white anther: A petunia mutant that abolishes pollen flavonol accumulation, induces male sterility, and is complemented by a chalcone synthase transgene

Towards male sterility in Pinus radiata--a stilbene synthase approach to genetically engineer nuclear male sterility

A male cone-specific promoter from Pinus radiata D. Don (radiata pine) was used to express a stilbene synthase gene (STS) in anthers of transgenic Nicotiana tabacum plants, resulting in complete male sterility in 70% of transformed plants. Three plants were 98%-99.9% male sterile, as evidenced by pollen germination. To identify the stage at which transgenic pollen first developed abnormally, tobacco anthers from six different developmental stages were assayed microscopically. Following the release of pollen grains from tetrads, transgenic pollen displayed an increasingly flake-like structure, which gradually rounded up during the maturation process. We further investigated whether STS expression may have resulted in an impaired flavonol or sporopollenin formation. A specific flavonol aglycone stain was used to demonstrate that significant amounts of these substances were produced only in late stages of normal pollen development, therefore excluding a diminished flavonol aglycone production as a reason for pollen ablation. A detailed analysis of the exine layer by transmission electron microscopy revealed minor structural changes in the exine layer of ablated pollen, and pyrolysis-gas chromatography-mass spectroscopy indicated that the biochemistry of sporopollenin production was unaffected. The promoter-STS construct may be useful for the ablation of pollen formation in coniferous gymnosperms and male sterility may potentially be viewed as a prerequisite for the commercial use of transgenic conifers.

Low temperature treatment at the young microspore stage induces protein changes in rice anthers

Male reproductive development in rice is very sensitive to various forms of environmental stresses including low temperature. A few days of cold treatment (<20 degrees C) at the young microspore stage induce severe pollen sterility and thus large grain yield reductions. To investigate this phenomenon, anther proteins at the early stages of microspore development, with or without cold treatment at 12 degrees C, were extracted, separated by two-dimensional gel electrophoresis, and compared. The cold-sensitive cultivar Doongara and the relatively cold-tolerant cultivar HSC55 were used. The abundance of 37 anther proteins was changed more than 2-fold after 1, 2, and 4 days of cold treatment in cv. Doongara. Among them, one protein was newly induced, 32 protein spots were up-regulated, and four protein spots were down-regulated. Of these 37 protein spots, we identified two anther-specific proteins (putative lipid transfer protein and Osg6B) and a calreticulin that were down-regulated and a cystine synthase, a beta-6 subunit of the 20 S proteasome, an H protein of the glycine cleavage system, cytochrome c oxidase subunit VB, an osmotin protein homologue, a putative 6-phosphogluconolactonase, a putative adenylate kinase, a putative cysteine proteinase inhibitor, ribosomal protein S12E, a caffeoyl-CoA O-methyltransferase, and a monodehydroascorbate reductase that were up-regulated. Identification of these proteins is available upon request. Accumulation of these proteins did not vary greatly after cold treatment in panicles of cv. Doongara or in the anthers of the cv. HSC55. The newly induced protein named Oryza sativa cold-induced anther protein (OsCIA) was identified as an unknown protein. The OsCIA protein was detected in panicles, leaves, and seedling tissues under normal growth conditions. Quantitative real time RT-PCR analysis of OsCIA mRNA expression showed no significant change between low temperature-treated and untreated plants. A possible regulatory role for the newly induced protein is proposed.

Engineering cytoplasmic male sterility via the chloroplast genome by expression of {beta}-ketothiolase

While investigating expression of the polydroxybutyrate pathway in transgenic chloroplasts, we addressed the specific role of beta-ketothiolase. Therefore, we expressed the phaA gene via the chloroplast genome. Prior attempts to express the phaA gene in transgenic plants were unsuccessful. We studied the effect of light regulation of the phaA gene using the psbA promoter and 5' untranslated region, and evaluated expression under different photoperiods. Stable transgene integration into the chloroplast genome and homoplasmy were confirmed by Southern analysis. The phaA gene was efficiently transcribed in all tissue types examined, including leaves, flowers, and anthers. Coomassie-stained gel and western blots confirmed hyperexpression of beta-ketothiolase in leaves and anthers, with proportionately high levels of enzyme activity. The transgenic lines were normal except for the male-sterile phenotype, lacking pollen. Scanning electron microscopy revealed a collapsed morphology of the pollen grains. Floral developmental studies revealed that transgenic lines showed an accelerated pattern of anther development, affecting their maturation, and resulted in aberrant tissue patterns. Abnormal thickening of the outer wall, enlarged endothecium, and vacuolation affected pollen grains and resulted in the irregular shape or collapsed phenotype. Reversibility of the male-sterile phenotype was observed under continuous illumination, resulting in viable pollen and copious amount of seeds. This study results in the first engineered cytoplasmic male-sterility system in plants, offers a new tool for transgene containment for both nuclear and organelle genomes, and provides an expedient mechanism for F(1) hybrid seed production.

Flavanols in somatic cell division and male meiosis of tea (Camellia sinensis) anthers

Young anthers excised from closed tea flower buds ( Camellia sinensis L.) were stained as fresh tissues with p-dimethylaminocinnamaldehyde reagent to localize flavanols associated with nuclei and chromosomes, apart from those flavanols stored in vacuoles. This staining reagent yields a blue colour for flavanols. In the nonsporogenic somatic cells of developing anthers, flavanols were found to be attached to chromosomes at all mitotic stages. Male meiosis started at a bud size of about 3.5 mm in diameter in pollen mother cells which displayed generally more or less pronounced blue nuclei and cytoplasm. The meiotic divisions from prophase I to telophase II were characterized by blue stained nuclei and chromosomes, but within the cytoplasm there was, if any, a random and very poor reaction for flavanols. Metaphase and telophase of meiotic divisions showed maximally condensed chromosomes staining dark blue. Early in telophase II, the cytoplasm was again stained blue; this faded at late tetrad stage. Flavanols of young mitotic and older non-mitotic anthers were determined using high pressure liquid chromatography--chemical reaction detection (HPLC-CRD). Catechin, epicatechin, B2, and epigallocatechin were minor compounds, whereas epicatechin gallate and epigallocatechin gallate were found in higher amounts. The major flavanol compound of the anthers, epicatechin gallate, exhibited a significant affinity to histone sulphate, as shown by UV-VIS spectroscopic titration.

AtMYB32 is required for normal pollen development in Arabidopsis thaliana

AtMYB32 gene is a member of the R2R3 MYB gene family coding for transcription factors in Arabidopsis thaliana. Its expression pattern was analysed using Northern blotting, in situ hybridization and promoter-GUS fusions. AtMYB32 is expressed in many tissues, but most strongly in the anther tapetum, stigma papillae and lateral root primordia. AtMYB32-GUS was induced in leaves and stems following wounding, and in root primordia by auxin. T-DNA insertion populations were screened and two insertion mutants were identified, both of which were partially male sterile, more than 50% of the pollen grains being distorted in shape and lacking cytoplasm. AtMYB4 is closely related to AtMYB32 and represses the CINNAMATE 4-HYDROXYLASE gene. Distorted pollen grains were produced in both AtMYB4 insertion mutant and overexpression lines. In an AtMYB32 insertion mutant, the transcript levels of the DIHYDROFLAVONOL 4-REDUCTASE and ANTHOCYANIDIN SYNTHASE genes decreased while the level of the CAFFEIC ACID 0-METHYLTRANSFERASE transcript increased. Change in the levels of AtMYB32 and AtMYB4 expression may influence pollen development by changing the flux along the phenylpropanoid pathways, affecting the composition of the pollen wall.

Modification of flavonoid biosynthesis in crop plants

Flavonoids comprise the most common group of polyphenolic plant secondary metabolites. In plants, flavonoids play an important role in biological processes. Beside their function as pigments in flowers and fruits, to attract pollinators and seed dispersers, flavonoids are involved in UV-scavenging, fertility and disease resistance. Since they are present in a wide range of fruits and vegetables, flavonoids form an integral part of the human diet. Currently there is broad interest in the effects of dietary polyphenols on human health. In addition to the potent antioxidant activity of many of these compounds in vitro, an inverse correlation between the intake of certain polyphenols and the risk of cardiovascular disease, cancer and other age related diseases has been observed in epidemiological studies. The potential nutritional effects of these molecules make them an attractive target for genetic engineering strategies aimed at producing plants with increased nutritional value. This review describes the current knowledge of the molecular regulation of the flavonoid pathway and the state of the art with respect to metabolic engineering of this pathway in crop plants.

Antisense phenotypes reveal a role for SHY, a pollen-specific leucine-rich repeat protein, in pollen tube growth

SHY, a pollen-specific gene identified in a screen for genes upregulated at pollen germination, encodes a leucine-rich repeat (LRR) protein that is predicted to be secreted. To test if SHY plays an important role during pollen germination, we generated transgenic plants expressing an antisense (AS) copy of the SHY cDNA in pollen. Primary transformants exhibited poor seed set, but homozygous lines could be identified. In these lines, nearly all pollen tubes failed to reach the ovules; tube growth was arrested at the apex of the ovary and the pollen tubes exhibited abnormal callose deposits throughout the tube and in the tips. We show that a SHY::eGFP fusion protein is targeted to the cell wall. The structure of the SHY protein is nearly identical to other extracellular matrix glycoproteins that are composed of LRRs, such as the polygalacturonase inhibitor proteins (PGIP) of plants. PGIPs may function as defense proteins by inhibiting fungal endo-polygalacturonases, but enzyme assays with extracts of AS-SHY pollen do not support such an inhibitor role for SHY. The tomato ortholog of SHY interacts with a tomato receptor kinase (LePRK2) in yeast two-hybrid and pull-down assays; this, and the AS-SHY phenotypes, suggest instead that SHY might function in a signal transduction pathway mediating pollen tube growth.

A maize QTL for silk maysin levels contains duplicated Myb-homologous genes which jointly regulate flavone biosynthesis

The maize p1 locus coincides with a major QTL (quantitative trait locus) determining levels of maysin, a C-glycosyl flavone that deters feeding by corn ear-worm. The p1 gene is tightly linked with a second gene, p2, and both genes encode similar Myb-domain proteins. We show here that maize cell cultures transformed with either the p1 or p2 genes expressed under a constitutive promoter accumulate transcripts for flavonoid biosynthetic genes, and synthesize phenylpropanoids and C-glycosyl flavones related to maysin. Additionally, maize plants that are deleted for the p1 gene have reduced maysin levels and moderate silk-browning reaction, whereas plants with a deletion of both p1 and p2 have non-detectable silk maysin and non-browning silks. We conclude that both p1 and p2 induce maysin biosynthesis in silk, although the two genes differ in their expression and pigmentation effects in other tissues. These results show that a QTL for flavone biosynthesis actually comprises two tightly linked genes with related functions.

Flavonol synthase gene expression during citrus fruit development

We isolated a cDNA clone (CitFLS) encoding flavonol synthase (FLS) from the satsuma mandarin (Citrus unshiu Marc.) fruit and investigated the steady state of CitFLS RNA expression during the fruit development. The CitFLS was 1274 bp long, encoded 335 amino acid residues, and belonged to a family of 2-oxoglutarate-dependent dioxygenases. The level of CitFLS transcript was higher in the young leaves than in the old leaves, and it was high at the early developmental stage and low at the mature stage in the juice sacs/segment epidermis (edible part). On the other hand, the CitFLS transcript increased in the peel during fruit maturation. These results indicated that the satsuma mandarin CitFLS was differentially regulated in the developmental stage and in a tissue-specific manner. Additionally, satsuma mandarin peel tissues produced rutin (a flavonol glycoside) from an exogenous dihydroquercetin (taxifolin), indicating the ability of these tissues to produce flavonols.

Cytoplasmic accumulation of flavonoids in flower petals and its relevance to yellow flower colouration

It is widely accepted that the mix of flavonoids in the cell vacuole is the source of flavonoid based petal colour, and that analysis of the petal extract reveals the nature and relative levels of vacuolar flavonoid pigments. However, it has recently been established with lisianthus flowers that some petal flavonoids can be excluded from the vacuolar mix through deposition in the cell wall or through complexation with proteins inside the vacuole, and that these flavonoids are not readily extractable. The present work demonstrates that flavonoids can also be compartmented within the cell cytoplasm. Using adaxial epidermal peels from the petals of lisianthus (Eustoma grandiflorum), Lathyrus chrysanthus and Dianthus caryophyllus, light and laser scanning confocal microscopy studies revealed a significant concentration of petal flavonoids in the cell cytoplasm of some tissues. With lisianthus, flavonoid analyses of isolated protoplasts and vacuoles were used to establish that ca 14% of petal flavonoids are located in the cytoplasm (cf. 30% in the cell wall and 56% in the vacuole). The cytoplasmic flavonoids are predominantly acylated glycosides (cf. non-acylated in the cell wall). Flavonoid aggregation on a cytoplasmic protein substrate provides a rational mechanism to account for how colourless flavonoid glycosides can produce yellow colouration in petals, and perhaps also in other plant parts. High vacuolar concentrations of such flavonoids are shown to be insufficient.

Novel anther-specific myb genes from tobacco as putative regulators of phenylalanine ammonia-lyase expression

Two cDNA clones (NtmybAS1 and NtmybAS2) encoding MYB-related proteins with strong sequence similarity to petunia (Petunia hybrida) PhMYB3 were isolated from a tobacco (Nicotiana tabacum cv Samsun) pollen cDNA library. Northern blot and in situ hybridization revealed that NtmybAS transcripts are specifically expressed in both sporophytic and gametophytic tissues of the anther including tapetum, stomium, vascular tissue, and developing pollen. Random binding site selection assays revealed that NtMYBAS1 bound to DNA sequences closely resembling consensus MYB binding sites MBSI and MBSIIG, with a higher affinity for MBSI. Transient expression analyses of the N-terminal MYB domain demonstrated the presence of functional nuclear localization signals, and full-length NtMYBAS1 was able to activate two different phenylalanine ammonia-lyase promoters (PALA and gPAL1) in tobacco leaf protoplasts. Similar analysis of truncated NtmybAS1 cDNAs identified an essential, C-terminal trans-activation domain. Further in situ hybridization analyses demonstrated strict co-expression of NtmybAS and gPAL1 in the tapetum and stomium. Despite abundant expression of NtmybAS transcripts in mature pollen, gPAL1 transcripts were not detectable in pollen. Our data demonstrate that NtMYBAS1 is a functional anther-specific transcription factor, which is likely to be a positive regulator of gPAL1 expression and phenylpropanoid synthesis in sporophytic, but not in gametophytic, tissues of the anther.

Induction of male sterility in plants by metabolic engineering of the carbohydrate supply

Extracellular invertase mediates phloem unloading via an apoplastic pathway. The gene encoding isoenzyme Nin88 from tobacco was cloned and shown to be characterized by a specific spatial and temporal expression pattern. Tissue-specific antisense repression of Nin88 under control of the corresponding promoter in tobacco results in a block during early stages of pollen development, thus, causing male sterility. This result demonstrates a critical role of extracellular invertase in pollen development and strongly supports the essential function of extracellular sucrose cleavage for supplying carbohydrates to sink tissues via the apoplast. The specific interference with phloem unloading, the sugar status, and metabolic signaling during pollen formation will be a potentially valuable approach to induce male sterility in various crop species for hybrid seed production.