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.     

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.     

Flowering genes in Metrosideros fit a broad herbaceous model encompassing Arabidopsis and Antirrhinum

Molecular studies were conducted on Metrosideros excelsa to determine if the current genetic models for flowering with regard to inflorescence and floral meristem identity genes in annual plants were applicable to a woody perennial. MEL , MESAP1 and METFL1 , the fragments of LEAFY ( LFY ), APETALA1 ( AP1 ) and TERMINAL FLOWER1 ( TFL1 ) equivalents, respectively, were isolated from M. excelsa . Temporal expression patterns showed that MEL and MESAP1 exhibited a bimodal pattern of expression. Expression exhibited during early floral initiation in autumn was followed by down-regulation during winter, and up-regulation in spring as floral organogenesis occurred. Spatial expression patterns of MEL showed that it had greater similarity to FLORICAULA ( FLO ) than to LFY , whereas MESAP1 was more similar to AP1 than SQUAMOSA . The interaction between MEL and METFL1 was more similar to the interaction between FLO and CENTRORADIALIS than that between LFY and TFL1 . Consequently, the three genes from M. excelsa fit a broader herbaceous model encompassing Antirrhinum as well as Arabidopsis , but with differences, such as the bimodal pattern of expression seen with MEL and MESAP1 . In mid-winter, at the time when both MEL and MESAP1 were down-regulated, GA₁ was below the level of detection in M. excelsa buds. Even though application of gibberellin inhibits flowering in members of the Myrtaceae, MEL was responsive to gibberellin with expression in juvenile plants up-regulated by GA₃. However, MESAP1 was not up-regulated indicating that meristem competence was also probably required to promote flowering in M. excelsa .     

Cloning and characterization of a FLORICAULA/LEAFY ortholog, PFL, in polygamous papaya

The homologous genes FLORICAULA （FLO） in Antirrhinum and LEAFY （LFY） in Arabidopsis are known to regulate the initiation of flowering in these two distantly related plant species. These genes are necessary also for the expression of downstream genes that control floral organ identity. We used Arabidopsis LFY cDNA as a probe to clone and sequence a papaya ortholog of LFY, PFL. It encodes a protein that shares 61% identity with the Arabidopsis LFY gene and 71% identity with the LFY homologs of the two woody tree species： California sycamore （Platanus racemosa） and black cottonwood （Populus trichocarpa）. Despite the high sequence similarity within two conserved regions, the N-terminal proline-rich motif in papaya PFL differs from other members in the family. This difference may not affect the gene function of papaya PFL, since an equally divergent but a functional LFY ortholog NEEDLY of Pinus radiata has been reported. Genomic and BAC Southern analyses indicated that there is only one copy of PFL in the papaya genome. In situ hybridization experiments demonstrated that PFL is expressed at a relatively low level in leaf primordia, but it is expressed at a high level in the floral meristem. Quantitative PCR analyses revealed that PFL was expressed in flower buds of all three sex types - male, female, and hermaphrodite with marginal difference between hermaphrodite and unisexual flowers. These data suggest that PFL may play a similar role as LFY in flower development and has limited effect on sex differentiation in papaya.     

LEAFY同源基因研究进展

LEAFY(LFY)同源基因存在于所有的陆生植物中,在植物花发育早期表达,并在花发育过程中抑制茎端分生组织的营养生长,调控花分生组织和花器官的形成,使转LFY基因植株提前开花,LFY同源基因与其上下游基因共同调控花发育过程.LFY同源基因的蛋白质结构在不同物种间保守性很高,但它们的表达部位差异很大.该文总结了近年来国内外已经克隆到的LFY同源基因的表达、功能及其在果树、花卉、粮食作物上的应用,以期为植物花发育的深入研究提供参考.     

Heterochronic development of the floret meristem determines grain number per spikelet in diploid, tetraploid and hexaploid wheats

Background and Aims

The inflorescence of grass species such as wheat, rice and maize consists of a unique reproductive structure called the spikelet, which is comprised of one, a few, or several florets (individual flowers). When reproductive growth is initiated, the inflorescence meristem differentiates a spikelet meristem as a lateral branch; the spikelet meristem then produces a floret meristem as a lateral branch. Interestingly, in wheat, the number of fertile florets per spikelet is associated with ploidy level: one or two florets in diploid, two or three in tetraploid, and more than three in hexaploid wheats. The objective of this study was to identify the mechanisms that regulate the architecture of the inflorescence in wheat and its relationship to ploidy level.

Methods

The floral anatomy of diploid (Triticum monococcum), tetraploid (T. turgidum ssp. durum) and hexaploid (T. aestivum) wheat species were investigated by light and scanning electron microscopy to describe floret development and to clarify the timing of the initiation of the floret primordia. In situ hybridization analysis using Wknox1, a wheat knotted1 orthologue, was performed to determine the patterning of meristem formation in the inflorescence.

Key Results

The recessive natural mutation of tetraploid (T. turgidum ssp. turgidum) wheat, branching head (bh), which produces branched inflorescences, was used to demonstrate the utility of Wknox1 as a molecular marker for meristematic tissue. Then an analysis of Wknox1 expression was performed in diploid, tetraploid and hexaploid wheats and heterochronic development of the floret meristems was found among these wheat species.

Conclusions

It is shown that the difference in the number of floret primordia in diploid, tetraploid and hexaploid wheats is caused by the heterochronic initiation of floret meristem development from the spikelet meristem.Key words: Triticum, wheat, inflorescence, spikelet, floret, meristem, heterochrony, heterochronic development, knotted1, polyploidy     

ABERRANT PANICLE ORGANIZATION 2/RFL, the rice ortholog of Arabidopsis LEAFY, suppresses the transition from inflorescence meristem to floral meristem through interaction with APO1

The temporal and spatial control of meristem identity is a key element in plant development. To better understand the molecular mechanisms that regulate inflorescence and flower architecture, we characterized the rice aberrant panicle organization 2 (apo2) mutant which exhibits small panicles with reduced number of primary branches due to the precocious formation of spikelet meristems. The apo2 mutants also display a shortened plastochron in the vegetative phase, late flowering, aberrant floral organ identities and loss of floral meristem determinacy. Map-based cloning revealed that APO2 is identical to previously reported RFL gene, the rice ortholog of the Arabidopsis LEAFY (LFY) gene. Further analysis indicated that APO2/RFL and APO1, the rice ortholog of Arabidopsis UNUSUAL FLORAL ORGANS, act cooperatively to control inflorescence and flower development. The present study revealed functional differences between APO2/RFL and LFY. In particular, APO2/RFL and LFY act oppositely on inflorescence development. Therefore, the genetic mechanisms for controlling inflorescence architecture have evolutionarily diverged between rice (monocots) and Arabidopsis (eudicots).     

Inflorescence, spikelet, and floral development in Panicum maximum and Urochloa plantaginea (Poaceae)

Inflorescence development in Panicum maximum and Urochloa plantaginea was comparatively studied with scanning electron and light microscopy to test the transfer of P. maximum to Urochloa and to look for developmental features applicable to future cladistic studies of the phosphoenol pyruvate carboxykinase (PCK) subtype of C(4) photosynthesis clade (P. maximum and some species of Brachiaria, Chaetium, Eriochloa, Melinis, and Urochloa). Eleven developmental features not discernable in the mature inflorescence were found: direction of branch differentiation; origins of primary branches; apical vs. intercalary development of the main axis; direction of spikelet differentiation; direction of glume, lemma and palea differentiation; position of the lower glume (in some cases); size of the floret meristem; pattern of distal floret development; pattern of gynoecium abortion; differential pollen development between proximal and distal floret; and glume elongation. Inflorescence homologies between P. maximum and U. plantaginea are also clarified. Panicum maximum and U. plantaginea differ not only in their mature inflorescence structure but also in eight fundamental developmental features that exclude P. maximum from Urochloa. The following developmental events are related to sex expression: size of floret meristem, gynoecium abortion, pollen development delay in the proximal floret, glume elongation and basipetal floret maturation at anthesis.     

Isolation and expression analysis of a LEAFY/FLORICAULA homolog and its promoter from London plane (Platanus acerifolia Willd.)

The LEAFY/FLORICAULA (LFY/FLO) homologous genes are necessary for normal flower development in diverse angiosperm species. To understand the genetic and molecular mechanisms underlying floral initiation and development in Platanaceae, an early divergent eudicot family consisting of large monoecious trees, we isolated a homolog of LFY/FLO, PlacLFY, and its promoter from London plane (Platanus acerifolia). PlacLFY is 1,419?bp in length, with an ORF of 1,122?bp encoding a predicted polypeptide of 374 amino acids and 5'/3'-UTR of 54 and 213?bp, respectively. The putative PlacLFY protein showed a high degree of identity (56-84?%) with LFY/FLO homologs from other species, including two highly conserved regions, the N and C domains, and a less conserved amino-terminal proline-rich region. Real-time PCR analysis showed that PlacLFY was expressed mainly in male inflorescences from May of the first year to March of next year, with the highest expression level in December, and in female inflorescences from June to April of next year. PlacLFY mRNA was also detected strongly in subpetiolar buds of December from 4-year-old and adult trees, and slightly in stem of young seedling and young leaf of adult plant. Additionally, we cloned 1,138?bp promoter sequence of PlacLFY and we drove GUS expression in transgenic tobacco by the chimerical pPlacLFY::GUS construction. Histological GUS staining analysis indicated that PlacLFY promoter can drive GUS gene expression in shoot apex, stem, young leaf and petiole, flower stalk, petal tip, and young/semi-mature fruits of transgenic tobacco, which is almost identical to the expression pattern of PlacLFY in London plane. The results revealed that the PlacLFY gene isolated from London plane is expressed not only in reproductive organ but also in vegetative organs. Moreover, this expression pattern is consistent with the expression pattern in tobacco of a GUS reporter gene under the control of the potential promoter region of PlacLFY.     

Interactions among APETALA1, LEAFY, and TERMINAL FLOWER1 specify meristem fate.

Upon floral induction, the primary shoot meristem of an Arabidopsis plant begins to produce flower meristems rather than leaf primordia on its flanks. Assignment of floral fate to lateral meristems is primarily due to the cooperative activity of the flower meristem identity genes LEAFY (LFY), APETALA1 (AP1), and CAULIFLOWER. We present evidence here that AP1 expression in lateral meristems is activated by at least two independent pathways, one of which is regulated by LFY. In lfy mutants, the onset of AP1 expression is delayed, indicating that LFY is formally a positive regulator of AP1. We have found that AP1, in turn, can positively regulate LFY, because LFY is expressed prematurely in the converted floral meristems of plants constitutively expressing AP1. Shoot meristems maintain an identity distinct from that of flower meristems, in part through the action of genes such as TERMINAL FLOWER1 (TFL1), which bars AP1 and LFY expression from the influorescence shoot meristem. We show here that this negative regulation can be mutual because TFL1 expression is downregulated in plants constitutively expressing AP1. Therefore, the normally sharp phase transition between the production of leaves with associated shoots and formation of the flowers, which occurs upon floral induction, is promoted by positive feedback interactions between LFY and AP1, together with negative interactions of these two genes with TFL1.     

Autophagy regulated by day length determines the number of fertile florets in wheat

SUMMARY: The wheat spikelet meristem differentiates into up to 12 floret primordia, but many of them fail to reach the fertile floret stage at anthesis. We combined microarray, biochemical and anatomical studies to investigate floret development in wheat plants grown in the field under short or long days (short days extended with low-fluence light) after all the spikelets had already differentiated. Long days accelerated spike and floret development and greening, and the expression of genes involved in photosynthesis, photoprotection and carbohydrate metabolism. These changes started while the spike was in the light-depleted environment created by the surrounding leaf sheaths. Cell division ceased in the tissues of distal florets, which interrupted their normal developmental progression and initiated autophagy, thus decreasing the number of fertile florets at anthesis. A massive decrease in the expression of genes involved in cell proliferation, a decrease in soluble carbohydrate levels, and an increase in the expression of genes involved in programmed cell death accompanied anatomical signs of cell death, and these effects were stronger under long days. We propose a model in which developmentally generated sugar starvation triggers floret autophagy, and long days intensify these processes due to the increased carbohydrate consumption caused by the accelerated plant development.     

Genetic Interactions That Regulate Inflorescence Development in Arabidopsis

In Arabidopsis, floral meristems arise in continuous succession directly on the flanks of the inflorescence meristem. Thus, the pathways that regulate inflorescence and floral meristem identity must operate both simultaneously and in close spatial proximity. The TERMINAL FLOWER 1 (TFL1) gene of Arabidopsis is required for normal inflorescence meristem function, and the LEAFY (LFY), APETALA 1 (AP1), and APETALA 2 (AP2) genes are required for normal floral meristem function. We present evidence that inflorescence meristem identity is promoted by TFL1 and that floral meristem identity is promoted by parallel developmental pathways, one defined by LFY and the other defined by AP1/AP2. Our analysis suggests that the acquisition of meristem identity during inflorescence development is mediated by antagonistic interactions between TFL1 and LFY and between TFL1 and AP1/AP2. Based on this study, we propose a simple model for the genetic regulation of inflorescence development in Arabidopsis. This model is discussed in relation to the proposed interactions between the inflorescence and the floral meristem identity genes and in regard to other genes that are likely to be part of the genetic hierarchy regulating the establishment and maintenance of inflorescence and floral meristems.