Search

Although phosphorus is an essential factor for proper plant growth in natural environments, an excess of phosphate in water sources causes serious pollution. In this paper we describe transgenic plants which hyperaccumulate inorganic phosphate (Pi) and which may be used to reduce environmental water pollution by phytoremediation. AtPHR1, a transcription factor for a key regulator of the Pi starvation response inArabidopsis thaliana, was overexpressed in the ornamental garden plantsTorenia, Petunia, and Verbena.The transgenic plants showed hyperaccumulation of Pi in leaves and accelerated Pi absorption rates from hydroponic solutions. Large-scale hydroponic experiments indicated that the enhanced ability to absorb Pi in transgenic torenia (AtPHR1) was comparable to water hyacinth a plant that though is used for phytoremediation causes overgrowth problems.

Rose petals contain 3-glucosylated anthocyanidins and flavonols. We isolated three flavonoid 3-glucosyltransferase (UF3GT) homolog genes (RhUF3GT1, RhUF3GT2, and RhUF3GT3) from rose. RhUF3GT1 encoded an amino acid sequence that is almost identical to the reported rose partial UF3GT homologs and highly homologous to strawberry and apple UF3GTs. Recombinant RhUF3GT1 expressed in yeast catalyzes 3-glucosylation of anthocyanidins but not flavonols. RhUF3GT1 was not expressed in the petals of many cultivars even when anthocyanin biosynthesis was active, while it was expressed in the mature petals of cultivars that synthesize cyanidin 3-glucoside in the mature petals. RhUF3GT2 and RhUF3GT3, sharing 79% identity, exhibit only 42% and 41% identities to RhUF3GT1, respectively, and are distantly related to strawberry and apple UF3GTs. They were expressed in coordination with the flavonol synthase gene in the petal. The recombinant RhUF3GT2 expressed in yeast catalyzed 3-glucosylation of flavonol much more efficiently than that of anthocyanidins. We suggest that RhUF3GT2 catalyzes flavonol 3-glucosylation in rose petals and that it also contributes to accumulation of anthocyanidin 3-glucoside in the petals.

4,2′,4′,6′-Tetrahydroxychalcone (THC) 2′-glucoside that confers yellow color to the petals of carnation, cyclamen, and catharanthus is biosynthesized by the action of UDP-glucose-dependent THC 2′-glucosyltransferase (THC2′GT). We isolated 18 types of full-length cDNA encoding GT-like sequences from carnation petals. Expression of these cDNA in Escherichia coli identified five cDNAs encoding THC2′GT that were different from the previously isolated THC2′GT. We also isolated a cDNA encoding THC2′GT from both catharanthus and cyclamen. These THC2′GT cDNAs were introduced to petunia. Transgenic petunia that expressed three of the GTs produced THC 2′-glucoside, which indicated that they function as THC2′GT in vivo. These cDNAs could be useful molecular tools to yield yellow flower color, although the amount accumulated in the transgenic petals was too small to alter the flower color in this study.

The release of genetically modified plants into the environment can only occur after permission is obtained from the relevant regulatory authorities. This permission will only be obtained after extensive risk assessment shows comparable risk of impact to the environment and biodiversity as compared to non-transgenic host plants. Two transgenic rose (Rosa×hybrida) lines, whose flowers were modified to a bluer colour as a result of accumulation of delphinidin-based anthocyanins, have been trialed in greenhouses and the field in both Japan and Australia. Flower colour modification was due to expression of genes of a viola flavonoid 3′,5′-hydroxylase and a torenia anthocyanin 5-acyltransferase. In all trials it was shown that the performance of the two transgenic lines, as measured by their growth characters, was comparable to the host untransformed variety. Biological assay showed that the transgenic lines did not produce allelopathic compounds. In Japan, seeds from wild rose species that had grown in close proximity to the transgenic roses did not carry either a Rosa×hybrida specific marker gene or the transgenes. In hybridization experiments using transgenic rose pollen and wild rose female parents, the transgenes were not detected in the seed obtained, though there was a low frequency of seed set. The transgene was also not transmitted when Rosa×hybrida cultivars were used as females. In in situ hybridization analysis transgene transcripts were only detected in the epidermal cells in the petals of the transgenic roses. In combination, the breeding and in situ analysis results show that the transgenic roses contain the transgene only in the L1 layer cells and not in the L2 layer cells that generate reproductive cells. General release permissions have been granted for both transgenic lines in Japan and one is now commercially produced.

Flower color is mainly determined by anthocyanins. Rosa hybrida lacks violet to blue flower varieties due to the absence of delphinidin-based anthocyanins, usually the major constituents of violet and blue flowers, because roses do not possess flavonoid 3',5'-hydoxylase (F3'5'H), a key enzyme for delphinidin biosynthesis. Other factors such as the presence of co-pigments and the vacuolar pH also affect flower color. We analyzed the flavonoid composition of hundreds of rose cultivars and measured the pH of their petal juice in order to select hosts of genetic transformation that would be suitable for the exclusive accumulation of delphinidin and the resulting color change toward blue. Expression of the viola F3'5'H gene in some of the selected cultivars resulted in the accumulation of a high percentage of delphinidin (up to 95%) and a novel bluish flower color. For more exclusive and dominant accumulation of delphinidin irrespective of the hosts, we down-regulated the endogenous dihydroflavonol 4-reductase (DFR) gene and overexpressed the Irisxhollandica DFR gene in addition to the viola F3'5'H gene in a rose cultivar. The resultant roses exclusively accumulated delphinidin in the petals, and the flowers had blue hues not achieved by hybridization breeding. Moreover, the ability for exclusive accumulation of delphinidin was inherited by the next generations.

Homologues of the flavonoid 3� ,5� -hydroxylase (F3� 5� H) gene, a key gene determining flower color, were obtained from a Verbena hybrida (verbena) cultivar Temari Violet, verbena cultivar Tapien Pink, and Clitoria ternatea (butterfly pea). The expression of the Temari and butterfly pea homologues in yeast confirmed that they encoded F3� 5� H. The two genes under the control of an enhanced cauliflower mosaic virus 35S promoter were introduced into verbena Temari Sakura. Some of the transgenic verbena plants had elevated delphinidin contents and flower color altered toward violet. Interestingly, the butterfly pea F3� 5� H gene yielded more delphinidin and gave clearer flower color change than the verbena Temari gene in the transgenic verbena plants. The results indicate that the choice of the gene source should be considered to obtain strong phenotypic changes, even if the genes encode the same enzymatic activity. We also cloned some flavonoid biosynthetic genes from verbenas. The potential usefulness of verbena in the phytomonitoring of environmental pollutants is also discussed.

Flower color is mainly determined by the structure of flavonoids, a group of secondary metabolites of plants. The biosynthetic pathway and the genes involved in the pathway are well characterized such that it is possible to change flower color by engineering the pathway by overexpression of heterologous genes and/or suppression of endogenous genes in transgenic plants. Trimming an unnecessary pathway by suppression of endogenous genes is often essential to achieve successful engineering of the pathway and the resultant accumulation of desirable compounds. RNAi by transcription of double-stranded RNA (dsRNA) is a powerful and efficient method to command such suppression and is widely used for artificial gene suppression in transgenic plants.

Flower color is mainly determined by the structure of flavonoids, a group of secondary metabolites of plants. The biosynthetic pathway and the genes involved in the pathway are well characterized such that it is possible to change flower color by engineering the pathway by overexpression of heterologous genes and/or suppression of endogenous genes in transgenic plants. Trimming an unnecessary pathway by suppression of endogenous genes is often essential to achieve successful engineering of the pathway and the resultant accumulation of desirable compounds. RNAi by transcription of double-stranded RNA (dsRNA) is a powerful and efficient method to command such suppression and is widely used for artificial gene suppression in transgenic plants.