Over 20 loci related to flowering time and inflorescence development have been identified in pea Table 1. Initial work on genetic control of flowering resolved several loci from existing variation among various cultivars of garden and field pea, while other loci were subsequently identified through characterization of induced mutants and specific mutant screens Murfet, ; Weller et al. Two major loci are known that delay flowering under non-inductive SD.
Only a single, naturally occurring mutant hr allele has been identified, but for SN both naturally occurring and induced mutant alleles have been described Liew et al. In contrast, the main naturally occurring sn allele has a more restricted distribution and occurs only within the subset of lines carrying hr , implying a more recent origin Liew et al.
Numerous allelic variants of LF are known, including both naturally occurring and induced mutant alleles. Accessions in which the LF gene is deleted or inactivated by nonsense mutation show extremely early, photoperiod-insensitive initiation of flowering Murfet, ; Foucher et al. Dominant alleles of E confer early initiation of flowering in some genetic backgrounds, but this effect shows complex interactions with other loci and incomplete penetrance.
Allelic differences at the HR , SN , LF , and E loci interact to specify an extremely wide range of flowering times in plants in non-inductive conditions. This range extends from the genotype lf sn which may flower as early as node 7 and is completely insensitive to photoperiod, to genotype LF SN HR e which flowers relatively late under LD and may not flower at all under SD Murfet, ; Weller et al.
Interestingly, most mutagenesis programs in pea have been conducted in spring-flowering hr cultivars, and in some cases in lines that also carry sn or lf alleles, and many are also likely to carry derived alleles at the E locus. Mutants isolated from these programs therefore carry at least one additional mutation affecting flowering time, and potentially as many as four.
In addition to the SN and HR loci, several other photoperiod response loci have been identified through analysis of induced mutants. In the presence of hr , these mutations confer complete photoperiod insensitivity for flowering and other traits King and Murfet, ; Arumingtyas and Murfet, Loci involved in promoting flowering in pea under inductive LD conditions have also been identified.
Mutants for the phyA photoreceptor were first identified in screens for seedling photomorphogenesis and subsequently shown to have a LD-specific late-flowering phenotype Weller et al. The phyA mutants are largely insensitive to LD, although day extensions with artificial light rich in blue or far-red wavelengths can result in earlier flowering, implying a contribution from other photoreceptors Weller et al.
While phyB and cry1 might seem plausible candidates for this activity, evidence from phyB and cry1 mutants suggests that neither is fundamentally involved in promotion of flowering Weller et al. The importance of PHYA is underlined by the dominant early flowering photoperiod-insensitive phyA-3D mutant, which has a higher level of phyA protein due to increased protein stability Weller et al.
The late1 mutants are similar to phyA mutants with respect to their effect on flowering and photoperiod responsiveness, but have only mild photomorphogenic defects. Mutations at other pea loci affect the flowering transition without significantly interfering with the overall ability of the plant to respond to daylength. Under LD, gigas and vegetative1 veg1 and veg2 mutants do not produce flowers, but instead show a profuse outgrowth of aerial vegetative branches Murfet, ; Hecht et al.
At least 10 loci that affect flowering-related characteristics in the SDP soybean have now been described Table 1. Cultivars grown at lower latitudes experience a longer growing season and are relatively late to mature, whereas expansion to higher latitudes and completion of the growth cycle within the short summer growing season have required a reduction in sensitivity to the inhibitory effects of LD.
The well-known E series of maturity loci E1 to E9 confer early flowering and maturity, particularly under non-inductive LD conditions Cober and Morrison, ; Watanabe et al. With the exception of E6 and the recently described E9 locus, the early flowering alleles at the E loci are recessive Watanabe et al. Addition of early flowering alleles at these loci results in incrementally earlier flowering under LD and improved adaptation to short summers at high latitudes.
Genetic control of the delayed juvenile trait is not currently clear. One study proposed a single locus J , with recessive alleles conferring late flowering in SD Ray et al.
Flowering time in soybean has also been analyzed as a quantitative trait, and several QTL have been identified, many of which are likely to correspond to known maturity loci Watanabe et al.
Several of the E loci have been characterized physiologically. As in pea, the effects of individual soybean loci have mostly been examined in genetic backgrounds already carrying hypomorphic alleles at other loci, making it difficult to gain a full picture of the action and interactions of any given locus. Nevertheless, five of the eight loci E1 , E3 , E4 , E7, E8 appear to specifically affect photoperiod responsiveness Cober et al.
In particular, differential sensitivity of E3 and E4 loci to light quality of an artificial LD implicated them in the phytochrome system Cober et al. Soybean contains four PHYA genes that consist of two pairs of homeologs, with E3 and E4 representing different homeolog pairs.
The homeolog of E4 , PHYA1 , is apparently functional, whereas the homeolog of E3 carries a deletion and is probably a pseudogene Watanabe et al. In most plant systems, phyA is important for de-etiolation under continuous far-red FR light. Interestingly, loss of E4 function reduced but did not abolish de-etiolation under FR, whereas loss of E3 function had no effect on this response, even in the absence of E4 Liu et al. The presence of phyA1 may also explain why the e3 e4 mutant still shows delayed flowering in response to photoperiod extensions rich in far-red light.
However, it is clear that three other loci, E1 , E7 , and E8 also contribute to this response Cober and Voldeng, ; Cober et al. Although recessive e2 alleles can promote flowering under both LD and SD, E2 is also reported to enhance the photoperiod response and clearly contributes to early flowering and latitudinal adaptation Jiang et al.
However, one of the most significant developments to emerge from analysis of soybean flowering loci has been the recent molecular characterization of the E1 gene. E1 has a major role in natural variation for flowering in soybean and has the largest effect among the E loci Yamanaka et al. Positional cloning of E1 revealed that it possesses a region of weak similarity to the plant-specific B3 domain, in addition to a helix-turn-helix domain and a nuclear localization signal, all suggesting a probable role as a transcription factor Xia et al.
FT genes are of particular interest for understanding flowering time control, in view of their well-documented roles in integration of environmental signals for flowering and in signaling from the site of photoperiod detection in the leaf to the site of flower formation at the shoot apex Pin and Nilsson, Genes in the FTc group are the most divergent, and are distinguished from Arabidopsis FT and most other FT genes by substitution of several conserved residues.
In pea and Medicago , study of expression patterns and mutant phenotypes suggest that FTa and FTb genes are expressed in leaves and may be important for targets of vernalization and photoperiod responses respectively, whereas FTc genes are expressed only in the shoot apex and may contribute to the integration signals from leaf-expressed FT genes Hecht et al. All pea FT genes can promote flowering to some extent when expressed in transgenic Arabidopsis Hecht et al.
In addition, its expression correlates strongly with production of a graft-transmissible flowering stimulus Hecht et al. However, the timing of FTa1 induction is delayed relative to FTb2 , and gigas mutants have inflorescence identity defects but respond strongly to daylength.
This suggests that the two main FT genes expressed in leaves in pea may both signal to the shoot apex but have different developmental roles.
In addition, it appears that pea FT genes may regulate each other Hecht et al. Models summarizing interactions between flowering genes in control of flowering time in pea and soybean. Genes that promote flowering are shown in green, and those that inhibit flowering are shown in red. LDs, long days; SDs, short days. Expression of both genes is induced in leaves under inductive SD photoperiods, and both promote flowering when overexpressed in either Arabidopsis or soybean itself Kong et al.
Whereas FT2a appears to be qualitatively regulated by photoperiod, FT5a is expressed to some extent even in LD suggesting its role may not be restricted to photoperiod response Kong et al. Surprisingly, one of the four soybean FTb genes, FT4 , shows an opposite pattern of regulation and appears to act as a repressor of flowering.
FT4 is induced in leaves in LD, and is able to delay flowering when overexpressed in Arabidopsis Zhai et al. Interestingly, FT4 carries a substitution of a highly conserved glycine GR in an important functional region, and the significance of this is supported by the fact that the same residue is also substituted in another FT known to repress flowering; FT1 in sugar beet Pin et al.
However, within the legumes the existence of a repressive FT may be unique to soybean, as all other legume FTb-type sequences known to date carry the canonical glycine in this position. In species as diverse as Arabidopsis and rice, the photoperiod-dependent induction of FT genes relies on interactions between light perception and the circadian clock Andres and Coupland, ; Brambilla and Fornara, ; Song et al.
It is likely that this is also the case in legumes, in view of the fact that orthologs of PHYA a photoreceptor and GI a gene affecting clock function are important regulators of photoperiodic flowering in both pea and soybean Weller et al.
The importance of the clock for legume photoperiod responsiveness is further reinforced by the fact that the pea HR , DNE , and SN genes are all orthologs of clock genes and influence clock function Liew et al.
Expression of soybean FT2a is elevated by e2 , e3 , and e4 alleles under long days Kong et al. In the pea late1 and phyA mutants, expression of FTb2 is not detected and other FT genes are more weakly expressed Hecht et al. These interactions are summarized in Figure 1.
Unfortunately, we do not yet have a clear picture of how these clock and photoreceptor inputs are integrated to provide photoperiod-specific regulation of FT genes. LD-specific induction of FT is achieved through transcriptional and post-translational regulation of CO.
Soybean contains four genes orthologous to Arabidopsis CO Wong et al. The situation in the galegoid legumes is simpler, with only a single CO ortholog COLa known to be present Hecht et al. While the function of this gene has not been directly investigated in pea, its expression is not misregulated in either late1 Psgi or dne Pself4 mutants, despite strong defects in FT regulation, flowering and photoperiod responsiveness in both mutants Hecht et al.
In addition, expression analyses, transgenic studies and characterization of specific mutants in M. The available evidence so far thus indicates that legume CO -like genes may not have a major role in integration of responses to photoperiod in the temperate legume group, and suggests that some alternative mechanism must be operating.
One possibility is that the E1 gene may participate in this role. E1 is specifically expressed under LD, where it shows a strong diurnal expression rhythm, and is also transcriptionally regulated by E4 Xia et al. In future, it will be interesting to learn whether E1 is also regulated by E2 and E3 , and whether E1 orthologs also regulate flowering in the temperate legumes. However, even if E1 does participate as a key integrator in the soybean photoperiod response mechanism, the fact that it is a repressor of FT suggests that there still may be undiscovered components required for upregulation of FT genes in inductive conditions.
It is possible that legume E1 and GI orthologs might interact in a similar way. Another scenario may be that the DNA binding and protein-interacting properties of CO may be partitioned into two separate proteins, as has recently been suggested in sugarbeet, where a CCT-domain pseudo-response regulator PRR protein similar to the C-terminal domain of CO and a B-box zinc finger protein similar to the N-terminal domain of CO may interact to confer CO function Dally et al. The relatively close taxonomic relationship between these two species make them an attractive model for understanding the evolution of this difference in photoperiod response mode.
Regulation of flowering by vernalization is a phenomenon widespread across annual species from temperate regions, but is thought to have evolved independently in different plant lineages. As a result, the genes and genetic mechanisms conferring vernalization responsiveness are likely to differ across different groups Kim et al.
In legumes, the first insight into the genetic control of vernalization response has come from work in M. FTa2 is induced during exposure to cold, but FTa1 is only expressed following return to warm conditions.
Loss-of-function FTa1 insertion mutants are insensitive to vernalization but retain sensitivity to photoperiod, indicating that FTa1 is necessary for response to vernalization, and may be the key target in the legume vernalization pathway Figure 1 ; Laurie et al.
Interestingly, lines carrying insertions close to but not within FTa1 coding regions show dominant inheritance of early, vernalization-independent flowering, which suggests that FTa1 is normally subject to repression by adjacent regulatory regions and that this repression is normally overcome by vernalization Jaudal et al. Flowering time control is a key issue in adaptation of many other crop legumes Summerfield and Roberts, b ; Nelson et al.
The majority of these loci have been detected as QTL, but in some cases they have been amenable to classical genetic analyses and Mendelian inheritance has been defined. TABLE 2. While all of this variation is naturally arising and therefore likely to be adaptive, several loci can be singled out for a particularly significant contribution to adaptation and range expansion. One example is the SDP common bean Phaseolus vulgaris , where, like soybean, expansion to higher latitudes has been accompanied by earlier flowering under LD and a reduction in photoperiod responsiveness Gepts and Debouck, A significant proportion of this variation can be attributed to PPD , a mendelized locus on LG1, where recessive alleles confer reduced photoperiod response and early flowering under LD Koinange et al.
A second example is narrow-leafed lupin Lupinus angustifolius , where the acquisition of early, vernalization insensitive flowering conditioned by dominant alleles at the the Ku locus has been integral to deployment of this crop for Mediterranean climates with mild winters in which a vernalization requirement would not be met Nelson et al.
A third example is lentil Lens culinaris , where an early flowering variant at the Sn locus has had an important role in developing early flowering cultivars for water-limited environments and broadening the genetic base of lentil in south Asia Sarker and Erskine, There is significant potential for translation of insights from the pea and soybean systems to achieve a better understanding of other legume species.
This potential reflects advances in two areas. First, functional and phylogenetic analyses of flowering genes and gene families in pea and soybean can improve the identification of plausible candidate genes for particular loci. Second, availability of sequenced genomes and gene-based genetic maps have improved the technical ability to identify and evaluate candidate genes under QTL, and to identify those QTL that may have conserved locations across several species.
In addition the fact that certain desirable traits such as early, photoperiod-insensitive flowering or determinate growth can result from simple monogenic loss-of-function mutations means that in species where such variants do not already exist it may be feasible to generate them through mutagenesis.
Within the legumes the value of a comparative approach is most clearly shown by recent findings that in several species, determinate inflorescence architecture is conferred by mutation of specific TFL1 genes [described by Benlloch et al. A second example is the recent identification of the lentil SN locus as the ortholog of pea HR Weller et al. In addition to these well-established examples, it is now also possible to identify other potentially conserved QTL and to begin to speculate about their nature Figure 2.
Relative locations of flowering time gene homologs and quantitative trait loci QTL in the chickpea genome. QTL are shown as vertical bars, and the markers delimiting them are indicated by gray boxes. References for each QTL can be found in Table 2. The most prominent case is the existence of a conserved major flowering time QTL in a region syntenic with a section of Medicago chromosome 7 containing a tandem array of FTa and FTc genes.
This region is now implicated in control of flowering time in numerous members of the temperate legume clade, including M. As described above, functional studies in Medicago and pea have demonstrated the importance of FT genes in this cluster for vernalization responsiveness and other aspects of flowering Laurie et al. Evidence from Medicago and pea indicates that FTa and FTc genes promote flowering, and a simple loss-of-function mutation would therefore be expected to be recessive and late-flowering.
This also suggests that the causal genetic changes could be varied and complex, potentially involving variation in copy number or alteration to promoter or other regulatory regions, as observed for gain-of-function mutations affecting flowering time genes in cereals e. A closer look at individual species provides additional illustrations of a comparative approach to candidate gene identification. The first example is common bean where, in addition to several flowering time QTL Table 2 , two loci controlling photoperiod response have also been characterized; Ppd and Hr Gu et al.
A second bean locus contributing to photoperiod sensitivity, Hr , is less well-defined but is positioned toward the other end of the same linkage group Gu et al.
The second example is chickpea, where classical genetic analyses have distinguished four Mendelian loci, named Early flowering 1 Efl1 to Efl4 Gaur et al. Recessive alleles at these loci confer early flowering and at least two are likely to be widespread within the chickpea germplasm and have a major impact on flowering time adaptation. In addition, many linkage studies have been performed in chickpea, and six flowering time QTL have been defined in LG1, 2, 3, 4, and 8 Table 2.
Some of these studies have involved parents known to carry efl1 or efl2 , suggesting that QTL may correspond to major loci in some cases. This, together with the availability of the chickpea genome sequence Jain et al. For example, Efl1 was defined in a cross using the early flowering line ICCV2 Kumar and van Rheenen, , which has also been used as a common parent in several studies reporting a major QTL Cho et al.
Although its genomic location is still uncertain as markers near the QTL have been variously assigned to LG3, 4, 5, 6, and 8 , the clearest and most recent indication is given by Jamalabadi et al. Recessive alleles at the photoperiod response locus Efl2 are present in the cultivar ICC Or et al.
This last gene has also been highlighted as a candidate near the chromosome 7 QTL in M. Detailed genetic studies in model legumes are bringing rapid progress to our understanding of flowering time control, and have identified significant similarities and differences with other plant groups.
At the same time, new genome sequences and dense, gene-based genetic maps are accelerating the translation of these insights to a range of other legume species, and giving new insight into how flowering time gene functions may be conserved or diversified within this important group of crops. It now seems reasonable to expect that genes underlying most of the major flowering time loci in legumes will be identified within the next 5—10 years, which will help improve our understanding of how they work together to provide adaptation to specific locations and agronomic constraints.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. We thank F. Sussmilch and R. Xue, W. Download references. You can also search for this author in PubMed Google Scholar. Correspondence to Christian Jung.
Reprints and Permissions. Jung, C. Flowering time regulation: Agrochemical control of flowering. Nature Plants 3, Download citation. Published : 06 April Anyone you share the following link with will be able to read this content:. Sorry, a shareable link is not currently available for this article.
Provided by the Springer Nature SharedIt content-sharing initiative. Advanced search. Skip to main content Thank you for visiting nature. Subjects Flowering Molecular engineering in plants. Access through your institution. However, these structures do not form petals or stamens and show helical phyllotaxy rather than the typical arrangement of whorls Schultz and Haughn, , ; Huala and Sussex, ; Weigel et al.
Mutations in AP1 also have a stronger effect on flowers that develop at early positions on the shoot. However, ap1 mutant structures are less affected than the shoots formed in lfy mutants; ap1 flowers are determinate, like those of the wild type, but form secondary flowers in the axils of the outer organs that develop within the primary flower. Therefore, the mature ap1 flower has a complex, branched structure that contains several individual flowers Irish and Sussex, ; Bowman et al.
The reduced requirement for LFY and AP1 in later flowers is probably caused by other floral meristem identity genes compensating for their loss of function. Redundancy is also evident between AP1 and CAL , so that mutations in the CAL gene do not cause a phenotype in otherwise wild-type plants but greatly enhance the effect of ap1 mutations Bowman et al.
Also, the effect of ap2 mutations on floral meristem identity was observed because ap2 mutations enhance the phenotype s of ap1 an d lfy mutants Schultz and Haughn, ; Shannon and Meeks-Wagner, This redundancy between the four floral meristem identity genes indicates that they have partially overlapping functions. These observations led to the suggestion that in wild-type plants the four genes act collectively, enhancing each others' expression and acting additively on target genes to promote floral meristem identity Fig.
This additive activity might enable plants to make a sharp transition between vegetative and reproductive development Bowman et al.
The redundancy and cooperation between floral meristem identity genes has made the roles of individual genes difficult to study. However, despite the interrelationships between them, ectopic and high-level expression of a single flower meristem identity gene can be sufficient to specify floral development.
In Arabidopsis plants ectopically expressing LFY or AP1 , lateral meristems that normally would be shoots are converted into axillary flowers Mandel and Yanofsky, ; Weigel and Nilsson, In addition, the shoot apical meristem of 35 S :: LFY and 35 S :: AP1 plants is determinate, forming a terminal flower similar to that of terminal flower tfl mutants see below.
These results demonstrate that both LFY and AP1 are sufficient to convert shoot meristems into flowers. The TFL gene influences meristem identity, but it has the reverse effect of LFY : in tfl mutants the apical shoot meristem and axillary shoot meristems become converted to floral meristems in which the LFY and AP1 genes are ectopically expressed Shannon and Meeks-Wagner, ; Alvarez et al.
In addition, tfl mutants flower early, suggesting a role for TFL during vegetative development to influence the timing of the transition to flowering Table I. The early flowering of tfl mutants seems to be the result of an earlier commitment to flowering, since tfl mutants are committed to flower after exposure to 5 d of LD conditions, whereas 7 d of LD conditions are required for the wild type Bradley et al.
The TFL gene was recently cloned and shown to encode a protein with similarity to animal phosphatidylethanolamine-binding proteins Bradley et al. TFL is expressed in a group of cells lying just below the apical dome of the meristem.
In wild-type plants TFL mRNA is detected from d 2 or 3, but expression is weak up to the point of commitment d 7 , after which it increases Bradley et al.
These mutants form reproductive structures such as stigmatic papillae and ovule-like structures on the surfaces of their cotyledons, and the shoot produces no rosette leaves but often forms carpelloid structures with features of ovules, and terminates in a pistil or flower Sung et al.
Weak mutant alleles of emf1 and emf2 form recognizable leaves, but they are small and sessile. AP1 is ectopically expressed in the shoot meristem and leaves of plants carrying weak emf alleles, as well as in the shoot apex, hypocotyl, and cotyledons of plants carrying strong emf alleles Chen et al.
Therefore, the EMF genes appear to negatively regulate the transition from vegetative to reproductive development, and to negatively regulate the expression of AP1 in vegetative tissue.
The extreme early flowering of emf mutants led to the suggestion that EMF genes are central repressors of flowering with activities that decline during plant development; when their activity falls below a certain threshold, plants undergo the transition from rosette development to inflorescence development, and from inflorescence development to the formation of single flowers Chen et al.
Flowering-time mutants display their major effects on the duration of vegetative development, whereas mutations in floral meristem identity genes disrupt floral development. Therefore, flowering-time genes are often assumed to act before floral meristem identity genes and, generally, to lead to their activation. The relationships between these two groups of genes have been studied genetically by making double mutants, and to a lesser extent at the molecular level by examining the effect of overexpression of flowering-time genes on meristem identity gene expression.
A complex relationship between flowering-time genes and floral meristem identity genes is emerging from these studies. In general, the effects of lfy or ap1 mutations are enhanced by mutations or conditions that delay flowering. Also, the lfy mutation is completely recessive under LD conditions, but under SD conditions the heterozygote is impaired in the maintenance of floral meristem identity Okamuro et al.
Furthermore, several mutations causing late flowering broadly enhance the effect of lfy or ap1 see below , again indicating a close relationship between genes that promote flowering and the action of floral meristem identity genes Putterill et al. The promotion of flowering by some treatments seems at least partially to act by causing an increase in the transcription of the LFY gene. Similarly, by utilizing transgenic plants in which CO activity could be regulated, it has been shown that activation of CO causes expression of LFY just as rapidly as exposure to LD conditions, and therefore at least one function of the LD-responsive pathway is to activate LFY Simon et al.
Strong expression of LEAFY from the cauliflower mosaic virus 35 S promoter was insufficient to cause flower development without the formation of several vegetative nodes, suggesting that the shoot apical meristem must also become competent to respond to LEAFY expression Weigel and Nilsson, The shoot meristem's ability to respond to LEAFY is also regulated by day length and flowering-time genes.
Exposure to LD conditions and the action of flowering-time genes may activate meristem identity genes that act cooperatively with LEAFY to confer floral identity on meristems see below or, alternatively, some flowering-time genes might act in the meristem to facilitate the action of the floral meristem identity genes. Genetic experiments suggest that some flowering-time genes do not act through LFY but through other floral meristem identity genes.
For example, co , fve, and fpa mutations enhance the lfy phenotype, but the double mutants formed lfy -like flowers late in development Putterill et al. The ft and fwa mutations also enhance the ap1 phenotype, but this enhancement is not as strong as that of lfy. Flowering-time genes are also likely to be involved in the increased expression of TFL that occurs around the time of commitment to flowering, as the activation of CO leads to increased expression of TFL Simon et al.
Also, the effects of the tfl mutation are weakened by environmental conditions such as SD that delay the onset of flowering Shannon and Meeks-Wagner, More recently, it was shown that at least some mutations that cause late flowering delay the determinate phenotype of tfl mutants, so that the double mutants form a terminal flower after producing more lateral flowers than produced in tfl mutants Ray et al.
The double mutants also flower with a similar number of leaves as the late-flowering parents, indicating that the genes affected in the late-flowering mutants are required for the early flowering seen in tfl mutants.
Expression of both CO and LFY are tightly regulated so that small changes in their activity affect flowering time or shoot morphology. The promotion of flowering by CO in response to LD conditions is probably regulated by transcriptional control of CO , because the gene is expressed at higher levels under LD conditions than under SD conditions. Furthermore, CO expression seems to be poised at a critical level in LD-grown seedlings: reducing the dosage of the gene in heterozygotes leads to a delay in flowering, whereas increasing CO dosage in transgenic plants carrying the wild-type CO gene causes an acceleration in flowering time.
In addition, its overexpression in 35 S :: CO transgenic plants is sufficient to promote very early flowering under SD and LD conditions, and flowering of these plants is insensitive to day length. Maintaining a balance in expression levels between different flowering-time genes might be important in enabling plants to flower in response to environmental conditions, so that increasing the dosage of the CO gene reduces the response to day length, and expression of CO from the 35 S promoter abolishes environmental regulation of flowering time.
Quantitative regulation of LFY expression is also important for the proper regulation of flowering time and the node at which flowers are first formed. Under SD conditions LFY is expressed at initially low levels and increases gradually during the long period of vegetative growth. Tight regulation of LEAFY gene expression is therefore important in the regulation of flowering time and in defining shoot morphology.
The relationships between flowering time and floral meristem identity genes are complex and complicated by functional redundancy. Recently, the functions of individual genes have become clearer through the use of gain-of-function transgenes.
Further genetic analyses with such transgenic plants should enable the function of single genes to be studied in the absence of redundant functions. For example, the inactivation of the autonomous and GA flowering-time pathways in a 35 S :: CO background should allow the function of the LD-responsive pathway to be studied in the absence of other pathways.
The study of flowering-time genes is also complicated by the lack of knowledge of the timing during plant development or the tissues in which they act. Recent analysis shows that plants are committed to flower within a week of sowing under LD conditions. This suggests that flowering-time genes act early in development. This hypothesis is supported by the phenotype conferred by conditional gi alleles, which indicate that GI acts 3 d after germination Araki and Komeda, Also, the addition of dexamethasone to 35 S ::CO:GR plants at d 7 after sowing produces a phenotype very similar to that of the wild type, which is consistent with CO acting around d 7 Simon et al.
The tissues in which the flowering-time genes are required to activate flowering have not been studied extensively in Arabidopsis.
Grafting experiments with pea have distinguished between genes that act in the leaf and those that act in the meristem Weller et al. In the case of FCA , homozygous mutant sectors were made in a heterozygous background and suggest that the gene product acts non-cell-autonomously to influence flowering time Furner et al.
Further data on the time of action of these genes and the tissues in which they are required will allow the relationship between the function of flowering-time genes and that of floral meristem identity genes to be established more accurately. Finally, genetic evidence for the existence of redundant flowering-time pathways is strong and consistent, and molecular relationships between gene products in the same pathway are starting to emerge. The use of gain-of-function transgenes and additional demonstrations that mutations in certain genes influence the activity of other genes in the same pathway should help to determine the order in which flowering-time genes act.
How the LD, autonomous, and GA pathways act additively to regulate flowering time also awaits the isolation of common targets for these pathways, and a better understanding of how they interact with the floral meristem identity genes. Plant J 2 Google Scholar.
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