Genetic interactions in the control of flowering time and reproductive structure development in tomato (Solanum lycopersicum)

Different tomato (Solanum lycopersicum) mutants, affected in flowering time, reproductive structure or plant architecture, were crossed to produce double mutants in order to investigate gene interactions in flowering regulation in this autonomous species with a sympodial growth habit. The compound inflorescence: uniflora, uniflora: self pruning, uniflora: blind, and jointless: uniflora double mutants all produced solitary flowers like their uniflora parent, instead of inflorescences. All double mutants were late flowering. uniflora: blind and uniflora: self pruning had flowering times intermediate between those of their two parents. jointless: uniflora and compound inflorescence: uniflora flowered later than uniflora, the mutant with the most delayed flowering. All double mutants developed strong lateral shoots at node levels approximately corresponding to the level at which their parent cultivars initiated their first reproductive structure, which is a typical trait of uniflora. These results suggest that the UNIFLORA gene acts upstream of the other investigated genes in controlling flowering in tomato, and that floral transition of the primary shoot and floral transition of sympodial segments are regulated differently.     

<Emphasis Type="Italic">SINGLE FLOWER TRUSS</Emphasis> regulates the transition and maintenance of flowering in tomato

The characterisation of the single flower truss ( sft) mutant phenotype of tomato ( Lycopersicon esculentum Mill.), as well as its genetic interactions with other mutations affecting FALSIFLORA ( FA) and SELF PRUNING ( SP) genes, has revealed that SFT is a key gene in the control of floral transition and floral meristem identity. The single sft mutation produces a late-flowering phenotype in both long-day and short-day conditions. In combination with fa, a mutation affecting the tomato gene orthologous to LFY, sft completely blocks the transition to flowering in this species. Thus, the phenotype of the sft fa double mutants indicates that SFT and FA participate in two parallel pathways that regulate the switch from vegetative to reproductive phase in tomato, and that both genes are indispensable for flowering. On the other hand, the replacement of flowers by vegetative shoots observed in the sft inflorescence suggests that SFT regulates flower meristem identity during inflorescence development of tomato. In addition to these two main functions, SFT is involved in the development of both flowers and sympodial shoots of tomato. First, the mutation produces a partial conversion of sepals into leaves in the first floral whorl, and a reduction in the number of floral organs, particularly carpels. Secondly, the sympodial development in the mutant plants is altered, which can be related to the interaction between SFT and SP, a gene controlling the number of nodes in sympodial shoots. In fact, we have found that the sft phenotype is epistatic to that of sp, and that the level of SP mRNA in the apical buds of sft around flowering is reduced. SFT can therefore co-ordinate the regulation of two simultaneous developmental processes in the tomato apical shoot, the promotion of flowering in one sympodial segment and the vegetative development of the next segment.     

Characterization of tomato (Solanum lycopersicum L.) mutants affected in their flowering time and in the morphogenesis of their reproductive structure

The impact of the season on flowering time and the organization and morphogenesis of the reproductive structures are described in three tomato mutants: compound inflorescence (s), single flower truss (sft), and jointless (j), respectively, compared with their wild-type cultivars Ailsa Craig (AC), Platense (Pl), and Heinz (Hz). In all environmental conditions, the sft mutant flowered significantly later than its corresponding Pl cultivar while flowering time in j was only marginally, but consistently, delayed compared with Hz. The SFT gene and, to a lesser extent, the J gene thus appear to be constitutive flowering promoters. Flowering in s was delayed in winter but not in summer compared with the AC cultivar, suggesting the existence of an environmentally regulated pathway for the control of floral transition. The reproductive structure of tomato is a raceme-like inflorescence and genes regulating its morphogenesis may thus be divided into inflorescence and floral meristem identity genes as in Arabidopsis. The s mutant developed highly branched inflorescences bearing up to 200 flowers due to the conversion of floral meristems into inflorescence meristems. The S gene appears to be a floral meristem identity gene. Both sft and j mutants formed reproductive structures containing flowers and leaves and reverting to a vegetative sympodial growth. The SFT gene appears to regulate the identity of the inflorescence meristem of tomato and is also involved, along with the J gene, in the maintenance of this identity, preventing reversion to a vegetative identity. These results are discussed in relation to knowledge accumulated in Arabidopsis and to domestication processes.     

Inflorescence structure and control of flowering time and duration by light in buckwheat (Fagopyrum esculentum Moench)

Morphogenesis of the reproductive structure of buckwheat and the impact of light conditions on flowering time and duration have been investigated using the variety 'La Harpe'. Inflorescences were initiated acropetally, in leaf axils, by the shoot apical meristem until its arrest of functioning which was accompanied by the abortion of the last inflorescence produced. The buckwheat inflorescence is a compound raceme that produces laterally flowered cymose clusters, the number of which was affected by the position of the inflorescence along the main stem. Similarly, the number of flowers in a lateral cluster was dependent on the inflorescence's position on the stem. The development of each inflorescence was stopped as its meristem stopped functioning and, in a situation reminiscent of the shoot apical meristem, the latest initiated cyme aborted. The development of each cyme was also terminated with the abortion of a few young flowers. The variety 'La Harpe' is a facultative short-day plant: the number of nodes generated before the initiation of the first inflorescence and the number of days from sowing to macroscopic appearance of this inflorescence were reduced in 8 h days as compared with 16 h days. The number of inflorescences, and thus flowering duration, was also strongly reduced by short days. It was unaffected by light irradiance in 8 h days while, in 16 h days, it was prolonged when light intensity was increased, suggesting the interaction of two different mechanisms for its regulation. Buckwheat is a distylous species, but inflorescence structure and flowering behaviour were not affected by floral morph.     

Universal florigenic signals triggered by FT homologues regulate growth and flowering cycles in perennial day-neutral tomato

The transition from vegetative to floral meristems in higher plants is programmed by the coincidence of internal and environmental signals. Classic grafting experiments have shown that leaves, in response to changing photoperiods, emit systemic signals, dubbed 'florigen', which induce flowering at the shoot apex. The florigen paradigm was conceived in photoperiod-sensitive plants: nevertheless it implies that although activated by different stimuli in different flowering systems, the signal is common to all plants. Tomato is a day-neutral, perennial plant, with sympodial and modular organization of its shoots and thus with reiterative regular vegetative/reproductive transitions. SINGLE FLOWER TRUSS a regulator of flowering-time and shoot architecture encodes the tomato orthologue of FT, a major flowering integrator gene in Arabidopsis. SFT generates graft-transmissible signals which complement the morphogenetic defects in sft plants, substitute for light dose stimulus in tomato and for contrasting day-length requirements in Arabidopsis and MARYLAND MAMMOTH tobacco. It is discussed how systemic signals initiated by SFT interact with the SELF PRUNING gene to regulate vegetative to reproductive (V/R) transitions in the context of two flowering systems, one for primary apices and the other for sympodial shoots.     

FALSIFLORA, the tomato orthologue of FLORICAULA and LEAFY, controls flowering time and floral meristem identity

Characterization of the tomato falsiflora mutant shows that fa mutation mainly alters the development of the inflorescence resulting in the replacement of flowers by secondary shoots, but also produces a late-flowering phenotype with an increased number of leaves below first and successive inflorescences. This pattern suggests that the FALSIFLORA (FA) locus regulates both floral meristem identity and flowering time in tomato in a similar way to the floral identity genes FLORICAULA (FLO) of Antirrhinum and LEAFY (LFY) of Arabidopsis. To analyse whether the fa phenotype is the result of a mutation in the tomato FLO/LFY gene, we have cloned and analysed the tomato FLO/LFY homologue (TOFL) in both wild-type and fa plants following a candidate gene strategy. The wild-type gene is predicted to encode a protein sharing 90% identity with NFL1 and ALF, the FLO/LFY-like proteins in Nicotiana and Petunia, and about 80 and 70% identity with either FLO or LFY. In the fa mutant, however, the gene showed a 16 bp deletion that results in a frameshift mutation and in a truncated protein. The co-segregation of this deletion with the fa phenotype in a total of 240 F2 plants analysed supports the idea that FA is the tomato orthologue to FLO and LFY. The gene is expressed in both vegetative and floral meristems, in leaf primordia and leaves, and in the four floral organs. The function of this gene in comparison with other FLO/LFY orthologues is analysed in tomato, a plant with a sympodial growth habit and a cymose inflorescence development.     

A Developmental Pattern of Flowering in Colored <Emphasis Type="Italic">Zantedeschia</Emphasis> spp.: Effects of Bud Position and Gibberellin

The restricted flowering of colored cultivars ofZantedeschia is a consequence of developmental constraints imposed by apical dominance of the primary bud on secondary buds in the tuber, and by the sympodial growth of individual shoots. GA₃ enhances flowering inZantedeschia by increasing the number of flowering shoots per tuber and inflorescences per shoot. The effects of gibberellin on the pattern of flowering and on the developmental fate of differentiated inflorescences along the tuber axis and individual shoot axes were studied in GA₃ and Uniconazole-treated tubers. Inflorescence primordia and fully developed (emerged) floral stems produced during tuber storage and the plant growth period were recorded. Days to flowering, percent of flowering shoots and floral stem length decreased basipetally along the shoot and tuber axes. GA₃ prolonged the flowering period and increased both the number of flowering shoots per tuber and the differentiated inflorescences per shoot. Activated buds were GA₃ responsive regardless of meristem size or age. Uniconazole did not inhibit inflorescence differentiation but inhibited floral stem elongation. The results suggest that GA₃ has a dual action in the flowering process: induction of inflorescence differentiation and promotion of floral stem elongation. The flowering pattern could be a result of a gradient in the distribution of endogenous factors involved in inflorescence differentialtion (possibly GAs) and in floral stem growth. This gradient along the tuber and shoot axes is probably controlled by apical dominance of the primary bud. Online publication: 7 April 2005     

Thorn and hook ontogeny in Artabotrys hexapetalus (Annonaceae)

Artabotrys hexapetalus is widely planted in the tropics and is known as "climbing ylang-ylang," an ornamental liana or woody climber. New natural sprouts, or water shoots, and those induced by the damage of Hurricane Andrew (24 August 1992) were collected and fixed in formalin/acidic acid/alcohol. Seeds from these plants were planted and grown in a greenhouse where seedling morphology was observed and young material collected and fixed. The development of lateral plagiotropic and orthotropic shoots was studied using both epi-illumination light microscopy and scanning electron microscopy. A series of buds develops in the axils of leaves on the orthotropic shoot. At the lateral margins of the axillary shelf, plagiotropic shoots form that will develop into either vegetative shoots, or thorns, or sympodial shoots that bear hooks and flowers. In between the two marginal buds, a series of median vertical buds develop that either remain dormant or grow out as renewal orthotropic shoots. Previous work that suggested that the plagiotropic shoot buds were displaced out of the median vertical series of supernumerary buds is not supported. The sympodial development of plagiotropic branches as inflorescence hooks is documented.     

QTL analysis of morphological traits in eggplant and implications for conservation of gene function during evolution of solanaceous species

An interspecific F(2) population from a cross between cultivated eggplant, Solanum melongena, and its wild relative, S. linnaeanum, was analyzed for quantitative trait loci (QTL) affecting leaf, flower, fruit and plant traits. A total of 58 plants were genotyped for 207 restriction fragment length polymorphism (RFLP) markers and phenotyped for 18 characters. One to eight loci were detected for each trait with a total of 63 QTL identified. Overall, 46% of the QTL had allelic effects that were the reverse of those predicted from the parental phenotypes. Wild alleles that were agronomically superior to the cultivated alleles were identified for 42% of the QTL identified for flowering time, flower and fruit number, fruit set, calyx size and fruit glossiness. Comparison of the map positions of eggplant loci with those for similar traits in tomato, potato and pepper revealed that 12 of the QTL have putative orthologs in at least one of these other species and that putative orthology was most often observed between eggplant and tomato. Traits showing potential orthology were: leaf length, shape and lobing; days to flowering; number of flowers per inflorescence; plant height and apex, leaf and stem hairiness. The functionally conserved loci included a major leaf lobing QTL ( llob6.1) that is putatively orthologous to the potato leaf ( c) and/or Petroselinum ( Pts) mutants of tomato, two flowering time QTL ( dtf1.1, dtf2.1) that also have putative counterparts in tomato and four QTL for trichomes that have potential orthologs in tomato and potato. These results support the mounting evidence of conservation of gene function during the evolution of eggplant and its relatives from their last common ancestor and indicate that this conservation was not limited to domestication traits.     

Mechanisms of floral repression in Arabidopsis

In the past two years, several early-flowering genes have been shown to encode putative chromatin-associated proteins in Arabidopsis. These proteins probably function as epigenetic silencers that repress the promotion of flowering and flower organ identity genes, and thereby maintain vegetative growth. As the plant matures, levels of the floral promoters increase despite the continued presence of floral repressors. High levels of the floral promoters are somehow able to overcome floral repression and to activate flower development. Further characterization of mutants that have impairments in either floral promoters or floral repressors revealed that these mutants not only display defects in flowering time but also have altered inflorescence architectures. These findings indicate that these flowering genes also regulate other aspects of shoot development and may be used to study the mechanism of shoot growth pattern.     

Inflorescence morphology and flower development in Pinguicula alpina and P. vulgaris (Lentibulariaceae: Lamiales): monosymmetric flowers are always lateral and occurrence of early sympetaly

Earlier interpretations of shoot morphology and flower position in Pinguicula are controversial, and data on flower development in Lentibulariaceae are scarce. We present scanning electron microscopy about the vegetative shoot, inflorescence and flower development in Pinguicula alpina and P. vulgaris. Analysis of original data and the available literature leads to the conclusion that the general pattern of shoot branching and inflorescence structure is uniform in all the Pinguicula species studied so far. The inflorescence is a sessile terminal umbel that is sometimes reduced to a solitary pseudoterminal flower. Flower-subtending bracts are either cryptic or present as tiny scales. A next order lateral shoot develops in the axil of the uppermost leaf, below the umbel. It is usually though not always homodromous, i.e., the direction of the phyllotaxy spiral is the same as in the main shoot. Among Pinguicula species that overwinter as a hibernaculum, the initiation of floral organs takes place in the same year as flowering in P. vulgaris, and 1?year earlier in P. alpina. Early congenital petal fusion (??early?? sympetaly) is documented in Pinguicula, though most other members of Lamiales exhibit ??late?? sympetaly. Sporadic occurrence of rudiments of two posterior stamens in Pinguicula is confirmed. A speculation is made that, in angiosperms, monosymmetric flowers cannot be terminal on shoots bearing more than two (or three) phyllomes.     

SURVEY OF SHOOT ORGANIZATION IN THE ARACEAE

Shoot organization is examined in 87 species from 29 genera representing all six subfamilies of the Araceae and of Acorus, which has been placed in a separate family. Within each taxonomic group examined, the details of shoot organization are presented, including the types of segments and articles which make up the shoot, the degree of expansion of leaf blades, and the placement of buds along the shoots. Literature on shoot organization of the 29 genera is reviewed. The degree of correlation between shoot organization characteristics and systematic groupings is examined, and the utility of these characteristics for systematics is evaluated. It is found that within the taxa observed, the pattern of shoot organization provides a distinctive “fingerprint” at the generic or sectional level, sufficient for determination of the group. Some patterns which appear are pointed out: taxa with bisexual flowers usually produce a single inflorescence at the terminus of a vegetative article. A few taxa with bisexual flowers produce pairs of inflorescences at the ends of articles. Multiple inflorescences (more than two) at an article terminus occur only among taxa with unisexual flowers. Multiple inflorescences are associated with anisophyllous or homeophyllous sympodial growth, while single or paired inflorescences are associated with homeophyllous or intermittent homeophyllous sympodial growth. These patterns might be understood as the result of selection for flexibility of reproductive effort and of seasonal reproduction.     

High frequency early in vitro flowering of <Emphasis Type="Italic">Dendrobium</Emphasis> Madame Thong-In (Orchidaceae)

We have successfully developed a method to induce early in vitro flowering of the self-pollinated seedlings of a tropical orchid hybrid, Dendrobium Madame Thong-In. Transition of vegetative shoot apical meristem to inflorescence meristem was observed when young protocorms were cultured in modified KC liquid medium. In contrast, protocorms cultured on Gelrite-solidified medium only produced axillary shoots and roots. CW was required to trigger the transitional shoot apical meristem and BA enhanced inflorescence stalk initiation and flower bud formation. However, normal flower development was deformed in liquid medium but developed fully upon transferring to two-layered (liquid over Gelrite-solidified) medium. Under optimal condition, in vitro flowering was observed about 5 months after seed sowing. Segregation of flower colours was observed in these seedlings and seedpods formed upon artificial pollination of the in vitro flowers.