Role of Floral Homeotic Genes in the Morphology of Forchlorfenuron-induced Paracorollas in Torenia fournieri Lind

Ectopic expression of AtNF-YA6-VP16 in petals results in a novel petal phenotype in Torenia fournieri

A novel Torenia phenotype having separate petals was obtained by the combination of NF-YA6-VP16 with a floral organ-specific promoter. Genetic engineering techniques helped in obtaining novel flower colors and shapes, in particular, by introducing functionally modified transcription factors (TFs) to ornamental flower species. Herein, we used functionally modified Arabidopsis TFs fused with the repression domain SRDX and the activation domain VP16 to screen for novel floral traits in Torenia fournieri Lind (torenia). We avoided undesired phenotypes unrelated to flowers by expressing these TFs through a floral organ-specific promoter belonging to the class-B genes, GLOBOSA (TfGLO). Fourteen constructs were produced to express functionally modified Arabidopsis TFs in which each of SRDX and VP16 was fused into 7 TFs that were used for the collective transformation of Torenia plants. Among the obtained transgenic plants, phenotypes with novel floral traits reflected in separate petals within normally gamopetalous flower lines. Sequencing analysis revealed that the transgenic plants contained nuclear factor-YA6 (NF-YA6) fused with the VP16. In the margin between the lips of the petals and tube in the TfGLOp:NF-YA6-VP16 plants, staminoid organs have been developed to separate petals. In the petals of the TfGLOp:NF-YA6-VP16 plants, the expression of a Torenia class C gene, PLENA (TfPLE), was found to be ectopically increased. Moreover, expression of TfPLE-VP16 under the control of the TfGLO promoter brought a similar staminoid phenotype observed in the TfGLOp:NF-YA6-VP16 plants. These results suggest that the introduction of the TfGLOp:NF-YA6-VP16 induced TfPLE expression, resulting in the formation of staminoid petals and separation of them.

Genetic analysis and population structure of wild and cultivated wishbone flower (Torenia fournieri Lind.) lines related to specific floral color

Background

The wishbone flower or Torenia fournieri Lind., an annual from tropical Indochina and southern China, is a popular ornamental plant, and many interspecific (T. fournieri × T. concolor) hybrid lines have been bred for the international market. The cultivated lines show a pattern of genetic similarity that correlates with floral color which informs on future breeding strategies. This study aimed to perform genetic analysis and population structure of cultivated hybrid lines comparing with closely related T. concolor wild populations.

Methods

We applied the retrotransposon based iPBS marker system for genotyping of a total of 136 accessions from 17 lines/populations of Torenia. These included 15 cultivated lines of three series: Duchess (A, B, C); Kauai (D, E, F, G, H, I, J); Little Kiss (K, L, M, N, P) and two wild T. concolor populations (Q and R). PCR products from each individual were applied to estimate the genetic diversity and differentiation between lines/populations.

Results

Genotyping results showed a pattern of genetic variation differentiating the 17 lines/populations characterized by their specific floral colors. The final PCoA analysis, phylogenetic tree construction, and Bayesian population structural bar plot all showed a clear subdivision of lines/populations analysed. The 15 cultivated hybrid lines and the wild population Q that collected from a small area showed the lowest genetic variability while the other wild population R which sampled from a larger area had the highest genetic variability.

Discussion

The extremely low genetic variability of 15 cultivated lines indicated that individual line has similar reduction in diversity/heterozygosity from a bottleneck event, and each retained a similar (but different from each other) content of the wild genetic diversity. The genetic variance for the two wild T. concolor populations could be due to our varied sampling methods. The two wild populations (Q, R) and the cultivated hybrid lines (I, K, M, N, P) are genetically more closely related, but strong positive correlations presented in cultivated lines A, C, E, M, and N. These results could be used to guide future Torenia breeding.

Conclusions

The genetic variation and population structure found in our study showed that cultivated hybrid lines had similar reduction in diversity/heterozygosity from a bottleneck event and each line retained a similar (but different from each other) content of the wild genetic diversity, especially when strong phenotypic selection of floral color overlaps. Generally, environmental factors could induce transposon activation and generate genetic variability which enabled the acceleration of the evolutionary process of wild Torenia species. Our study revealed that wild Torenia populations sampled from broad geographic region represent stronger species strength with outstanding genetic diversity, but selective breeding targeting a specific floral color decreased such genetic variability.

Generation of Novel Floral Traits Using a Combination of Floral Organ-Specific Promoters and a Chimeric Repressor in Torenia fournieri Lind

In this study, we attempted to develop a new biotechnological method for the efficient modification of floral traits. Because transcription factors play an important role in determining floral traits, chimeric repressors, which are generated by attaching a short transcriptional repressor domain to transcription factors, have been widely used as effective tools for modifying floral traits in many plant species. However, the overexpression of these chimeric repressors by the Cauliflower mosaic virus 35S promoter sometimes causes undesirable morphological alterations to other organs. We attempted simultaneously to generate new floral traits and avoid such quality loss by examining five additional floral organ-specific promoters, one Arabidopsis thaliana promoter and four Torenia fournieri promoters, for the expression of the chimeric repressor of Arabidopsis TCP3 (AtTCP3), whose overexpression drastically alters floral traits but also generates dwarf phenotypes and deformed leaves. We found that the four torenia promoters exhibited particularly strong activity in the petals but not in the leaves, and that the combination of these floral organ-specific promoters with the chimeric repressor of AtTCP3 caused changes in the color, color patterns and cell shapes of petals, whilst avoiding other unfavorable phenotypes. Interestingly, each promoter that we used in this study generated characteristic and distinguishable floral traits. Thus, the use of different floral organ-specific promoters with different properties enables us to generate diverse floral traits using a single chimeric repressor without changing the phenotypes of other organs.

Mutation in Torenia fournieri Lind. UFO homolog confers loss of TfLFY interaction and results in a petal to sepal transformation

We identified a Torenia fournieri Lind. mutant (no. 252) that exhibited a sepaloid phenotype in which the second whorls were changed to sepal-like organs. This mutant had no stamens, and the floral organs consisted of sepals and carpels. Although the expression of a torenia class B MADS-box gene, GLOBOSA (TfGLO), was abolished in the 252 mutant, no mutation of TfGLO was found. Among torenia homologs such as APETALA1 (AP1), LEAFY (LFY), and UNUSUAL FLORAL ORGANS (UFO), which regulate expression of class B genes in Arabidopsis, only accumulation of the TfUFO transcript was diminished in the 252 mutant. Furthermore, a missense mutation was found in the coding region of the mutant TfUFO. Intact TfUFO complemented the mutant phenotype whereas mutated TfUFO did not; in addition, the transgenic phenotype of TfUFO-knockdown torenias coincided with the mutant phenotype. Yeast two-hybrid analysis revealed that the mutated TfUFO lost its ability to interact with TfLFY protein. In situ hybridization analysis indicated that the transcripts of TfUFO and TfLFY were partially accumulated in the same region. These results clearly demonstrate that the defect in TfUFO caused the sepaloid phenotype in the 252 mutant due to the loss of interaction with TfLFY.