. Floral bud formation is initiated in early summer and bloom in the following spring. While the apple genome contains orthologs of most genes known to regulate flowering in Arabidopsis, floral induction in apple is regulated mainly through an autonomous pathway influenced by endogenous developmental cues, including age (juvenility), hormones, and sugars. Recent studies on the molecular basis of floral induction suggest that plant phosphatidyl ethanolamine-binding protein (PEBP)-family genes, orthologs of TERMINAL FLOWER 1 and FLOWERING LOCUS T, are involved in the control of shoot meristem identity and play central roles in the transition from vegetative to reproductive phases. This review summarizes recent progress on plant PEBP-like genes and proposes a model for the seasonal growth cycle in apple shoot apices.
Apple MdMADS13 has a transcription factor with MADS domain. Moreover, it is expressed specifically at petals and carpels. The product forms a dimer with MdPISTILLATA (MdPI) protein as a class B gene ...for floral organ formation. Reportedly, in parthenocarpic cultivars of apple (Spencer Seedless, Wellington Bloomless, Wickson and Noblow) the MdPI function is lost by genome insertion of retrotransposon, which cultivars show a homeotic mutation of floral organs, petals to sepals and stamens to carpels. Apple fruit is pome from receptacle tissue, and MdSEPALLATA (MdMADS8/9) and AGAMOUS homologues MdMADS15/22 involved in the fruit development, the transgenic apple suppressed these gene showed poor fruit development and abnormal flower formation. This article describes that the MdMADS13 retained expression after blossom and small fruits of parthenocarpic cultivars. Yeast two-hybrid experiment showed specific binding between MdPI and MdMADS13 proteins. Furthermore, transgenic Arabidopsis with 35S::MdMADS13 have malformed stamens and carpels. These results suggest strongly that MdMADS13 is related to flower organ formation as a class B gene with MdPI.
In higher eukaryotes, RAD51 functions as an essential protein in homologous recombination and recombinational repair of DNA double strand breaks. During these processes, RAD51 catalyzes homologous ...pairing between single-stranded DNA and double-stranded DNA. Japonica cultivars of rice (Oryza sativa) encode two RAD51 proteins, RAD51A1 and RAD51A2, whereas only one RAD51 exists in yeast and mammals. However, the functional differences between RAD51A1 and RAD51A2 have not been elucidated, because their biochemical properties have not been characterized. In the present study, we purified RAD51A1 and RAD51A2, and found that RAD51A2 robustly promotes homologous pairing in vitro. RAD51A1 also possesses homologous-pairing activity, but it is only about 10% of the RAD51A2 activity. Both RAD51A1 and RAD51A2 bind to ssDNA and dsDNA, and their DNA binding strictly requires ATP, which modulates the polymer formation activities of RAD51A1 and RAD51A2. These findings suggest that although both RAD51A1 and RAD51A2 have the potential to catalyze homologous pairing, RAD51A2 may be the major recombinase in rice.
The apple (
Malus
×
domestica
Borkh.) is one of the commercially important fruit crops in the worldwide. The apple has a relatively long juvenile period (up to 4 years) and a long reproductive ...period between the flower initiation and the mature fruit (14–16 months), which prevent the fruit breeding. Therefore, the understanding of the flowering system is important to improve breeding efficiency in the apple. In this study, to examine the temporal and spatial expression patterns of the floral genes,
MdTFL1
,
MdAP1
(
MdMASD5
)
,
AFL2
, and
MdFT
, we conducted in situ hybridization analysis in the apple shoot apex. In vegetative phase,
MdTFL1
was expressed on the rib meristem zone. When vegetative meristem began converting into inflorescence meristem, the expression level of
MdTFL1
was drastically decreased. At the early stage of inflorescence meristem, the expression levels of
AFL2
,
MdFT
, and
MdAP1
were up-regulated in the leaf primordia and the upper region of cell layers on the shoot apex. In late stage, the expression levels of
AFL2
and
MdAP1
were up-regulated in the young floral primordia. At a more advanced stage, high expression of
MdAP1
was observed in the inflorescence primordium through the inner layer of sepal primordia and the outer layer of receptacle primordia and floral axis. Our results suggest that
AFL2
,
MdFT
, and
MdAP1
affect to convert from the vegetative meristem into the inflorescence meristem after the decline of
MdTFL1
expression. After that,
AFL2
and
MdAP1
promote the formation of the floral primordia and floral organs.
Transgenic apple plants that overexpressed Arabidopsis FLOWERING LOCUS T (FT) under the control of the rolC promoter showed early flowering, while the introduction of FT driven by the 35S promoter ...induced flower development directly from transgenic apple callus in vitro, but vegetative growth was not maintained and the explant died. GFP-FT fusion proteins were detectable in transgenic apple tissues but never caused early flowering in the transformants. Under the control of the rolC promoter, the fused protein was localized in vascular tissues and fluorescence was detectable in companion cells in the stem and petiole. The transgenic apple lines that expressed AtFT driven by the rolC promoter showed differences in inflorescence architecture and floral organ number from those typical of nontransformed apple. Flowers of transgenic apple lines often contained more numerous petals, fewer stamens, and no pistils, and the pollen grains were incapable of germinating. The transgenic apple lines showed perpetual flowering without the requirement for low temperature and obvious photoperiodism. The expression patterns of six floral meristem identity genes and floral organ genes in flowers of transgenic apple lines were investigated. Among the floral meristem identity genes, expression of MdFT2 was suppressed and that of AFL2 was dramatically enhanced in the transformants. Of the floral organ genes, expression of MdMADS13, which functions as a class B gene, was unchanged from that of the flower of wild type one, whereas expression of MdMADS-NB, a possible class C gene, was suppressed.
In parthenocarpic cultivars of apple (Malus×domestica Borkh.), MdPISTILLATA (MdPI) expression has been suppressed by retrotransposon insertion into the MdPI genome. In this study, transgenic apple ...lines were produced that exhibited the same level of MdPI depression. The 1P-2 promoter from the MdPI genome, which specifies its expression in the petals and stamens, was used for antisense-MdPI expression, and rolC:AtFT was included to accelerate flowering. The transgenic apple with rolC:AtFT/1P-2:antisense-MdPI showed homeotic changes in the floral organs, whereby petals and stamens were replaced with sepals and pistils, respectively. Line 9-2 of this transgenic apple also showed strong suppression of MdPI. Some individuals from this line had deformed floral organs, suggesting that the homeotic changes were incomplete. Other transformants of line 9-2 that had double sepals in the first and second whorls, and many pistils in the third and fourth whorls, as seen in apple cultivars with class B mutations, which demonstrated MdPI functioned for floral organs formation same as Arabidopsis PISTILLATA gene. The transgenic apples set parthenocarpic fruits (15.7%). However, precocious transgenic apples with rolC:AtFT exhibited more parthenocarpy (14-27%). This indicates that MdPI depression cannot explain fruit formation in parthenocarpic cultivars of apple, and so some other as yet unidentified genes must be responsible.
To select cultivars to produce doubled haploids (DHs) efficiently in an anther culture, a total of 28 apple cultivars (25 cultivars and 3 rootstock cultivars) were investigated to assess their callus ...formation rate, embryo formation rate, shoot formation rate from embryo, and efficiency of shoot multiplication and acclimatization. The callus formation rates of ‘Red Astrachan’, ‘Kinsei’, ‘Tsugaru’, ‘Golden Delicious’ (‘GD’), ‘American Summer Pearmain’ (‘ASP’), ‘Gala’, ‘Rome Beauty’, and ‘Jonathan’ were high, but ‘JM2’ and ‘King of Tompkins’ formed no callus. Regarding the embryo formation rates, ‘ASP’, ‘Rome Beauty’, ‘M.9’, and ‘Starking Delicious’ (‘SD’) were high, but ‘GD’, ‘JM2’, ‘King of Tompkins’, ‘Ralls Janet’, and ‘Smith’s Cider’ formed no embryo. Regarding the shoot formation rate from embryos, ‘Senshu’, ‘Tsugaru’, and ‘Sansa’ showed high values, but ‘Fuji’, ‘Jonathan’, and ‘Wijcik’ did not form shoots at all. The rate of shoot formation from anthers showed a high–low order of ‘Senshu’, ‘SD’, ‘ASP’, and ‘Tsugaru’. Multiplication and acclimatization of the shoots were not easy. Many individuals died. Eventually, only ‘Senshu’ and ‘SD’ acclimatized. Therefore, cultivars that produced DHs efficiently were ‘Senshu’ and ‘SD’. The results indicated that all processes of “embryo formation”, “shoot induction”, and “shoot multiplication and acclimatization” are important for obtaining DHs in apple anther cultures.
Background
The TERMINAL FLOWER 1 (TFL1) gene of Arabidopsis plays an important role in regulating flowering time and in maintaining the fate of inflorescence meristem (IM). TFL1 is a homologue of ...CENTRORADIALIS (CEN) from Antirrhinum, which is only involved in IM maintenance. Recent mutational studies and the genome project revealed that TFL1 belongs to a small gene family in Arabidopsis, in which functional divergence may have occurred among the members.
Results
We found a new member of the TFL1 gene family, which is mapped on chromosome 2 of Arabidopsis. The predicted protein sequence encoded by this gene is more closely related to that of CEN than other Arabidopsis TFL1 homologues (and therefore named ATC for ArabidopsisthalianaCENTRORADIALIS homologue). Transgenic plants constitutively expressing the ATC gene (35S::ATC), in either wild‐type or tfl1 mutant backgrounds, showed a phenotype similar to that observed in transgenic plants constitutively expressing the TFL1 gene. However, in contrast to TFL1, the expression of ATC was only detected in the hypocotyl of young plants, and not in the IM. In addition, an atc loss‐of‐function mutant, isolated by screening a T‐DNA library, showed no phenotypes that were similar to those of tfl1 mutants.
Conclusion
The phenotypes of transgenic plants over‐expressing ATC suggest that the ATC protein can functionally substitute for TFL1. However, the pattern and level of expression and the loss‐of‐function phenotype indicate that ATC does not participate in the regulation of IM identity, but rather has a role that is different from that of TFL1.
Fruit trees, such as apple (Malus x domestica Borkh.), are woody perennial plants with a long juvenile phase. The biological analysis for the regulation of flowering time provides insights into the ...reduction of juvenile phase and the acceleration of breeding in fruit trees. In Arabidopsis, LIKE HETEROCHROMATIN PROTEIN1 (LHP1) is involved in epigenetic silencing of the target genes such as flowering genes. We isolated and characterized twin apple LHP1 homolog genes, MdLHP1a and MdLHP1b. These genes may have been generated as a result of ancient genome duplication. Although the putative MdLHP1 proteins showed lower similarity to any other known plant LHP1 homologs, a chromo domain, a chromo shadow domain, and the nuclear localization signal motifs were highly conserved among them. RT-PCR analysis showed that MdLHP1a and MdLHP1b were expressed constantly in developing shoot apices of apple trees throughout the growing season. Constitutive expression of MdLHP1a or MdLHP1b could compensate for the pleiotropic phenotype of lhp1/tfl2 mutant, suggesting that apple LHP1 homolog genes are involved in the regulation of flowering time and whole-plant growth. Based on these results, LHP1 homolog genes might have rapidly evolved among plant species, but the protein functions were conserved, at least between Arabidopsis and apple.
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