Arabidopsis genes AS1, AS2, and JAG negatively regulate boundary-specifying genes to promote sepal and petal development

Boundary formation is crucial for organ development in multicellular eukaryotes. In higher plants, boundaries that separate the organ primordia from their surroundings have relatively low rates of cell proliferation. This cellular feature is regulated by the actions of certain boundary-specifying genes, whose ectopic expression in organs can cause inhibition of organ growth. Here, we show that the Arabidopsis thaliana ASYMMETRIC LEAVES1 and 2 (AS1 and AS2) and JAGGED (JAG) genes function in the sepal and petal primordia to repress boundary-specifying genes for normal development of the organs. Loss-of-function as1 jag and as2 jag double mutants produced extremely tiny sepals and petals. Analysis of a cell-cycle marker HISTONE4 revealed that cell division in sepal primordia of the double mutant was inhibited. Moreover, these abnormal sepals and petals exhibited ectopic overexpression of the boundary-specifying genes PETAL LOSS (PTL) and CUP-SHAPED COTYLEDON1 [corrected] and 2 (CUC1 and CUC2). Loss of PTL or CUC1 and CUC2 functions in the as1 jag background could partially rescue the tiny sepal and petal phenotypes, supporting the model that the tiny sepal/petal phenotypes are caused, at least in part, by ectopic expression of boundary-specifying genes. Together, our data reveal a previously unrecognized fundamental regulation by which AS1, AS2, and JAG act to define sepal and petal from their boundaries.     

Initiation of axillary and floral meristems in Arabidopsis

Shoot development is reiterative: shoot apical meristems (SAMs) give rise to branches made of repeating leaf and stem units with new SAMs in turn formed in the axils of the leaves. Thus, new axes of growth are established on preexisting axes. Here we describe the formation of axillary meristems and floral meristems in Arabidopsis by monitoring the expression of the SHOOT MERISTEMLESS and AINTEGUMENTA genes. Expression of these genes is associated with SAMs and organ primordia, respectively. Four stages of axillary meristem development and previously undefined substages of floral meristem development are described. We find parallels between the development of axillary meristems and the development of floral meristems. Although Arabidopsis flowers develop in the apparent absence of a subtending leaf, the expression patterns of AINTEGUMENTA and SHOOT MERISTEMLESS RNAs during flower development suggest the presence of a highly reduced, "cryptic" leaf subtending the flower in Arabidopsis. We hypothesize that the STM-negative region that develops on the flanks of the inflorescence meristem is a bract primordium and that the floral meristem proper develops in the "axil" of this bract primordium. The bract primordium, although initially specified, becomes repressed in its growth.     

棉花MADS框蛋白基因(GhMADS1)的克隆

作为转录因子,MADS框蛋白基因在植物花器官发育中有着重要的功能。为研究棉花花器官发育的分子机理,以棉花花器官突变体CHV1(cotton homeotic variant)和徐州142正常植株为材料,利用棉花EST数据库资料,通过EST序列整合,从陆地棉徐州142花蕾中克隆出一个MADS框蛋白的编码区段,GenBank登录号为AF538965。该片段(GhMADS1)长713bp,包含一个711bp的开放阅读框,推导的氨基酸序列(236个氨基酸)与葡萄、烟草、矮牵牛、拟南芥和金鱼草等的AGL2组MADS框蛋白有很高的序列相似性。系统进化分析同样将GhMADS1基因归人AGt2组MADS框蛋白。RT-PCR分析显示,该基因在陆地棉的花瓣、雄蕊、胚珠和纤维中表达,特别是在花瓣中表达量最高,而在根、茎、叶等营养器官和棉花同源异型突变体CHV1(所有花器官均变为苞叶状叶性器官)的变异花蕾中不表达。这些结果说明GhMADS1基因可能在棉花花器官发育中有着重要的功能。     

Isolation and characterization of a Brassica napus cDNA corresponding to a B-class floral development gene

B-class floral homeotic genes are required for the proper formation and identity of petals and stamens in dicot flowers. A partial cDNA clone encoding a B-class gene, BnAP3 (Brassica napus APETALA3), was isolated from a B. napus cDNA library derived from young inflorescence meristems. The 5' region of the cDNA was retrieved by RACE. The deduced amino acid sequence of the full-length clone exhibited high similarity to APETALA3 of Arabidopsis thaliana and functionally homologous proteins from other species. 5' RACE and Southern analysis suggests that BnAP3 has multiple alleles in B. napus. Expression analysis assayed by RT-PCR shows that BnAP3 is expressed in floral tissues, as well as non-floral tissues such as root and bract. Transformation of wild-type A. thaliana and B. napus plants with BnAP3 under the control of a promoter specific to reproductive organs converts carpels to stamens, while the expression of this construct in A. thaliana plants mutant for AP3 restores the development of third-whorl stamens in addition to directing a carpel to stamen conversion in the fourth whorl.     

Conversion of perianth into reproductive organs by ectopic expression of the tobacco floral homeotic gene NAG1.

Mutations in the AGAMOUS (AG) gene of Arabidopsis thaliana result in the conversion of reproductive organs, stamens and carpels, into perianth organs, sepals and petals. We have isolated and characterized the putative AG gene from Nicotiana tabacum, NAG1, whose deduced protein product shares 73% identical amino acid residues with the Arabidopsis AG gene product. RNA tissue in situ hybridizations show that NAG1 RNA accumulates early in tobacco flower development in the region of the floral meristem that will later give rise to stamens and carpels. Ectopic expression of NAG1 in transgenic tobacco plants results in a conversion of sepals and petals into carpels and stamens, respectively, indicating that NAG1 is sufficient to convert perianth into reproductive floral organs.     

The role of gene tepal-like bract (TLB) in the separation between the bracts and perianth in Fagopyrum esculentum moench

The morphological and genetic studies of the tlb mutant of common buckwheat Fagopyrum esculentum from the collection of the All-Russia Research Institute of Legumes and Groat Crops have demonstrated that gene TLB controls an important stage of flower development-it determines the lower boundary of simple perianths. The loss of TLB gene function leads to changes in the structure of the bract from scale-like to tepal-like. Gene TLB is assumed to limit, on the basal side, the region of the expression of genes determining the development of flower organs as petals.     

FRL1 is required for petal and sepal development in Arabidopsis

A novel flower mutant, frl1 (frill 1) was isolated in Arabidopsis thaliana. The frl1 mutant has serrated petals and sepals but the other floral and vegetative organs appear to be normal. To analyse the role of the FRL1 gene, morphological, cytological and double mutant analyses were carried out. The frl1 flower had broader petals and sepals as compared with the wild-type. The distal region of frl1 petals contained fewer epidermal cells but their size was variable and generally larger than that in the wild-type. However, no significant difference was found in the basal region. Observations of the early petal development revealed that the morphology of the developing frl1 petal was normal until the middle of stage 9, but the frl1 phenotype became apparent in stages later than 10. Furthermore, larger nuclei with varied sizes were observed in the distal region of frl1 petals, but not in this region in wild-type petals. This strongly suggests that abnormal endo-reduplication had occurred. These observations indicate that the frl1 mutation affects the number of cell divisions and the subsequent cell expansion during the late stage of petal lamina formation, and that FRL1 might be maintaining the mitotic state or suppressing the transition to the endo-reduplication cycle. Double mutants with the homeotic mutants apetala3-1 and agamous showed additive phenotypes. Ectopic petals in the third whorl of fr11 ag flowers were serrated, indicating that the FRL1 gene acts in petal and sepal development in an organ-specific manner.     

The Arabidopsis floral homeotic gene PISTILLATA is regulated by discrete cis-elements responsive to induction and maintenance signals

PISTILLATA is a B-class floral organ identity gene required for the normal development of petals and stamens in Arabidopsis. PISTILLATA expression is induced in the stage 3 flowers (early expression) and is maintained until anthesis (late expression). To explore in more detail the developmentally regulated gene expression of PISTILLATA, we have analyzed the PISTILLATA promoter using uidA (beta)-glucuronidase gene) fusion constructs (PI::GUS) in transgenic Arabidopsis. Promoter deletion analyses suggest that early PISTILLATA expression is mediated by the distal region and that late expression is mediated by the proximal region. Based on the PI::GUS expression patterns in the loss- and gain-of-function alleles of meristem or organ identity genes, we have shown that LEAFY and UNUSUAL FLORAL ORGANS induce PISTILLATA expression in a flower-independent manner via a distal promoter, and that PISTILLATA and APETALA3 maintain PISTILLATA expression (autoregulation) in the later stages of flower development via a proximal promoter. In addition, we have demonstrated that de novo protein synthesis is required for the PISTILLATA autoregulatory circuit.     

The lateral organ boundaries gene defines a novel,plant-specific gene family

The LATERAL ORGAN BOUNDARIES (LOB) gene in Arabidopsis defines a new conserved protein domain. LOB is expressed in a band of cells at the adaxial base of all lateral organs formed from the shoot apical meristem and at the base of lateral roots. LOB encodes a predicted protein that does not have recognizable functional motifs, but that contains a conserved domain (the LOB domain) that is present in 42 other Arabidopsis proteins and in proteins from a variety of other plant species. Proteins showing similarity to the LOB domain were not found outside of plant databases, indicating that this unique protein may play a role in plant-specific processes. Genes encoding LOB domain proteins are expressed in a variety of temporal- and tissue-specific patterns, suggesting that they may function in diverse processes. Loss-of-function LOB mutants have no detectable phenotype under standard growth conditions, suggesting that LOB is functionally redundant or required during growth under specific environmental conditions. Ectopic expression of LOB leads to alterations in the size and shape of leaves and floral organs and causes male and female sterility. The expression of LOB at the base of lateral organs suggests a potential role for LOB in lateral organ development.     

Novel receptor-like kinase ALE2 controls shoot development by specifying epidermis in Arabidopsis

The epidermis plays crucial roles in the development of various organs and in water retention in both animals and plants. In Arabidopsis thaliana, the subtilase ABNORMAL LEAF SHAPE 1 (ALE1) and the Arabidopsis homolog of the Crinkly4 (ACR4) receptor-like protein kinase (RLK) have been implicated in the intercellular communication that is required for surface functions of the epidermis. We have identified a novel mutant gene in Arabidopsis, ale2, which is associated with various epidermal defects, including disorganization of epidermis-related tissues, defects in the leaf cuticle and the fusion of organs. ALE2 encodes a previously uncharacterized RLK with a cluster of basic amino acid residues followed by a cysteine-containing sequence in the putative extracellular domain. Our genetic investigations suggest that ALE2 and ACR4 function in the same process, whereas ALE1 has a different mode of action, and that these three genes play partially overlapping roles in positively regulating protoderm-specific gene expression and for the formation of leafy organs. We propose that at least two modes of intercellular communication facilitate the specification of epidermis, thereby promoting shoot organogenesis in Arabidopsis.     

Arabidopsis thaliana FIMBRIATA PETIOLES gene, controlling cell division and growth in floral organs

A new mutant, fimbriata petioles (fip), of Arabidopsis thaliana was obtained by chemical mutagenesis. The mutant is characterized by unusual anomalies of floral organs. Clusters of very large cells formed in the distal region of sepals and petals, which created fringed edges of these organs. An analysis of the morphology of the floral organs and leaves of the fip as 1 double mutant revealed a complementary interaction of the ASYMMETRIC LEAVES1 (AS1) and FIMBRIATA PETIOLES (FIP) genes. It was assumed that the FIP gene, together with the AS1 gene, controls cell proliferation, preventing their premature entry into endocycle.