Analysis of the APETALA3- and PISTILLATA-like genes in Hedyosmum orientale (Chloranthaceae) provides insight into the evolution of the floral homeotic B-function in angiosperms

Functional analysis of a PISTILLATA-like gene CcMADS20 involved in floral organs specification in citrus

PISTILLATA (PI), as a member of MADS-box transcription factor, plays an important role in petal and stamen specification in Arabidopsis. However, little is known about PI-like genes in citrus. To understand the molecular mechanism of PI during the developmental process of citrus flower, a PI-like gene CcMADS20 was isolated from Citrus Clemantina. Sequence alignment and phylogenetic analysis revealed that CcMADS20 had relatively high similarity with PI-like homolog and was classified in the core dicotyledonous group. The temporal and spatial expression analyses showed that CcMADS20 was specifically expressed in petal and stamen of citrus flower, which was consistent with PI expression pattern in Arabidopsis. Protein interaction revealed that CcMADS20 could form heterodimer with AP3-like proteins. Furthermore, ectopic overexpression of CcMADS20 in Arabidopsis resulted in transformation of sepals into petal-like structure, as observed in other plants overexpressing a functional PI-like homolog. Additionally, promoter fragments of CcMADS20 were also cloned in the representative 21 citrus varieties. Interestingly, four types of promoters were discovered in these citrus varieties, resulting from two stable insert/deletion fragments (Locus1 and Locus2). The homo/hetero-zygosity of promoter alleles in each variety was strongly related to the evolutionary origin of citrus. Four promoters activity analysis indicated that Locus1 presence inhibited CcMADS20 transcriptional activity and Locus2 presence promoted its transcriptional activity. These findings suggested that CcMADS20 determines petal and stamen development during the evolutionary process of citrus and four promoters discovered, as effective genetic markers, are valuable for citrus breeding practices.

Functional identification of the different regions in B-class floral homeotic MADS-box proteins IiAP3 and IiPI from Isatis indigotica

APETALA3 (AP3) and PISTILLATA (PI) are B-class MADS-box floral homeotic genes of Arabidopsis and are involved in specifying the identity of petals and stamens. In the present work, IiAP3 and IiPI, the respective orthologous genes of AP3 and PI, were cloned from Isatis indigotica. By expressing in ap3-6 and pi-1 homozygous mutant and in wild-type Arabidopsis under the control of AP3 promoter or CaMV 35S promoter, we demonstrated that IiAP3 and IiPI were functionally equivalent to AP3 and PI of Arabidopsis. Referring to previous reports and the research results in the present work, expression patterns of AP3 and PI homologs are not the same in different angiosperms possessing diverse floral structures. It suggests that the alterations in expression may contribute to the changing morphology of flowers. To further determine the relationship between IiAP3 and IiPI, the coding sequences of the different structural regions in these two proteins were swapped with each other, and the data collected from transgenic Arabidopsis plants of the chimeric constructs suggested that MADS domain was irreplaceable for the function of IiAP3, K domain of IiAP3 was involved in specifying the identity of stamens, K domain of IiPI was mainly related to the formation of petals, and C-terminal region of IiPI was involved in characterization of stamens. In addition, a complete KC region of these two proteins was more effective in phenotypic complementation of the mutants.

Characterization of PISTILLATA-like Genes and Their Promoters from the Distyly Fagopyrum esculentum

Arabidopsis PISTILLATA (PI) encodes B-class MADS-box transcription factor (TF), and works together with APETALA3 (AP3) to specify petal and stamen identity. However, a small-scale gene duplication event of PI ortholog was observed in common buckwheat and resulted in FaesPI_1 and FaesPI_2. FaesPI_1 and FaesPI_2 were expressed only in the stamen of dimorphic flower (thrum and pin) of Fagopyrum esculentum. Moreover, intense beta-glucuronidase (GUS) staining was found in the entire stamen (filament and anther) in pFaesPI_1::GUS transgenic Arabidopsis, while GUS was expressed only in the filament of pFaesPI_2::GUS transgenic Arabidopsis. In addition, phenotype complementation analysis suggested that pFaesPI_1::FaesPI_1/pFaesPI_2::FaesPI_2 transgenic pi-1 Arabidopsis showed similar a flower structure with stamen-like organs or filament-like organs in the third whorl. This suggested that FaesPI_2 only specified filament development, but FaesPI_1 specified stamen development. Meanwhile, FaesPI_1 and FaesPI_2 were shown to function redundantly in regulating filament development, and both genes work together to require a proper stamen identity. The data also provide a clue to understanding the roles of PI-like genes involved in floral organ development during the early evolution of core eudicots and also suggested that FaesPI_1 and FaesPI_2 hold the potential application in bioengineering to develop a common buckwheat male sterile line.

De novo transcriptome assembly and comparative analysis between male and benzyladenine-induced female inflorescence buds of Plukenetia volubilis

Plukenetia volubilis is a promising oilseed crop due to its seeds being rich in unsaturated fatty acids, especially alpha-linolenic acid. P. volubilis is monoecious, with separate male and female flowers on the same inflorescence. We previously reported that male flowers were converted to female flowers by exogenous cytokinin (6-benzyladenine, 6-BA) treatment in P. volubilis. To identify candidate genes associated with floral sex differentiation of P. volubilis, we performed de novo transcriptome assembly and comparative analysis on control male inflorescence buds (MIB) and female inflorescence buds (FIB) induced by 6-BA using Illumina sequencing technology. A total of 57,664 unigenes with an average length of 979 bp were assembled from 104.1 million clean reads, and 45,235 (78.45%) unigenes were successfully annotated in the public databases. Notably, Gene Ontology analyses revealed that 4193 and 3880 unigenes were enriched in the categories of reproduction and reproductive processes, respectively. Differential expression analysis identified 1385 differentially expressed unigenes between MIB and FIB, of which six unigenes related to cytokinin and auxin signaling pathways and 16 important transcription factor (TF) genes including MADS-box family members were identified. In particular, several unigenes encoding important TFs, such as homologs of CRABS CLAW, RADIALIS-like 1, RADIALIS-like 2, HECATE 2, WUSCHEL-related homeobox 9, and SUPERMAN, were expressed at higher levels in FIB than in MIB. The expression patterns of the 36 selected unigenes revealed by transcriptome analysis were successfully validated by quantitative real-time PCR. This study not only provides comprehensive gene expression profiles of P. volubilis inflorescence buds, but also lays the foundation for research on the molecular mechanism of floral sex determination in P. volubilis and other monoecious plants.

The ABCs of flower development: mutational analysis of AP1/FUL-like genes in rice provides evidence for a homeotic (A)-function in grasses

The well-known ABC model describes the combinatorial interaction of homeotic genes in specifying floral organ identities. While the B- and C-functions are highly conserved throughout flowering plants and even in gymnosperms, the A-function, which specifies the identity of perianth organs (sepals and petals in eudicots), remains controversial. One reason for this is that in most plants that have been investigated thus far, with Arabidopsis being a remarkable exception, one does not find recessive mutants in which the identity of both types of perianth organs is affected. Here we report a comprehensive mutational analysis of all four members of the AP1/FUL-like subfamily of MADS-box genes in rice (Oryza sativa). We demonstrate that OsMADS14 and OsMADS15, in addition to their function of specifying meristem identity, are also required to specify palea and lodicule identities. Because these two grass-specific organs are very likely homologous to sepals and petals of eudicots, respectively, we conclude that there is a floral homeotic (A)-function in rice as defined previously. Together with other recent findings, our data suggest that AP1/FUL-like genes were independently recruited to fulfil the (A)-function in grasses and some eudicots, even though other scenarios cannot be excluded and are discussed.

The C-Terminal Sequence and PI motif of the Orchid (Oncidium Gower Ramsey) PISTILLATA (PI) Ortholog Determine its Ability to Bind AP3 Orthologs and Enter the Nucleus to Regulate Downstream Genes Controlling Petal and Stamen Formation

This study focused on the investigation of the effects of the PI motif and C-terminus of the Oncidium Gower Ramsey MADS box gene 8 (OMADS8), a PISTILLATA (PI) ortholog, on floral organ formation. 35S::OMADS8 completely rescued and 35S::OMADS8-PI (with the PI motif deleted) partially rescued petal/stamen formation, whereas these deficiencies were not rescued by 35S::OMADS8-C (C-terminal 29 amino acids deleted) in pi-1 mutants. OMADS8 could interact with Arabidopsis APETALA3 (AP3) and enter the nucleus. The nuclear entry efficiency was reduced for OMADS8-PI/AP3 and OMADS8-C/AP3. OMADS8 could also interact with OMADS5/OMADS9 (the Oncidium AP3 ortholog) and enter the nucleus with an efficiency only slightly affected by the deletion of the C-terminal sequence or PI motif. However, the stability of the OMADS8/OMADS5 and OMADS8/OMADS9 complexes was significantly reduced by deletion of the C-terminal sequence or PI motif. Further analysis indicated that the expression of genes downstream of AP3/PI (BNQ1/BNQ2/GNC/At4g30270) was compensated by 35S::OMADS8 and 35S::OMADS8-PI to a level similar to wild-type plants but was not affected by 35S::OMADS8-C in the pi-1 mutants. A similar FRET (fluorescence resonance energy transfer) efficiency was observed for Arabidopsis AGAMOUS (AG) and the Oncidium AG ortholog OMADS4 for OMADS8, OMADS8-PI and OMADS8-C. These results indicated that the OMADS8 PI motif and C-terminus were valuable for the interaction of OMADS8 with the AP3 orthologs to form higher order heterotetrameric complexes that regulated petal/stamen formation in both Oncidium orchids and transgenic Arabidopsis. However, the C-terminal sequence and PI motif were dispensable for the interaction of OMADS8 with the AG orthologs.

CsPI from the perianthless early-diverging Chloranthus spicatus show function on petal development in Arabidopsis thaliana

BACKGROUND

In the floral ABC model, B-class genes comprised of DEFICIENS (DEF)/APETALA3 (AP3) and GLOBOSA (GLO)/PISTILLATA (PI) had been proposed to involve in second and third whorl floral organ development. However, less is known about the function of B-class genes from early-diverging angiosperms. Chloranthaceae is one of the early-diverging angiosperm families. In this study, we characterized the role of the PI-like gene CsPI cloned from Chloranthus spicatus which have the simplest perianthless bisexual flowers.

RESULTS

The expression profile analysis reveals high levels of CsPI mRNA in stamens in Chloranthus spicatus, with weak distribution in leaves and other floral organs. Nevertheless, CsPI rescued both stamen and petal development in Arabidopsis thaliana pi-1 mutants and caused partially conversion of sepals into petaloid organs in wild-type Arabidopsis thaliana plants. Yeast two-hybrid analysis showed that CsPI can form not only homodimers but also heterodimers with proteins encoded by Arabidopsis thaliana and Chloranthus spicatus AP3-like genes.

CONCLUSIONS

These results suggested that CsPI has an ancestral function on stamen development and that CsPI has capability to specify petal development in Arabidopsis thaliana. The finding indicates that the activity of the encoded PI-like proteins is highly conserved between the early-diverging Chloranthus and Arabidopsis. Moreover, our results appear to suggest that B-function genes may not play a role in perianth development in Chloranthus spicatus.