Genetic control of fasciation in pea (<Emphasis Type="Italic">Pisum sativum</Emphasis> L.)

The inheritance and manifestation of fasciation character in three fasciated lines of common pea Pisum sativum L. were investigated. All studied forms are characterized by abnormal enlargement of stem apical meristem leading to distortions in shoot structure. It was estimated that fasciation in mutant Shtambovyi is connected with recessive mutation in gene FAS, which was localized in linkage group III using morphological and molecular markers. It was demonstrated that fasciation in cultivar Rosacrone and line Lupinoid is caused by recessive mutation of the same gene (FA). The peculiar architecture of inflorescence in the Lupinoid line is a result of interaction of two recessive mutations (det fa). Investigation of interaction of mutations fa and fas revealed that genes FA and FAS control consequential stages of apical meristem specialization. Data on incomplete penetrance and varying expressivity were confirmed for the mutant allele fa studied.     

Expression of fasciation mutation in apical meristems of soybean, Glycine max (Leguminosae)

The phenotype of the apical meristem was used to examine the effect of fasciation mutation at the f locus in different genetic backgrounds in soybean Glycine max (L.) Merr. Comparisons of meristem development in fasciation mutant and wild type were conducted with scanning electron microscope (SEM) on isogenic lines BARC-11-11-ff and BARC-11-11-FF at postgermination and early vegetative stages. Studies of apical meristems of three independently originated fasciation mutants, PI 83945-4, PI 243541, and T173, were carried out at vegetative and early floral transition stages. Corolla Fasciation, the extreme mutant phenotype, was used for comparison of meristem development. Enlargement of the apical meristem and shortened plastochron were observed in the mutant lines 2 d after germination. Similar to Corolla Fasciation, in PI 83945-4, PI 243541, and T173, enlargement of the apical meristem was followed by growth along one axis at the V3 stage and establishment of a ridge-like meristem at the V4 stage. Influence of pedigree on the expression of the fasciation phenotype was demonstrated by different growth patterns (subangular vs. ridge-like) of the apical meristem in BARC-11-11-ff and PI 243541 with the same f gene. During transition of the apical meristem from vegetative to reproductive stage in all mutant lines further production of leaf primordia ceased. The developmental pattern of the apical meristems suggests that the f locus may have the same allele in fasciation mutants of independent origin in soybean.     

<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.     

Effect of the auxin polar transport inhibitor on the morphogenesis of leaves and generative structures during fasciation in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> (L.) Heynh.

An increase in the proliferative activity of a shoot apical meristem (SAM) and the further accumulation of a pool of undifferentiated cells (fasciation) results in phyllotaxis changes. In the case of Arabidopsis thaliana, a typical spiral leaf arrangement is replaced by an opposite or verticillate one (depending on the level of a fasciation manifestation). Pistil development in mutant plants is accompanied by the appearance of a group of undifferentiated meristematic cells in its central part. The addition of N-1-naphthylphthalamic acid (NPA) causes an increase in the meristem volume and number of stipules in both mutant and control plants. The NPA effect on the floral morphogenesis results in a significant growth of meristemic cell pool. The interaction of different mechanisms of a meristem volume control is discussed.     

Molecular mapping of the fasciation mutation in soybean, Glycine max (Leguminosae)

The spontaneous fasciation mutation generates novel developmental diversity in cultivated soybean, Glycine max (L.) Merrill. An increased apical dominance in the mutant inhibits axillary buds, causes a branchless phenotype, and restricts reproduction to shoot apices. The fasciation mutation is encoded by a recessive (f) allele at a single locus. The mutation, despite its importance in soybean development, has no locus assignment on previously reported molecular maps of soybean. A population of 70 F(2) progeny was derived from a cross between 'Clark 63' and the fasciation mutant. More than 700 molecular markers (amplified restriction fragment length polymorphisms [AFLPs], random amplified polymorphic DNAs [RAPDs], restriction fragment length polymorphisms [RFLPs], and simple sequence repeats [SSRs]) were used in mapping of the fasciation phenotype. Twenty linkage groups (LGs) corresponding to the public soybean molecular map are represented on the Clark 63 × fasciation mutant molecular map that spans 3050 centimorgans (cM). The f locus was mapped on LG D1b+W and linked with two AFLPs and four SSR markers (Satt005, Satt141, Satt600, and Satt703). No linkage was found between the f locus and several cDNA polymorphic loci between the wild type and the mutant. The known map position of the f locus and demonstration of the mutant phenotype from early postembryonic throughout reproductive stages provide an excellent resource for investigations of molecular mechanisms affecting soybean ontogeny.     

Genetic and morphological characterization of the barley <Emphasis Type="Italic">uniculm2</Emphasis> (<Emphasis Type="Italic">cul2</Emphasis>) mutant

Axillary meristem growth and development help define plant architecture in barley (Hordeum vulgare L). Plants carrying the recessive uniculm2 (cul2) mutation initiate vegetative axillary meristem development but fail to develop tillers. In addition, inflorescence axillary meristems develop into spikelets, but the spikelets at the distal end of the inflorescence have an altered phyllotaxy and are sometimes absent. Double mutant combinations of cul2 and nine other recessive mutations that exhibit low to high tiller number phenotypes resulted in a uniculm vegetative phenotype. One exception was the occasional multiple shoots produced in combination with granum-a; a high tillering mutant that occasionally produces two shoot apical meristems. These results show that the CUL2 gene product plays a role in the development of axillary meristems into tillers but does not regulate the development of vegetative apical meristems. Moreover, novel double-mutant inflorescence phenotypes were observed with cul2 in combination with the other mutants. These data show that the wild-type CUL2 gene product is involved in controlling proper inflorescence development and that it functions in combination with some of the other genes that affect branching. Our genetic analysis indicates that there are genetically separate but not distinct regulatory controls on vegetative and inflorescence axillary development. Finally, to facilitate future positionally cloning of cul2, we positioned cul2 on chromosome 6(6H) of the barley RFLP map.     

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.     

On the potential for evolutionary change in meristematic cell lineages through intraorganismal selection

In the absence of sexual recombination somatic mutations represent the only source of genetic variation in clonally propagating plants. We analyse the probability of such somatic mutations in the shoot apical meristem being fixed in descendant generations of meristems. A model of meristem cell dynamics is presented for the unstratified shoot apical meristem. The fate of one mutant initial is studied for a two- and three-celled shoot apical meristem. The main parameters of the model are the number of apical initials, the time between selection cycles, number of selection cycles and cell viability of the mutant genotype. As the number of mitotic divisions per selection cycle and number of selection cycles increases the chimeric state dissipates and the probability of mutation fixation approaches an asymptote. The value of this fixation asymptote depends primarily on cell viability, while the time to reach it is mainly influenced by the total number of mitotic divisions as well as the number of initials. In contrast to the presumed operation of Muller’s Ratchet in plants the chimeric state may represent an opportunity for deleterious mutations to be eliminated through intraorganismal selection or random drift. We conclude that intraorganismal selection not only can be a substantial force for the elimination of deleterious mutations, but also can have the potential to confer an evolutionary change through a meristematic cell lineage alone.     

Repression of floral meristem fate is crucial in shaping tomato inflorescence

Tomato is an important crop and hence there is a great interest in understanding the genetic basis of its flowering. Several genes have been identified by mutations and we constructed a set of novel double mutants to understand how these genes interact to shape the inflorescence. It was previously suggested that the branching of the tomato inflorescence depends on the gradual transition from inflorescence meristem (IM) to flower meristem (FM): the extension of this time window allows IM to branch, as seen in the compound inflorescence (s) and falsiflora (fa) mutants that are impaired in FM maturation. We report here that Jointless (J), which encodes a MADS-box protein of the same clade than Short Vegetative Phase (SVP) and Agamous Like 24 (AGL24) in Arabidopsis, interferes with this timing and delays FM maturation, therefore promoting IM fate. This was inferred from the fact that j mutation suppresses the high branching inflorescence phenotype of s and fa mutants and was further supported by the expression pattern of J, which is expressed more strongly in IM than in FM. Most interestingly, FA--the orthologue of the Arabidopsis LEAFY (LFY) gene--shows the complementary expression pattern and is more active in FM than in IM. Loss of J function causes premature termination of flower formation in the inflorescence and its reversion to a vegetative program. This phenotype is enhanced in the absence of systemic florigenic protein, encoded by the Single Flower Truss (SFT) gene, the tomato orthologue of Flowering Locus T (FT). These results suggest that the formation of an inflorescence in tomato requires the interaction of J and a target of SFT in the meristem, for repressing FA activity and FM fate in the IM.     

The pleiotropic mutation dar1 affects plant architecture in Arabidopsis thaliana

Shoot architecture is shaped upon the organogenic activity of the shoot apical meristem (SAM). Such an activity relies on the balance between the maintenance of a population of undifferentiated cells in the centre of the SAM and the recruitment of organ founder cells at the periphery. A novel mutation in Arabidopsis thaliana, distorted architecture1 (dar1), is characterised by disturbed phyllotaxy of the inflorescence and consumption of the apical meristem late in development. SEM and light microscopy analyses of the dar1 SAM reveal an abnormal partitioning of meristematic domains, and mutations known to affect the SAM structure and function were found to interact with dar1. Moreover, the mutant shows an alteration of the root apical meristem (RAM) structure. Those observations support the hypothesis that DAR1 has a role in meristem maintenance and it is required for the normal development of Arabidopsis inflorescence during plant life.     

Flower and shoot fasciation: From phenomenology to the construction of models of apical meristem transformations

The review considers the phenomenon of fasciation arising due to the enlargement of shoot or floral meristem. The system of CLAVATA-WUSCHEL proteins controls the pool of stem cells in the surface layers of the shoot apical meristem. The analysis of the literature allowed clarifying the role of the WUSCHEL (WUS) gene in the creation of positional signals for the emergence of leaf primordia and procambial strands. The hypothesis is put forward about the new regulatory cascade PINHEAD/ZWILLE-WUSCHEL involved in the positional control of auxin fluxes and determining the optimal density of vascular bundles in the tissue volume. The examples of various types of shoot and flower fasciation are presented: radial, linear, and ring fasciation, defasciation). The model of anatomo-morphological changes accompanying fasciation is suggested.     

FASCIATA genes for chromatin assembly factor-1 in arabidopsis maintain the cellular organization of apical meristems

Postembryonic development of plants depends on the activity of apical meristems established during embryogenesis. The shoot apical meristem (SAM) and the root apical meristem (RAM) have similar but distinct cellular organization. Arabidopsis FASCIATA1 (FAS1) and FAS2 genes maintain the cellular and functional organization of both SAM and RAM, and FAS gene products are subunits of the Arabidopsis counterpart of chromatin assembly factor-1 (CAF-1). fas mutants are defective in maintenance of the expression states of WUSCHEL (WUS) in SAM and SCARECROW (SCR) in RAM. We suggest that CAF-1 plays a critical role in the organization of SAM and RAM during postembryonic development by facilitating stable maintenance of gene expression states.     

Gene NANA regulates cell proliferation in Arabidopsis thaliana shoot apical meristem without interaction with CLV1, CLV2, CLV3

A constancy of stem cell pool in shoot apical meristem of Arabidopsis thaliana is provided by a genetic regulation system with negative feedback loop based on the interaction of the gene WUS, which maintains indeterminate state of cells, with CLV genes, which restrict the level of WUS expression and stem cell pool size. clv mutations lead to an increase in the pool of stem cells in the apical and floral meristems and wus mutation leads to the opposite effect. Mutation na (nana), like wus mutation, causes premature termination of shoot apical meristem function, although it does not affect the activity of the flower meristem. To elucidate the role of NA in the control of shoot apical meristem functioning, the interaction of NA with CLV genes were investigated. Additive phenotype of double mutants na clv1, na clv2-1, and na clv3-2 indicates that the NA gene makes an independent contribution to the functioning of the shoot apical meristem. It is assumed that the NA gene controls apical meristem cell proliferation during the transition to the reproductive phase of plant development, acting much later and independently of the genes WUS-CLV.     

FASCIATION AND DICHOTOMOUS BRANCHING IN ECHINOCEREUS (CACTACEAE)

In Echinocereus reichenbachii dichotomous branching and fasciation (cresting) are rare events. Both were found together in only a few of many populations investigated and are interpreted as variants of a single phenomenon. They may occur at any stage of shoot development, but crest meristems arise most commonly on young branches among clusters of normal shoots. Sometimes they appear on unbranched young plants or seedlings, very rarely on older shoots. Dichotomy results from the division of an apical meristem into equal parts each of which functions independently, producing a forked shoot. Fasciation involves the extension of a single meristem into an apical ridge. The product is a flabellate shoot that becomes undulate if growth along the summit continues. In longisection linear meristems appear similar to radial sections of normal shoots; in median sagittal section they have a much extended central mother cell zone within which the cell pattern resembles a rib meristem. Although crest meristems become sluggish or even inactive with age, localized renewed growth may occur spontaneously or be induced by injury. In this species the random production of normal shoots from crest meristems (defasciation) was not observed, but if much or all of such a meristem is removed, branches may arise from lateral areoles, and these are always normal. It seems, therefore, that whatever induces fasciation in E. reichenbachii originates in and is restricted to the apical meristem and its immediate vicinity.     

The ULTRAPETALA gene controls shoot and floral meristem size in Arabidopsis

The regulation of proper shoot and floral meristem size during plant development is mediated by a complex interaction of stem cell promoting and restricting factors. The phenotypic effects of mutations in the ULTRAPETALA gene, which is required to control shoot and floral meristem cell accumulation in Arabidopsis thaliana, are described. ultrapetala flowers contain more floral organs and whorls than wild-type plants, phenotypes that correlate with an increase in floral meristem size preceding organ initiation. ultrapetala plants also produce more floral meristems than wild-type plants, correlating with an increase in inflorescence meristem size without visible fasciation. Expression analysis indicates that ULTRAPETALA controls meristem cell accumulation partly by limiting the domain of CLAVATA1 expression. Genetic studies show that ULTRAPETALA acts independently of ERA1, but has overlapping functions with PERIANTHIA and the CLAVATA signal transduction pathway in controlling shoot and floral meristem size and meristem determinacy. Thus ULTRAPETALA defines a novel locus that restricts meristem cell accumulation in Arabidopsis shoot and floral meristems.     

Analysis of the role of the late-flowering locus,GI, in the flowering of Arabidopsis thaliana

The mutation gigantea (gi) is recessive and belongs to the late-flowering mutations in Arabidopsis thaliana. The late-flowering mutations result in a pronounced delay in flowering due to a prolonged phase of vegetative growth, which is manifested by an increased number of primary foliage leaves in the rosette (i.e. vegetative nodes). To examine the nature of the gi mutation, detailed phenotypic analysis was carried out for three representative mutant alleles. The results indicate that gi mutants have a defect in the promotion of the floral induction process by long-day photoperiods and not in the flowering process per se. Temperature-shift experiments using a partially conditional allele were employed to determine the timing of the functional requirement for the product of the GI locus. The end of the deduced functional period corresponds to the period at which transition of the apical meristem from the vegetative to the reproductive phase occurs. Such timing is in good agreement with the postulated role of the GI locus. These results demonstrate that the GI locus is involved in the promotion of floral initiation (entrance of the meristem into the transitional stage) by long-day photoperiods.     

Further characterization of recessive suppression in yeast. Isolation of the low-temperature sensitive mutant of Saccharomyces cerevisiae defective in the assembly of 60 S ribosomal subunit

It has been shown that recessive suppressor mutations in the yeast Saccharomyces cerevisiae may cause sensitivity towards low temperatures (very slow growth or lack of growth at 10 degrees C). One of the sup 1 low temperature sensitive (Lts-) mutants, 26-125A-P-2156, was studied in detail. After a prolonged period of incubation (70 h) under restrictive conditions the protein synthesis apparatus in the mutant cells was irreversibly damaged. In addition, Lts- cells incubated under restrictive conditions synthesize unequal amounts of ribosomal subunits, the level of 60 S subunit being reduced. It has been suggested that the recessive suppression is mediated by a mutation in the gene coding for 60 S subunit component, probably a ribosomal protein. The mutation leads simultaneously to a defect in the assembly of 60 S subunit and to low-temperature sensitive growth of the mutant.     

LEAFY COTYLEDON1 Is an Essential Regulator of Late Embryogenesis and Cotyledon Identity in Arabidopsis

LEAFY COTYLEDON1 (LEC1) is an embryo defective mutation that affects cotyledon identity in Arabidopsis. Mutant cotyledons possess trichomes that are normally a leaf trait in Arabidopsis, and the cellular organization of these organs is intermediate between that of cotyledons and leaves from wild-type plants. We present several lines of evidence that indicate that the control of late embryogenesis is compromised by the mutation. First, mutant embryos are desiccation intolerant, yet embryos can be rescued before they dry to yield homozygous recessive plants that produce defective embryos exclusively. Second, although many genes normally expressed during embryonic development are active in the mutant, at least one maturation phase-specific gene is not activated. Third, the shoot apical meristem is activated precociously in mutant embryos. Fourth, in mutant embryos, several genes characteristic of postgerminative development are expressed at levels typical of wild-type seedlings rather than embryos. We conclude that postgerminative development is initiated prematurely and that embryonic and postgerminative programs operate simultaneously in mutant embryos. The pleiotropic effects of the mutation indicate that the LEC1 gene plays a fundamental role in regulating late embryogenesis. The role of LEC1 and its relationship to other genes involved in controlling late embryonic development are discussed.     

Mutations in two independent genes lead to suppression of the shoot apical meristem in maize

The shoot apical meristem (SAM), initially formed during embryogenesis, gives rise to the aboveground portion of the maize (Zea mays) plant. The shootless phenotype (sml) described here is caused by disruption of SAM formation due to the synergistic interaction of mutations at two genetic loci. Seedlings must be homozygous for both sml (shootmeristemless), and the unlinked dgr (distorted growth) loci for a SAM-less phenotype to occur. Seedlings mutant only for sml are impaired in their morphogenesis to different extents, whereas the dgr mutation alone does not have a recognisable phenotype. Thus, dgr can be envisaged as being a dominant modifier of sml and the 12 (normal):3 (distorted growth):1 (shoot meristemless) segregation observed in the F(2) of the double heterozygote is the result of the interaction between the sml and dgr genes. Other segregation patterns were also observed in the F(2), suggesting instability of the dgr gene. Efforts to rescue mutant embryos by growth on media enriched with hormones have been unsuccessful so far. However, mutant roots grow normally on medium supplemented with kinetin at a concentration that suppresses wild-type root elongation, suggesting possible involvement of the mutant in the reception or transduction of the kinetin signal or transport of the hormone. The shootless mutant appears to be a valuable tool with which to investigate the organization of the shoot meristem in monocots as well as a means to assay the origins and relationships between organs such as the scutellum, the coleoptile, and leaves that are initiated during the embryogenic process.