AGL24 acts in concert with SOC1 and FUL during Arabidopsis floral transition

Arabidopsis plants flower in response to long days (LDs). Exposure of leaves to inductive day lengths activates expression of FLOWERING LOCUS T (FT) protein which moves to the shoot apical meristem (SAM) to induce developmental reprogramming. SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) and FRUITFULL (FUL) are induced by FT at the apex. We previously screened the SAM for mRNAs of genes required to promote the floral transition in response to photoperiod, and conducted detailed expression and functional analyses on several putative candidates. Here, we show that expression of AGAMOUS-LIKE 24 (AGL24) is detected at the SAM under SD conditions and increases upon exposure to LDs. Mutations in AGL24 further delay flowering of a soc1 ful double mutant, suggesting that flowering is controlled by AGL24 partly independently of SOC1 and FUL.     

Conservation and divergence of APETALA1/FRUITFULL-like gene function in grasses: evidence from gene expression analyses

Duplicated APETALA1/FRUITFULL (AP1/FUL) genes show distinct but overlapping patterns of expression within rice (Oryza sativa) and within ryegrass (Lolium temulentum), suggesting discrete functional roles in the transition to flowering, specification of spikelet meristem identity, and specification of floral organ identity. In this study, we analyzed the expression of the AP1/FUL paralogues FUL1 and FUL2 across phylogenetically disparate grasses to test hypotheses of gene function. In combination with other studies, our data support similar roles for both genes in spikelet meristem identity, a general role for FUL1 in floral organ identity, and a more specific role for FUL2 in outer floral whorl identity. In contrast to Arabidopsis AP1/FUL genes, expression of FUL1 and FUL2 is consistent with an early role in the transition to flowering. In general, FUL1 has a wider expression pattern in all spikelet organs than FUL2, but both genes are expressed in all spikelet organs in some cereals. FUL1 and FUL2 appear to have multiple redundant functions in early inflorescence development. We hypothesize that sub-functionalization of FUL2 and interaction of FUL2 with LHS1 could specify lemma and palea identity in the grass floret.     

Bi-directional inflorescence development inArabidopsis thaliana: Acropetal initiation of flowers and basipetal initiation of paraclades

In this study we investigated Arabidopsis thaliana (L.) Heynh. inflorescence development by characterizing morphological changes at the shoot apex during the transition to flowering. Sixteen-hour photoperiods were used to synchronously induce flowering in vegetative plants grown for 30 d in non-inductive 8-h photoperiods. During the first inductive cycle, the shoot apical meristem ceased producing leaf primordia and began to produce flower primordia. The differentiation of paraclades (axillary flowering shoots), however, did not occur until after the initiation of multiple flower primordia from the shoot apical meristem. Paraclades were produced by the basipetal activation of buds from the axils of leaf primordia which had been initiated prior to photoperiodic induction. Concurrent with the activation of paraclades was the partial suppression of paraclade-associated leaf primordia, which became bract leaves. The suppression of bract-leaf primordia and the abrupt initiation of flower primordia during the first inductive photoperiod is indicative of a single phase change during the transition to flowering in photoperiodically induced Arabidopsis. Morphogenetic changes characteristic of the transition to flowering in plants grown continuously in 16-h photoperiods were qualitatively equivalent to the changes observed in plants which were photoperiodically induced after 30 d. These results suggest that Arabidopsis has only two phases of development, a vegetative phase and a reproductive phase; and that the production of flower primordia, the differentiation of paraclades from the axils of pre-existing leaf primordia and the elongation of internodes all occur during the reproductive phase.     

植物MADS-box 基因FRUITFULL(FUL)研究进展

植物MADS-box基因家族的不同成员在植物生长发育过程中起着非常重要的作用。拟南芥MADS-box 基因FRUITFULL(FUL) 在控制拟南芥开花时间、花分生组织分化、茎生叶形态以及心皮和果实的发育中起到重要作用。其他植物中,FUL的同源基因也在调控花发育,果实发育以及叶片发育等方面各自起到重要作用。本文综述了FUL基因及其同源基因的表达模式和功能,并就其在农作物及果树育种上的潜在应用价值进行了讨论。     

The quest for florigen: a review of recent progress

The photoperiodic induction of flowering is a systemic process requiring translocation of a floral stimulus from the leaves to the shoot apical meristem. In response to this stimulus, the apical meristem stops producing leaves to initiate floral development; this switch in morphogenesis involves a change in the identity of the primordia initiated and in phyllotaxis. The physiological study of the floral transition has led to the identification of several putative floral signals such as sucrose, cytokinins, gibberellins, and reduced N-compounds that are translocated in the phloem sap from leaves to the shoot apical meristem. On the other hand, the genetic approach developed more recently in Arabidopsis thaliana allowed the discovery of many genes that control flowering time. These genes function in 'cascades' within four promotive pathways, the 'photoperiodic', 'autonomous', 'vernalization', and 'gibberellin' pathways, which all converge on the 'integrator' genes SUPPRESSOR OF OVEREXPRESSION OF CO 1 (SOC1) and FLOWERING LOCUS T (FT). Recently, several studies have highlighted a role for a product of FT as a component of the floral stimulus or 'florigen'. These recent advances and the proposed mode of action of FT are discussed here.     

Antagonistic roles of SEPALLATA3, FT and FLC genes as targets of the polycomb group gene CURLY LEAF

In Arabidopsis, mutations in the Pc-G gene CURLY LEAF (CLF) give early flowering plants with curled leaves. This phenotype is caused by mis-expression of the floral homeotic gene AGAMOUS (AG) in leaves, so that ag mutations largely suppress the clf phenotype. Here, we identify three mutations that suppress clf despite maintaining high AG expression. We show that the suppressors correspond to mutations in FPA and FT, two genes promoting flowering, and in SEPALLATA3 (SEP3) which encodes a co-factor for AG protein. The suppression of the clf phenotype is correlated with low SEP3 expression in all case and reveals that SEP3 has a role in promoting flowering in addition to its role in controlling floral organ identity. Genetic analysis of clf ft mutants indicates that CLF promotes flowering by reducing expression of FLC, a repressor of flowering. We conclude that SEP3 is the key target mediating the clf phenotype, and that the antagonistic effects of CLF target genes masks a role for CLF in promoting flowering.     

Gibberellin-induced reorganization of spatial relationships of emerging leaf primordia at the shoot apical meristem in Hedera helix L.

The transition from spiral to distichous leaf arrangement during gibberellic-acid (GA₃)-induced rejuvenation in Hedera was studied in detail by scanning electron microscopy of the shoot apical meristem. The transition, which involves the initiation of about 14 new leaf primordia, is accomplished by progressive increments in the divergence angle between the leaf primordia from an initial average value of 138.9 ° until it approaches 180 °. This process is preceded, as well as accompanied, by an increased radial displacement of young leaf primordia away from the apical meristem. Although the width of the leaf primordia also increases, this is unlikely to be a causal factor since it occurs only late in the transition. The size of the primordium-free area of the apical meristem is also unlikely to be involved. Quantitative analysis shows that the divergence angle of consecutive leaf primordia commonly fluctuates between relatively large and small values. Thus the transitional stages form a spirodistichous arrangement in which the divergence angle within each pair of leaves is large relative to that between leaf pairs. The stimulation of the radial displacement of the leaf primordia and the associated phyllotactic transition may involve GA₃-induced modification in the spatial organization of cortical microtubules in the apical meristem and related changes in directional cell expansion.Abbreviations DA divergence angle - GA₃ gibberellic acid We thank Mr. Gilbert Ahlstrand for his advice regarding scanning electron microscopy. This paper is contribution of the University of Minnesota Agricultural Experimental Station No. 18,726.     

Activation of Basal Cells of the Apical Meristem during Sepal Formation in Tomato

Although great advances have been made in research on the regulation of primordium fate in the floral meristem, our understanding of the molecular events occurring during the floral transition remains incomplete. Via a careful analysis of the expression patterns of five genes encoding housekeeping functions during the floral transition in tomato (using both in situ hybridization and enzyme histochemistry), we identified a particular phase of floral development (sepal initiation) at which cells located toward the base of the meristem show a high level of cellular metabolism, whereas cells at the tip of the meristem dome show little activity. At other stages of floral development, the probes used showed genespecific patterns of expression generally consistent with our previous investigation of the vegetative apical meristem. Our data, in conjunction with other reports in the literature, enabled us to postulate that relative activation of basal cells of the meristem may be of general occurrence during the transition to flowering. Such a hypothesis could account for recent observations using periclinal chimeras on the effect of L3 genotypes on flower development and have a bearing on the expected mechanism by which the number of primordia generated by a floral meristem is regulated.     

Dual effects of miR156-targeted SPL genes and CYP78A5/KLUH on plastochron length and organ size in Arabidopsis thaliana

Leaves of flowering plants are produced from the shoot apical meristem at regular intervals, with the time that elapses between the formation of two successive leaf primordia defining the plastochron. We have identified two genetic axes affecting plastochron length in Arabidopsis thaliana. One involves microRNA156 (miR156), which targets a series of SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) genes. In situ hybridization studies and misexpression experiments demonstrate that miR156 is a quantitative, rather than spatial, modulator of SPL expression in leaf primordia and that SPL activity nonautonomously inhibits initiation of new leaves at the shoot apical meristem. The second axis is exemplified by a redundantly acting pair of cytochrome P450 genes, CYP78A5/KLUH and CYP78A7, which are likely orthologs of PLASTOCHRON1 of rice (Oryza sativa). Inactivation of CYP78A5, which is expressed at the periphery of the shoot apical meristem, accelerates the leaf initiation rate, whereas cyp78a5 cyp78a7 double mutants often die as embryos with supernumerary cotyledon primordia. The effects of both miR156-targeted SPL genes and CYP78A5 on organ size are correlated with changes in plastochron length, suggesting a potential compensatory mechanism that links the rate at which leaves are produced to final leaf size.     

Inflorescence meristem identity in rice is specified by overlapping functions of three AP1/FUL-like MADS box genes and PAP2, a SEPALLATA MADS box gene

In plants, the transition to reproductive growth is of particular importance for successful seed production. Transformation of the shoot apical meristem (SAM) to the inflorescence meristem (IM) is the crucial first step in this transition. Using laser microdissection and microarrays, we found that expression of PANICLE PHYTOMER2 (PAP2) and three APETALA1 (AP1)/FRUITFULL (FUL)-like genes (MADS14, MADS15, and MADS18) is induced in the SAM during meristem phase transition in rice (Oryza sativa). PAP2 is a MADS box gene belonging to a grass-specific subclade of the SEPALLATA subfamily. Suppression of these three AP1/FUL-like genes by RNA interference caused a slight delay in reproductive transition. Further depletion of PAP2 function from these triple knockdown plants inhibited the transition of the meristem to the IM. In the quadruple knockdown lines, the meristem continued to generate leaves, rather than becoming an IM. Consequently, multiple shoots were formed instead of an inflorescence. PAP2 physically interacts with MAD14 and MADS15 in vivo. Furthermore, the precocious flowering phenotype caused by the overexpression of Hd3a, a rice florigen gene, was weakened in pap2-1 mutants. Based on these results, we propose that PAP2 and the three AP1/FUL-like genes coordinately act in the meristem to specify the identity of the IM downstream of the florigen signal.