The SELF-PRUNING gene of tomato regulates vegetative to reproductive switching of sympodial meristems and is the ortholog of CEN and TFL1

Functional divergence of the TFL1-like gene family in Arabidopsis revealed by characterization of a novel homologue

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 Arabidopsis thaliana CENTRORADIALIS 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.

In vitro control of floral transition in tomato (Lycopersicon esculentum Mill.), the model for autonomously flowering plants, using the late flowering uniflora mutant

In vitro control of floral transition in tomato (Lycopersicon esculentum Mill.), the model plant for autonomously flowering species has been investigated using the late flowering mutant uniflora (uf). Apices collected from truly vegetative plants were cultivated on solid media supplemented with different combinations of growth regulators and chemicals. Several chemical factors implicated in the promotion of floral transition of the uf mutant have been identified: sucrose, cytokinins and nitrogenous nutrients have all to be supplied at optimal concentrations. In contrast, gibberellic acid was found to be inhibitory. These results are discussed in relation to knowledge accumulated on the nature of the flowering signals circulating, at floral transition, in other plants, especially in photoperiodic species. This study suggests that tomato could constitute an adequate model to investigate the genetic and physiological control of floral transition and contribute in unravelling pathways which are constitutively regulating this important step of plant life cycle.

Evolution of floral meristem identity genes. Analysis of Lolium temulentum genes related to APETALA1 and LEAFY of Arabidopsis

Flowering (inflorescence formation) of the grass Lolium temulentum is strictly regulated, occurring rapidly on exposure to a single long day (LD). During floral induction, L. temulentum differs significantly from dicot species such as Arabidopsis in the expression, at the shoot apex, of two APETALA1 (AP1)-like genes, LtMADS1 and LtMADS2, and of L. temulentum LEAFY (LtLFY). As shown by in situ hybridization, LtMADS1 and LtMADS2 are expressed in the vegetative shoot apical meristem, but expression increases strongly within 30 h of LD floral induction. Later in floral development, LtMADS1 and LtMADS2 are expressed within spikelet and floret meristems and in the glume and lemma primordia. It is interesting that LtLFY is detected quite late (about 12 d after LD induction) within the spikelet meristems, glumes, and lemma primordia. These patterns contrast with Arabidopsis, where LFY and AP1 are consecutively activated early during flower formation. LtMADS2, when expressed in transgenic Arabidopsis plants under the control of the AP1 promoter, could partially complement the organ number defect of the severe ap1-15 mutant allele, confirming a close relationship between LtMADS2 and AP1.

A TERMINAL FLOWER1-like gene from perennial ryegrass involved in floral transition and axillary meristem identity

Control of flowering and the regulation of plant architecture have been thoroughly investigated in a number of well-studied dicot plants such as Arabidopsis, Antirrhinum, and tobacco. However, in many important monocot seed crops, molecular information on plant reproduction is still limited. To investigate the regulation of meristem identity and the control of floral transition in perennial ryegrass (Lolium perenne) we isolated a ryegrass TERMINAL FLOWER1-like gene, LpTFL1, and characterized it for its function in ryegrass flower development. Perennial ryegrass requires a cold treatment of at least 12 weeks to induce flowering. During this period a decrease in LpTFL1 message was detected in the ryegrass apex. However, upon subsequent induction with elevated temperatures and long-day photoperiods, LpTFL1 message levels increased and reached a maximum when the ryegrass apex has formed visible spikelets. Arabidopsis plants overexpressing LpTFL1 were significantly delayed in flowering and exhibited dramatic changes in architecture such as extensive lateral branching, increased growth of all vegetative organs, and a highly increased trichome production. Furthermore, overexpression of LpTFL1 was able to complement the phenotype of the severe tfl1-14 mutant of Arabidopsis. Analysis of the LpTFL1 promoter fused to the UidA gene in Arabidopsis revealed that the promoter is active in axillary meristems, but not the apical meristem. Therefore, we suggest that LpTFL1 is a repressor of flowering and a controller of axillary meristem identity in ryegrass.

An alternative pathway to beta -carotene formation in plant chromoplasts discovered by map-based cloning of beta and old-gold color mutations in tomato

Carotenoid pigments in plants fulfill indispensable functions in photosynthesis. Carotenoids that accumulate as secondary metabolites in chromoplasts provide distinct coloration to flowers and fruits. In this work we investigated the genetic mechanisms that regulate accumulation of carotenoids as secondary metabolites during ripening of tomato fruits. We analyzed two mutations that affect fruit pigmentation in tomato (Lycopersicon esculentum): Beta (B), a single dominant gene that increases beta-carotene in the fruit, and old-gold (og), a recessive mutation that abolishes beta-carotene and increases lycopene. Using a map-based cloning approach we cloned the genes B and og. Molecular analysis revealed that B encodes a novel type of lycopene beta-cyclase, an enzyme that converts lycopene to beta-carotene. The amino acid sequence of B is similar to capsanthin-capsorubin synthase, an enzyme that produces red xanthophylls in fruits of pepper (Capsicum annum). Our results prove that beta-carotene is synthesized de novo during tomato fruit development by the B lycopene cyclase. In wild-type tomatoes B is expressed at low levels during the breaker stage of ripening, whereas in the Beta mutant its transcription is dramatically increased. Null mutations in the gene B are responsible for the phenotype in og, indicating that og is an allele of B. These results confirm that developmentally regulated transcription is the major mechanism that governs lycopene accumulation in ripening fruits. The cloned B genes can be used in various genetic manipulations toward altering pigmentation and enhancing nutritional value of plant foods.

JOINTLESS is a MADS-box gene controlling tomato flower abscission zone development

Abscission is a universal and dynamic process in plants whereby organs such as leaves, flowers and fruit are shed, both during normal development, and in response to tissue damage and stress. Shedding occurs by separation of cells in anatomically distinct regions of the plant, called abscission zones (AZs). During abscission, the plant hormone ethylene stimulates cells to produce enzymes that degrade the middle lamella between cells in the AZ. The physiology and regulation of abscission at fully developed AZs is well known, but the molecular biology underlying their development is not. Here we report the first isolation of a gene directly involved in the development of a functional plant AZ. Tomato plants with the jointless mutation fail to develop AZs on their pedicels and so abscission of flowers or fruit does not occur normally. We identify JOINTLESS as a new MADS-box gene in a distinct phylogenetic clade separate from those functioning in floral organs. We propose that a deletion in JOINTLESS accounts for the failure of activation of pedicel AZ development in jointless tomato plants.

A recombination hotspot delimits a wild-species quantitative trait locus for tomato sugar content to 484 bp within an invertase gene

In nature, genetic variation usually takes the form of a continuous phenotypic range rather than discrete classes. The genetic variation underlying quantitative traits results from the segregation of numerous interacting quantitative trait loci (QTLs), whose expression is modified by the environment. To uncover the molecular basis of this variation, we characterized a QTL (Brix9-2-5) derived from the green-fruited tomato species Lycopersicon pennellii. The wild-species allele increased glucose and fructose contents in cultivated tomato fruits in various genetic backgrounds and environments. Using nearly isogenic lines for the QTL, high-resolution mapping analysis delimited Brix9-2-5 to a single nucleotide polymorphism-defined recombination hotspot of 484 bp spanning an exon and intron of a fruit-specific apoplastic invertase. We suggest that the differences between the Brix9-2-5 alleles of the two species are associated with a polymorphic intronic element that modulates sink strength of tomato fruits. Our results demonstrate a link between naturally occurring DNA variation and a Mendelian determinant of a complex phenotype for a yield-associated trait.

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.

Stability and physicochemical properties of the bovine brain phosphatidylethanolamine-binding protein

The equilibrium behaviour of the bovine phosphatidylethanolamine-binding protein (PEBP) has been studied under various conditions of pH, temperature and urea concentration. Far-UV and near-UV CD, fluorescence and Fourier transform infrared spectroscopies indicate that, in its native state, PEBP is mainly composed of beta-sheets, with Trp residues mostly localized in a hydrophobic environment; these results suggest that the conformation of PEBP in solution is similar to the three-dimensional structure determined by X-ray crystallography. The pH-induced conformational changes show a transition midpoint at pH 3.0, implying nine protons in the transition. At neutral pH, the thermal denaturation is irreversible due to protein precipitation, whereas at acidic pH values the protein exhibits a reversible denaturation. The thermal denaturation curves, as monitored by CD, fluorescence and differential scanning calorimetry, support a two-state model for the equilibrium and display coincident values with a melting temperature Tm = 54 degrees C, an enthalpy change DeltaH = 119 kcal.mol-1 and a free energy change DeltaG(H2O, 25 degrees C) = 5 kcal.mol-1. The urea-induced unfolding profiles of PEBP show a midpoint of the two-state unfolding transition at 4.8 M denaturant, and the stability of PEBP is 4.5 kcal.mol-1 at 25 degrees C. Moreover, the surface active properties indicate that PEBP is essentially a hydrophilic protein which progressively unfolds at the air/water interface over the course of time. Together, these results suggest that PEBP is well-structured in solution but that its conformation is weakly stable and sensitive to hydrophobic conditions: the PEBP structure seems to be flexible and adaptable to its environment.

LEAF DEVELOPMENT IN ANGIOSPERMS

Leaves are produced in succession on the shoot apical meristem (SAM) of a plant. The three landmark stages in leaf morphogenesis include initiation, acquisition of suborgan identities, and tissue differentiation. The expression of various genes relative to these steps in leaf morphogenesis is described. KNOTTED-like homeobox (KNOX) genes, FLO/LFY, and floral homeotic genes may be involved in generation of leaf shape and complexity. The differences between compound leaves and simple leaves in gene expression characteristics and morphogenetic patterns are discussed.

Separation of shoot and floral identity in Arabidopsis

The overall morphology of an Arabidopsis plant depends on the behaviour of its meristems. Meristems derived from the shoot apex can develop into either shoots or flowers. The distinction between these alternative fates requires separation between the function of floral meristem identity genes and the function of an antagonistic group of genes, which includes TERMINAL FLOWER 1. We show that the activities of these genes are restricted to separate domains of the shoot apex by different mechanisms. Meristem identity genes, such as LEAFY, APETALA 1 and CAULIFLOWER, prevent TERMINAL FLOWER 1 transcription in floral meristems on the apex periphery. TERMINAL FLOWER 1, in turn, can inhibit the activity of meristem identity genes at the centre of the shoot apex in two ways; first by delaying their upregulation, and second, by preventing the meristem from responding to LEAFY or APETALA 1. We suggest that the wild-type pattern of TERMINAL FLOWER 1 and floral meristem identity gene expression depends on the relative timing of their upregulation.

Genetic control of branching in Arabidopsis and tomato

The patterns of axillary bud formation and the growth characteristics of side-shoots determine to a large extent the form of plants. Characterization of mutants in the monopodial plant Arabidopsis thaliana and in the sympodial tomato, as well as cloning of some of the respective genes, contributes to a better understanding of side-shoot development. Genes have been identified that influence the initiation of axillary meristems and the pattern of their subsequent development.

The making of a flower: control of floral meristem identity in IT>Arabidopsis/IT>

During the reproductive phase of a plant, shoot meristems follow one of two developmental programs to produce either flowers or vegetative shoots. The decision as to which meristems give rise to flowers, and when they do so, determines the general morphology of an inflorescence. Molecular and genetic research in Arabidopsis and other model species has identified several genes that control the identity that a meristem will adopt. These meristem identity genes are activated in response to developmental and environmental cues, and can be assigned to three basic categories: those required either to initiate or maintain the floral program in some meristems and those required to maintain the vegetative program in others.