Expression of tandem invertase genes associated with sexual and vegetative growth cycles in potato

Cell wall invertase 3 plays critical roles in providing sugars during pollination and fertilization in cucumber

In plants, pollen-pistil interactions during pollination and fertilization mediate pollen hydration and germination, pollen tube growth, and seed set and development. Cell wall invertases (CWINs) help provide the carbohydrates for pollen development; however, their roles in pollination and fertilization have not been well established. In cucumber (Cucumis sativus), CsCWIN3 showed the highest expression in flowers, and we further examined CsCWIN3 for functions during pollination to seed set. Both CsCWIN3 transcript and CsCWIN3 protein exhibited similar expression patterns in the sepals, petals, stamen filaments, anther tapetum, and pollen of male flowers, as well as in the stigma, style, transmitting tract, and ovule funiculus of female flowers. Notably, repression of CsCWIN3 in cucumber did not affect the formation of parthenocarpic fruit but resulted in an arrested growth of stigma integuments in female flowers and a partially delayed dehiscence of anthers with decreased pollen viability in male flowers. Consequently, the pollen tube grew poorly in the gynoecia after pollination. In addition, CsCWIN3-RNA interference plants also showed affected seed development. Considering that sugar transporters could function in cucumber fecundity, we highlight the role of CsCWIN3 and a potential close collaboration between CWIN and sugar transporters in these processes. Overall, we used molecular and physiological analyses to determine the CsCWIN3-mediated metabolism during pollen formation, pollen tube growth, and plant fecundity. CsCWIN3 has essential roles from pollination and fertilization to seed set but not parthenocarpic fruit development in cucumber.

Recent Advances in Molecular Improvement for Potato Tuber Traits

Potato is an important crop due to its nutritional value and high yield potential. Improving the quality and quantity of tubers remains one of the most important breeding objectives. Genetic mapping helps to identify suitable markers for use in the molecular breeding, and combined with transgenic approaches provides an efficient way for gaining desirable traits. The advanced plant breeding tools and molecular techniques, e.g., TALENS, CRISPR-Cas9, RNAi, and cisgenesis, have been successfully used to improve the yield and nutritional value of potatoes in an increasing world population scenario. The emerging methods like genome editing tools can avoid incorporating transgene to keep the food more secure. Multiple success cases have been documented in genome editing literature. Recent advances in potato breeding and transgenic approaches to improve tuber quality and quantity have been summarized in this review.

Genome-wide identification and expression profiling of invertase gene family for abiotic stresses tolerance in Poncirus trifoliata

BACKGROUND

Sucrose (Suc) hydrolysis is directly associated with plants tolerance to multiple abiotic stresses. Invertase (INV) enzymes irreversibly catalyze Suc degradation to produce glucose (Glc) and fructose (Frc). However, genome-wide identification and function of individual members of the INV gene family in Poncirus trifoliata or its Citrus relatives in response to abiotic stresses are not fully understood.

RESULTS

In this report, fourteen non-redundant PtrINV family members were identified in P. trifoliata including seven alkaline/neutral INV genes (PtrA/NINV1-7), two vacuolar INV genes (PtrVINV1-2), and five cell wall INV isoforms (PtrCWINV1-5). A comprehensive analysis based on the biochemical characteristics, the chromosomal location, the exon-intron structures and the evolutionary relationships demonstrated the conservation and the divergence of PtrINVs. In addition, expression analysis of INV genes during several abiotic stresses in various tissues indicated the central role of A/NINV7 among INV family members in response to abiotic stresses. Furthermore, our data demonstrated that high accumulation of Suc, Glc, Frc and total sugar contents were directly correlated with the elevated activities of soluble INV enzymes in the cold-tolerant P. trifoliata, C. ichangensis and C. sinensis, demonstrating the potential role of soluble INV enzymes for the cold tolerance of Citrus.

CONCLUSIONS

This work offered a framework for understanding the physiological role of INV genes and laid a foundation for future functional studies of these genes in response to abiotic stresses.

Novel SNP markers in InvGE and SssI genes are associated with natural variation of sugar contents and frying color in Solanum tuberosum Group Phureja

Background

Potato frying color is an agronomic trait influenced by the sugar content of tubers. The candidate gene approach was employed to elucidate the molecular basis of this trait in Solanum tuberosum Group Phureja, which is mainly diploid and represents an important genetic resource for potato breeding. The objective of this research was to identify novel genetic variants related with frying quality in loci with key functions in carbohydrate metabolism, with the purpose of discovering genetic variability useful in breeding programs. Therefore, an association analysis was implemented with 109 SNP markers identified in ten candidate genes.

Results

The analyses revealed four associations in the locus InvGE coding for an apoplastic invertase and one association in the locus SssI coding for a soluble starch synthase. The SNPs SssI-C_45711901T and InvGE-C_2475454T were associated with sucrose content and frying color, respectively, and were not found previously in tetraploid genotypes. The rare haplotype InvGE-A_2475187C_2475295A_2475344 was associated with higher fructose contents. Our study allowed a more detailed analysis of the sequence variation of exon 3 from InvGE, which was not possible in previous studies because of the high frequency of insertion-deletion polymorphisms in tetraploid potatoes.

Conclusion

The association mapping strategy using a candidate gene approach in Group Phureja allowed the identification of novel SNP markers in InvGE and SssI associated with frying color and the tuber sugar content measured by High Performance Liquid Chromatography (HPLC). These novel associations might be useful in potato breeding programs for improving quality traits and to increase crop genetic variability. The results suggest that some genes involved in the natural variation of tuber sugar content and frying color are conserved in both Phureja and tetraploid germplasm. Nevertheless, the associated variants in both types of germplasm were present in different regions of these genes. This study contributes to the understanding of the genetic architecture of tuber sugar contents and frying color at harvest in Group Phureja.

Electronic supplementary material

The online version of this article (doi:10.1186/s12863-017-0489-3) contains supplementary material, which is available to authorized users.

STP10 encodes a high-affinity monosaccharide transporter and is induced under low-glucose conditions in pollen tubes of Arabidopsis

Pollen tubes are fast growing, photosynthetically inactive cells. Their energy demand is covered by specific transport proteins in the plasma membrane that mediate the uptake of sugars. Here we report on the functional characterization of AtSTP10, a previously uncharacterized member of the SUGAR TRANSPORT PROTEIN family. Heterologous expression of STP10 cDNA in yeast revealed that the encoded protein catalyses the high-affinity uptake of glucose, galactose and mannose. The transporter is sensitive to uncouplers of transmembrane proton gradients, indicating that the protein acts as a hexose-H(+)symporter. Analyses of STP10 mRNA and STP10 promoter-reporter gene studies revealed a sink-specific expression pattern of STP10 in primordia of lateral roots and in pollen tubes. This restriction to sink organs is mediated by intragenic regions of STP10 qPCR analyses with cDNA of in vitro grown pollen tubes showed that STP10 expression was down-regulated in the presence of 50mM glucose. However, in pollen tubes of glucose-insensitive plants, which lack the glucose sensor hexokinase1 (HXK1), no glucose-induced down-regulation of STP10 expression was detected. A stp10T-DNA insertion line developed normally, which may point towards functional redundancy. The data presented in this paper indicate that a high-affinity glucose uptake system is induced in growing pollen tubes under low glucose conditions and that this regulation may occur through the hexokinase pathway.

SNPs in genes functional in starch-sugar interconversion associate with natural variation of tuber starch and sugar content of potato (Solanum tuberosum L.)

Starch accumulation and breakdown are vital processes in plant storage organs such as seeds, roots, and tubers. In tubers of potato (Solanum tuberosum L.) a small fraction of starch is converted into the reducing sugars glucose and fructose. Reducing sugars accumulate in response to cold temperatures. Even small quantities of reducing sugars affect negatively the quality of processed products such as chips and French fries. Tuber starch and sugar content are inversely correlated complex traits that are controlled by multiple genetic and environmental factors. Based on in silico annotation of the potato genome sequence, 123 loci are involved in starch-sugar interconversion, approximately half of which have been previously cloned and characterized. By means of candidate gene association mapping, we identified single-nucleotide polymorphisms (SNPs) in eight genes known to have key functions in starch-sugar interconversion, which were diagnostic for increased tuber starch and/or decreased sugar content and vice versa. Most positive or negative effects of SNPs on tuber-reducing sugar content were reproducible in two different collections of potato cultivars. The diagnostic SNP markers are useful for breeding applications. An allele of the plastidic starch phosphorylase PHO1a associated with increased tuber starch content was cloned as full-length cDNA and characterized. The PHO1a-HA allele has several amino acid changes, one of which is unique among all known starch/glycogen phosphorylases. This mutation might cause reduced enzyme activity due to impaired formation of the active dimers, thereby limiting starch breakdown.

Arabidopsis PTB1 and PTB2 proteins negatively regulate splicing of a mini-exon splicing reporter and affect alternative splicing of endogenous genes differentially

This paper examines the function of Arabidopsis thaliana AtPTB1 and AtPTB2 as plant splicing factors. The effect on splicing of overexpression of AtPTB1 and AtPTB2 was analysed in an in vivo protoplast transient expression system with a novel mini-exon splicing reporter. A range of mutations in pyrimidine-rich sequences were compared with and without AtPTB and NpU2AF65 overexpression. Splicing analyses of constructs in protoplasts and RNA from overexpression lines used high-resolution reverse transcription polymerase chain reaction (RT-PCR). AtPTB1 and AtPTB2 reduced inclusion/splicing of the potato invertase mini-exon splicing reporter, indicating that these proteins can repress plant intron splicing. Mutation of the polypyrimidine tract and closely associated Cytosine and Uracil-rich (CU-rich) sequences, upstream of the mini-exon, altered repression by AtPTB1 and AtPTB2. Coexpression of a plant orthologue of U2AF65 alleviated the splicing repression of AtPTB1. Mutation of a second CU-rich upstream of the mini-exon 3' splice site led to a decline in mini-exon splicing, indicating the presence of a splicing enhancer sequence. Finally, RT-PCR of AtPTB overexpression lines with c. 90 known alternative splicing (AS) events showed that AtPTBs significantly altered AS of over half the events. AtPTB1 and AtPTB2 are splicing factors that influence alternative splicing. This occurs in the potato invertase mini-exon via the polypyrimidine tract and associated pyrimidine-rich sequence.

Promoter regions of potato vacuolar invertase gene in response to sugars and hormones

Potato vacuolar acid invertase (StvacINV1) (β-fructofuranosidase; EC 3.2.1.26) has been confirmed to play an important role in cold-induced sweetening of potato tubers. However, the transcriptional regulation mechanisms of StvacINV1 are largely unknown. In this study, the 5'-flanking sequence of StvacINV1 was cloned and the cis-acting elements were predicted. Histochemical assay showed that the StvacINV1 promoter governed β-glucuronidase (GUS) expression in potato leaves, stems, roots and tubers. Quantitative analysis of GUS expression suggested that the activity of StvacINV1 promoter was suppressed by sucrose, glucose, fructose, and cold, while enhanced by indole-3-acetic acid (IAA), and gibberellic acid (GA3). Further deletion analysis clarified that the promoter regions from -118 to -551, -551 to -1021, and -1021 to -1521 were required for responding to sucrose/glucose, GA3, and IAA, respectively. These findings provide essential information regarding transcriptional regulation mechanisms of StvacINV1.

The SlFRK4 promoter is active only during late stages of pollen and anther development

Carbohydrates are essential for male gametophyte development. However, our understanding of the mechanism by which the sugar supply is controlled in the stamen is still in its infancy. We previously reported on the stamen-specific expression of the tomato (Solanum lycopersicum) sugar metabolic gene, fructokinase 4 (SlFRK4). Here, we present the cloning and the characterization of the SlFRK4 promoter and show its differential activation during anther development. We also show that the tissue-specific expression of SlFRK4 promoter is maintained in Arabidopsis thaliana. By histochemical analyses of the GUS reporter gene and DTA toxin driven by the SlFRK4 promoter, we show that the SlFRK4 promoter is gradually activated in pollen grains throughout the later stages of anther development and upon pollen germination. In addition, we analyzed the expression profile of SlFRK4 and other sugar metabolic genes and found that SlFRK4 and the invertase LIN7 are co-expressed in mature and germinated pollen. These findings point to the existence of a specialized mechanism in which carbohydrates are provided to the male gametophyte during the later stages of its development and suggest a valuable tool for manipulating the development of male gametophytes in crop species.

Systematic analysis of potato acid invertase genes reveals that a cold-responsive member, StvacINV1, regulates cold-induced sweetening of tubers

Acid invertase is believed to play a regulatory role during plant developmental processes and to respond to environmental stimuli. The expression profiles of the entire acid invertase family are not yet available for potato. By searching existing databases, it was determined that there are at least six acid invertase genes in potato, including four cell-wall invertase genes and two vacuolar invertase genes. They were subjected to comparative expression profiling in various organs of potato plants and in stored tubers to exploit their potential functions. The results revealed that each gene exhibited a unique expression pattern, which differed in transcript abundance or showed organ-specific features, pointing to the possible involvement of individual genes in plant development. The vacuolar invertase gene StvacINV1 had the highest expression level among three genes detected in the potato tubers. Further storage experiments showed that StvacINV1 was strongly induced by low temperatures, which is consistent with glucose accumulation in cold-stored tubers. Suppression of StvacINV1 by the antisense transformation in potato confirmed that lower StvacINV1 transcript abundance in transgenic tubers is related to lower reducing sugar content and lighter chip color in comparison with the wild type. The evidence strongly suggests that StvacINV1 is a gene involved in regulation of cold-induced sweetening of potato tubers. This provides an avenue for studying the mechanism involved in the regulation of the cold-induced sweetening trait and for agronomic enhancement.

Metabolically engineered male sterility in rapeseed (Brassica napus L.)

Male sterility is of special interest as a mechanism allowing hybrid breeding, especially in important crops such as rapeseed (Brassica napus). Male sterile plants are also suggested to be used as a biological safety method to prevent the spread of transgenes, a risk that is high in the case of rapeseed due to the mode of pollination, out-crossing by wind or insects, and the presence of related, cross-pollinating species in the surrounding ecosystem in Europe. Different natural occurring male sterilities and alloplasmic forms have been tried to be used in rapeseed with more or less success. Due to the difficulties and limitations with these systems, we present a biotechnological alternative: a metabolically engineered male sterility caused by interference with anther-specific cell wall-bound invertase. This is an essential enzyme for carbohydrate supply of the symplastically isolated pollen. The activity of this enzyme is reduced either by antisense interference or by expressing an invertase inhibitor under control of the anther-specific promoter of the invertase with the consequence of a strong decrease of pollen germination ability.

Natural diversity of potato (Solanum tuberosum) invertases

BACKGROUND

Invertases are ubiquitous enzymes that irreversibly cleave sucrose into fructose and glucose. Plant invertases play important roles in carbohydrate metabolism, plant development, and biotic and abiotic stress responses. In potato (Solanum tuberosum), invertases are involved in 'cold-induced sweetening' of tubers, an adaptive response to cold stress, which negatively affects the quality of potato chips and French fries. Linkage and association studies have identified quantitative trait loci (QTL) for tuber sugar content and chip quality that colocalize with three independent potato invertase loci, which together encode five invertase genes. The role of natural allelic variation of these genes in controlling the variation of tuber sugar content in different genotypes is unknown.

RESULTS

For functional studies on natural variants of five potato invertase genes we cloned and sequenced 193 full-length cDNAs from six heterozygous individuals (three tetraploid and three diploid). Eleven, thirteen, ten, twelve and nine different cDNA alleles were obtained for the genes Pain-1, InvGE, InvGF, InvCD141 and InvCD111, respectively. Allelic cDNA sequences differed from each other by 4 to 9%, and most were genotype specific. Additional variation was identified by single nucleotide polymorphism (SNP) analysis in an association-mapping population of 219 tetraploid individuals. Haplotype modeling revealed two to three major haplotypes besides a larger number of minor frequency haplotypes. cDNA alleles associated with chip quality, tuber starch content and starch yield were identified.

CONCLUSIONS

Very high natural allelic variation was uncovered in a set of five potato invertase genes. This variability is a consequence of the cultivated potato's reproductive biology. Some of the structural variation found might underlie functional variation that influences important agronomic traits such as tuber sugar content. The associations found between specific invertase alleles and chip quality, tuber starch content and starch yield will facilitate the selection of superior potato genotypes in breeding programs.

Anther-specific carbohydrate supply and restoration of metabolically engineered male sterility

Male-sterile plants are used in hybrid breeding as well as for gene confinement for genetically modified plants in field trials and agricultural production. Apart from naturally occurring mutations leading to male sterility, biotechnology has added new possibilities for obtaining male-sterile plants, although so far only one system is used in practical breeding due to limitations in propagating male-sterile plants without segregations in the next generation or insufficient restoration of fertility when fruits or seeds are to be harvested from the hybrid varieties. Here a novel mechanism of restoration for male sterility is presented that has been achieved by interference with extracellular invertase activity, which is normally specifically expressed in the anthers to supply the developing microspores with carbohydrates. Microspores are symplastically isolated in the locular space of the anthers, and thus an unloading pathway of assimilates via the apoplasmic space is mandatory for proper development of pollen. Antisense repression of the anther-specific cell wall invertase or interference with invertase activity by expressing a proteinacious inhibitor under the control of the anther-specific invertase promoter results in a block during early stages of pollen development, thus causing male sterility without having any pleiotropic effects. Restoration of fertility was successfully achieved by substituting the down-regulated endogenous plant invertase activity by a yeast invertase fused to the N-terminal portion of potato-derived vacuolar protein proteinase II (PiII-ScSuc2), under control of the orthologous anther-specific invertase promoter Nin88 from tobacco. The chimeric fusion PiII-ScSuc2 is known to be N-glycosylated and efficiently secreted from plant cells, leading to its apoplastic location. Furthermore, the Nin88::PiII-ScSuc2 fusion does not show effects on pollen development in the wild-type background. Thus, such plants can be used as paternal parents of a hybrid variety, thereby the introgression of Nin88::PiII-ScSuc2 to the hybrid is obtained and fertility is restored. In order to broaden the applicability of this male sterility/restoration system to other plant species, a phylogenic analysis of plant invertases(beta-fructofuranosidases) and related genes of different species was carried out. This reveals a specific clustering of the cell wall invertases with anther-specific expression for dicotyl species and another cluster for monocotyl plants. Thus, in both groups of plants, there seems to be a kind of co-evolution, but no recent common ancestor of these members of the gene family. These findings provide a helpful orientation to classify corresponding candidate genes in further plant species, in addition to the species analysed so far (Arabidopsis, tobacco, tomato, potato, carrots, rice, and wheat).

Extracellular invertase LIN6 of tomato: a pivotal enzyme for integration of metabolic, hormonal, and stress signals is regulated by a diurnal rhythm

To elucidate the regulation pattern of extracellular invertase LIN6 of tomato, the corresponding promoter has been cloned and the sink-tissue specific expression and its regulation by sugars, stress stimuli, growth regulators, and the diurnal rhythm is shown. The in situ analysis of transgenic tobacco plants expressing a LIN6 promoter::beta-glucuronidase reporter gene fusion demonstrates LIN6 expression in sink tissues, such as pollen grains and vascular tissues of leaves and stems. LIN6 is up-regulated in close proximity to wounded tissue, and by methyl jasmonate and abscisic acid, global signals known to modulate defence/stress response. Salicylic acid on the other hand, as well as acetyl salicylic acid, suppresses LIN6 expression, supporting the fact that LIN6 is an inducible compound of the defence/stress response pathway that is antagonistically regulated by jasmonates and salicylates. Induction of the LIN6 promoter in stable transformed BY2 suspension cultures by sucrose and the growth-promoting phytohormones cytokinin and auxin along histochemical expression data, showing LIN6 expression in germinating seeds and seedlings, indicates a role of LIN6 invertase during growth processes. In addition, LIN6 is regulated by a diurnal rhythm that drives LIN6 expression in subjective dawn. Transactivation assays with circadian oscillator elements of Arabidopsis Circadian Clock Associated 1 and Late Elongated Hypocotyl demonstrate functional interaction with the LIN6 promoter.

Interspecies compatibility of the anther specific cell wall invertase promoters from Arabidopsis and tobacco for generating male sterile plants

Histochemical GUS-staining and fluorometric analyses revealed strong tissue specific activities of the cell wall invertase promoters Nin88 from Nicotiana tabacum and AtcwINV2 from Arabidopsis thaliana that are restricted tightly to anthers and pollen, respectively. Both in A. thaliana and N. tabacum repression of invertase activity by anther specific RNA-interference turned out to be an efficient method to circumvent carbohydrate supply of the symplastically isolated pollen with subsequent strong decrease of pollen germination ability and seed setting. In the case of tobacco, comparable results were also obtained by expressing a proteinaceous invertase inhibitor, whereas this approach was less efficient in Arabidopis. The present study revealed that anther specific interference with invertase-activity in order to generate male sterile plants can be applied to members of the two different plant families Solanaceae (N. tabacum) and Brassicaceae (A. thalaina) and the strategy seems to be a general tool for practical application in hybrid breeding or as biological safety precautions. To elucidate the compatibility of the isolated promoters beyond plant families, we transferred the regulatory sequences into the respectively heterologous systems, i.e. the Nin88 promoter into Arabidopsis and the AtcwINV2 promoter into tobacco. The specificities of both promoters are maintained in the heterologous backgrounds, but their activities are strongly reduced as GUS-stainings of flowers and pollen revealed and fluorometrical quantification confirmed.

Heterologous expression and functional characterization of two hybrid poplar cell-wall invertases

The expression of two hybrid poplar cell-wall invertases (EC 3.2.1.26; PaxgINV1 and PaxgINV2) were previously shown to be spatially and temporally regulated in the vegetative tissues. The expression of PaxgINV1 was linked to processes relating to dormancy, while PaxgINV2 expression was prominent in tissues undergoing growth and expansion. In an effort to further elucidate the physiological roles of these key cell wall enzymes, PaxgINV1 and PaxgINV2 were heterologously expressed in the methylotrophic yeast Pichia pastoris. Three-dimensional predictive models of the poplar invertases revealed a structural channel containing both the conserved beta-fructofuranosidase and cell-wall invertase motifs, suggesting that this channel is the putative active site of these enzymes. Recombinant PaxgINV1 and PaxgINV2 had pH optima of 4.8 and 5.6 and temperature optima of 45 and 40 degrees C, respectively. Functional characterization revealed the ability for both enzymes to hydrolyze the fructose residue of sucrose, raffinose, stachyose and verbascose, with PaxgINV2 having higher specific activity for each of the substrates tested. The K(m) values of sucrose/raffinose/stachyose were 1.7/1.8/5.0 mM for PaxgINV1 and 1.6/1.7/1.9 mM for PaxgINV2, respectively. Activity analyses in the presence of various metal cations showed that PaxgINV2 was strongly inhibited by Cu(2+), Zn(2+) and Hg(2+), while PaxgINV1 was only weakly inhibited by these cations. The results from this study, coupled with previous expression data, suggest that PaxgINV1 and PaxgINV2 have distinct roles with respect to the physiology and development of hybrid poplar, specifically phloem unloading and processes related to dormancy and bud break.

OsCYT-INV1 for alkaline/neutral invertase is involved in root cell development and reproductivity in rice (Oryza sativa L.)

A short root mutant was isolated from an EMS-generated rice mutant library. Under normal growth conditions, the mutant exhibited short root, delayed flowering, and partial sterility. Some sections of the roots revealed that the cell length along the longitudinal axis was reduced and the cell shape in the root elongation zone shrank. Genetic analysis indicated that the short root phenotype was controlled by a recessive gene. Map-based cloning revealed that a nucleotide substitution causing an amino acid change from Gly to Arg occurred in the predicted rice gene (Os02g0550600). It coded an alkaline/neutral invertase and was homologous to Arabidopsis gene AtCyt-inv1. This gene was designated as OsCyt-inv1. The results of carbohydrate analysis showed an accumulation of sucrose and reduction of hexose in the Oscyt-inv1 mutant. Exogenously supplying glucose could rescue the root growth defects of the Oscyt-inv1 mutant. These results indicated that OsCyt-inv1 played important roles in root cell development and reproductivity in rice.

Effects of season-long high temperature growth conditions on sugar-to-starch metabolism in developing microspores of grain sorghum (Sorghum bicolor L. Moench)

High temperature stress-induced male sterility is a critical problem in grain sorghum (Sorghum bicolor L. Moench) that significantly compromises crop yields. Grain sorghum plants were grown season-long under ambient (30/20 degrees C, day-time maximum/night-time minimum) and high temperature (36/26 degrees C) conditions in sunlit Soil-Plant-Atmospheric-Research (SPAR) growth chambers. We report data on the effects of high temperature on sugar levels and expression profiles of genes related to sugar-to-starch metabolism in microspore populations represented by pre- and post-meiotic "early" stages through post-mitotic "late" stages that show detectable levels of starch deposition. Microspores from high temperature stress conditions showed starch-deficiency and considerably reduced germination, translating into 27% loss in seed-set. Sugar profiles showed significant differences in hexose levels at both "early" and "late" stages at the two temperature regimes; and most notably, undetectable sucrose and approximately 50% lower starch content in "late" microspores from heat-stressed plants. Northern blot, quantitative PCR, and immunolocalization data revealed a significant reduction in the steady-state transcript abundance of SbIncw1 gene and CWI proteins in both sporophytic as well as microgametophytic tissues under high temperature conditions. Northern blot analyses also indicated greatly altered temporal expression profiles of various genes involved in sugar cleavage and utilization (SbIncw1, SbIvr2, Sh1, and Sus1), transport (Mha1 and MST1) and starch biosynthesis (Bt2, SU1, GBSS1, and UGPase) in heat-stressed plants. Collectively, these data suggest that impairment of CWI-mediated sucrose hydrolysis and subsequent lack of sucrose biosynthesis may be the most upstream molecular dysfunctions leading to altered carbohydrate metabolism and starch deficiency under elevated growth temperature conditions.

Identification of microspore-active promoters that allow targeted manipulation of gene expression at early stages of microgametogenesis in Arabidopsis

BACKGROUND

The effective functional analysis of male gametophyte development requires new tools enabling the spatially and temporally controlled expression of both marker genes and modified genes of interest. In particular, promoters driving expression at earlier developmental stages including microspores are required.

RESULTS

Transcriptomic datasets covering four progressive stages of male gametophyte development in Arabidopsis were used to select candidate genes showing early expression profiles that were male gametophyte-specific. Promoter-GUS reporter analysis of candidate genes identified three promoters (MSP1, MSP2, and MSP3) that are active in microspores and are otherwise specific to the male gametophyte and tapetum. The MSP1 and MSP2 promoters were used to successfully complement and restore the male transmission of the gametophytic two-in-one (tio) mutant that is cytokinesis-defective at first microspore division.

CONCLUSION

We demonstrate the effective application of MSP promoters as tools that can be used to elucidate gametophytic gene functions in microspores in a male-specific manner.

Promoter activity of a putative pollen monosaccharide transporter in Petunia hybrida and characterisation of a transposon insertion mutant

For the growth of the male reproductive cells of plants, the pollen, the presence of sufficient sucrose or monosaccharides is of vital importance. From Petunia hybrida a pollen-specific putative monosaccharide transporter designated PMT1 (for petunia monosaccharide transporter) has been identified previously. The present work provides an in-depth analysis and characterisation of PMT1 in the context of pollen development with the GUS reporter gene and an insertion mutant. The promoter of the pollen-specific putative PMT1 gene has been isolated by inverse PCR and sequenced. Analysis of plants transformed with the promoter-GUS fusion confirmed the specificity of this gene, belonging to the late pollen-specific expressed genes. GUS activity was detected even after 24 h of in vitro pollen germination, at the pollen tube tip. To elucidate the importance of PMT1 for gametophyte development and fertilisation, we isolated a mutant plant containing a transposon insertion in the PMT1 gene by the dTph1 transposon-tagging PCR-based assay. The PMT1 mutant contained a dTph1 insertion in position 1474 bp of the transcribing part of the gene, before the last two transmembrane-spanning domains. Analysis of the progeny of the heterozygous mutant after selfing revealed no alterations in pollen viability and fertility. Mature pollen grains of a plant homozygous for the transposon insertion were able to germinate in vitro in a medium containing sucrose, glucose, or fructose, which indicates that PMT1 is not essential for pollen survival. Several explanations for these results are discussed in the present work.

Gibberellin-dependent induction of tomato extracellular invertase Lin7 is required for pollen development

The tomato extracellular invertase family comprises four members with different expression patterns. Among the three invertase isoenzymes expressed in floral tissues, Lin5, Lin6 and Lin7, the expression of Lin7 was previously shown to be restricted to the tapetum and pollen. Histochemical analysis of β-glucuronidase (GUS) reporter activity shows Lin7 expression in pollen and pollen tubes of corresponding transgenic plants. The physiological relevance of the identification of gibberellin-responsive cis-acting elements for induction of the Lin7 promoter is supported by the repression of Lin7 expression in pollen grains by the gibberellin biosynthesis inhibitor paclobutrazol. Functional approaches with transgenic tomato plants establish a link between gibberellin action and invertase function in the tapetum for pollen development: both tissue-specific antisense repression of extracellular Lin7 and ectopic inactivation of the biologically active GAs by expression of a GA2-oxidase under control of the Lin7 promoter result in germination deficient pollen. These complementary findings support the idea that the GA requirement of pollen development, pollen germination and pollen tube growth are linked to energy metabolism via the regulation of an extracellular invertase as a key enzyme for carbohydrate supply via an apoplasmic pathway.

Molecular cloning and expression analysis of the cell-wall invertase gene family in rice (Oryza sativa L.)

Cell-wall invertase (CIN) catalyzes the hydrolysis of sucrose into glucose and fructose for the supply of carbohydrates to sink organs via an apoplastic pathway. To study the CIN genes in rice (Oryza sativa L.), we isolated cDNA clones showing amino acid similarity to the plant cell wall invertase proteins from a search of rice sequence databases. Profile analyses revealed that the cloned genes are expressed in unique patterns in various organs. For example, transcripts of OsCIN1, OsCIN2, OsCIN4, and OsCIN7 were detected in immature seeds whereas OsCIN3 gene expression was flower-specific. Further transcript analysis of these genes expressed in developing seeds indicated that OsCIN1, OsCIN2, and OsCIN7 might play an important role involving sucrose partitioning to the embryo and endosperm. Sucrose, a substrate of CINs, induced the accumulation of OsCIN1 transcripts in excised leaves and OsCIN2 in immature seeds, while the level of OsCIN5 was significantly down-regulated in excised leaves treated with sucrose. Infecting the tissues with rice blast (Magnaporthe grisea) as a biotic stressor increased the expression of OsCIN1, OsCIN4, and OsCIN5, suggesting that these genes may participate in a switch in metabolism to resist pathogen invasion. These results demonstrate that OsCIN genes play diverse roles involving the regulation of metabolism, growth, development, and stress responses.

AtSTP11, a pollen tube-specific monosaccharide transporter in Arabidopsis

Pollen development, as well as pollen germination and pollen tube growth, requires a highly regulated supply of sugars. In this paper we describe the molecular, kinetic, and physiological characterization of AtSTP11, a new member of the H+/monosaccharide transporter family in Arabidopsis thaliana (L.) Heynh. Heterologous expression in yeast (Saccharomyces cerevisiae) showed that AtSTP11 is a high-affinity (Km = 25 microM), broad-spectrum, and uncoupler-sensitive monosaccharide transporter of the plasma membrane. In reverse transcription-polymerase chain reaction analyses we found that AtSTP11 expression is restricted to flowers. Furthermore, AtSTP11-promoter::GFP plants revealed that AtSTP11 expression is only found in pollen tubes. Using a specific antibody we could also detect the AtSTP11 protein exclusively in pollen tubes but not in other flower tissues or in pollen grains of any developmental stage. These results suggest that the newly identified AtSTP11 transporter plays a role in the supply of monosaccharides to growing pollen tubes.

DNA variation at the invertase locus invGE/GF is associated with tuber quality traits in populations of potato breeding clones

Starch and sugar content of potato tubers are quantitative traits, which are models for the candidate gene approach for identifying the molecular basis of quantitative trait loci (QTL) in noninbred plants. Starch and sugar content are also important for the quality of processed products such as potato chips and French fries. A high content of the reducing sugars glucose and fructose results in inferior chip quality. Tuber starch content affects nutritional quality. Functional and genetic models suggest that genes encoding invertases control, among other things, tuber sugar content. The invGE/GF locus on potato chromosome IX consists of duplicated invertase genes invGE and invGF and colocalizes with cold-sweetening QTL Sug9. DNA variation at invGE/GF was analyzed in 188 tetraploid potato cultivars, which have been assessed for chip quality and tuber starch content. Two closely correlated invertase alleles, invGE-f and invGF-d, were associated with better chip quality in three breeding populations. Allele invGF-b was associated with lower tuber starch content. The potato invertase gene invGE is orthologous to the tomato invertase gene Lin5, which is causal for the fruit-sugar-yield QTL Brix9-2-5, suggesting that natural variation of sugar yield in tomato fruits and sugar content of potato tubers is controlled by functional variants of orthologous invertase genes.

Expression analysis of a chicory fructan 1-exohydrolase gene reveals complex regulation by cold

The gene for a recently identified cDNA, 1-FEH IIa, encoding a fructan 1-exohydrolase was isolated and cloned from Cichorium intybus and a 1149 bp promoter fragment was characterized. An analysis of the genomic 1-FEH IIa sequence indicated that the gene (FEHIIa) consists of six introns and seven exons, which is similar to plant invertase genes. Like invertase genes, FEHIIa also contains the 9 nt mini-exon encoding the tripeptide DPN. A database search for cis-acting response elements within its promoter identified multiple elements that appear to have relevance to cold-induced expression of the gene in field-grown roots. Promoter analysis by transient expression assay demonstrated that the FEHIIa gene promoter is highly expressed in etiolated Cichorium leaves and cold-stored roots, which correlated well with the high level expression detected by RNA blot analysis. Cold also enhanced FEHIIa reporter gene expression in green leaves, however, the reporter gene activity was much lower compared with similar induction experiments in etiolated leaves. Promoter deletion analysis demonstrated the presence of potential cold-responsive ABRE and/or CRT/DRE elements in the -22 to -172 region, while regions -933 to -717 and -493 to -278 contain elements that can down-regulate expression at the conditions used. Characterization of the FEHIIa promoter may provide tools to study cold-induced expression and to increase freezing tolerance in agricultural crops.

Linkage disequilibrium mapping of a Verticillium dahliae resistance quantitative trait locus in tetraploid potato ( Solanum tuberosum) through a candidate gene approach

We have used the linkage disequilibrium mapping method to test for an association between a candidate gene marker and resistance to Verticillium dahliae in tetraploid potato. A probe derived from the tomato Verticillium resistance gene ( Ve1) identified homologous sequences ( StVe1) in potato, which in a diploid population map to chromosome 9, in a position analogous to that of the tomato resistance gene. When a molecular marker closely linked (1.5 cM) to the homologues was used as a candidate gene marker on 137 tetraploid potato genotypes (mostly North American cultivars), the association between the marker and resistance was confirmed ( P<0.001). The amount of phenotypic variation in resistance explained by the allele of the STM1051 marker was greater than 10% and 25% in two subpopulations that were inferred from coancestry data matrix. Cloning of homologues from the highly resistant potato cv. Reddale indicates that the resistance quantitative trait locus (QTL) comprises at least an eleven-member family, encoding plant-specific leucine-rich repeat proteins highly similar to the tomato Ve genes. The sequence analysis shows that all homologues are uninterrupted open reading frames and thus represent putative functional resistance genes. This is the first time that the linkage disequilibrium method has been used to find an association between a resistance gene and a candidate gene marker in tetraploid potato. We have shown that it is possible to map QTL directly on already available potato cultivars, without developing a new mapping population.

Linkage disequilibrium mapping of a Verticillium dahliae resistance quantitative trait locus in tetraploid potato ( Solanum tuberosum) through a candidate gene approach

We have used the linkage disequilibrium mapping method to test for an association between a candidate gene marker and resistance to Verticillium dahliae in tetraploid potato. A probe derived from the tomato Verticillium resistance gene ( Ve1) identified homologous sequences ( StVe1) in potato, which in a diploid population map to chromosome 9, in a position analogous to that of the tomato resistance gene. When a molecular marker closely linked (1.5 cM) to the homologues was used as a candidate gene marker on 137 tetraploid potato genotypes (mostly North American cultivars), the association between the marker and resistance was confirmed ( P<0.001). The amount of phenotypic variation in resistance explained by the allele of the STM1051 marker was greater than 10% and 25% in two subpopulations that were inferred from coancestry data matrix. Cloning of homologues from the highly resistant potato cv. Reddale indicates that the resistance quantitative trait locus (QTL) comprises at least an eleven-member family, encoding plant-specific leucine-rich repeat proteins highly similar to the tomato Ve genes. The sequence analysis shows that all homologues are uninterrupted open reading frames and thus represent putative functional resistance genes. This is the first time that the linkage disequilibrium method has been used to find an association between a resistance gene and a candidate gene marker in tetraploid potato. We have shown that it is possible to map QTL directly on already available potato cultivars, without developing a new mapping population.

Functional characterization and expression analyses of the glucose-specific AtSTP9 monosaccharide transporter in pollen of Arabidopsis

A genomic clone and the corresponding cDNA of a new Arabidopsis monosaccharide transporter AtSTP9 were isolated. Transport analysis of the expressed protein in yeast showed that AtSTP9 is an energy-dependent, uncoupler-sensitive, high-affinity monosaccharide transporter with a K(m) for glucose in the micromolar range. In contrast to all previously characterized monosaccharide transporters, AtSTP9 shows an unusual specificity for glucose. Reverse transcriptase-polymerase chain reaction analyses revealed that AtSTP9 is exclusively expressed in flowers, and a more detailed approach using AtSTP9 promoter/reporter plants clearly showed that AtSTP9 expression is restricted to the male gametophyte. AtSTP9 expression is not found in other floral organs or vegetative tissues. Further localization on the cellular level using a specific antibody revealed that in contrast to the early accumulation of AtSTP9 transcripts in young pollen, the AtSTP9 protein is only found weakly in mature pollen but is most prominent in germinating pollen tubes. This preloading of pollen with mRNAs has been described for genes that are essential for pollen germination and/or pollen tube growth. The pollen-specific expression found for AtSTP9 is also observed for other sugar transporters and indicates that pollen development and germination require a highly regulated supply of sugars.

Functional divergence of a syntenic invertase gene family in tomato, potato, and Arabidopsis

Comparative analysis of complex developmental pathways depends on our ability to resolve the function of members of gene families across taxonomic groups. LIN5, which belongs to a small gene family of apoplastic invertases in tomato (Lycopersicon esculentum), is a quantitative trait locus that modifies fruit sugar composition. We have compared the genomic organization and expression of this gene family in the two distantly related species: tomato and Arabidopsis. Invertase family members reside on segmental duplications in the near-colinear genomes of tomato and potato (Solanum tuberosum). These chromosomal segments are syntenically duplicated in the model plant Arabidopsis. On the basis of phylogenetic analysis of genes in the microsyntenic region, we conclude that these segmental duplications arose independently after the separation of the tomato/potato clade from Arabidopsis. Rapid regulatory divergence is characteristic of the invertase family. Interestingly, although the processes of gene duplication and specialization of expression occurred separately in the two species, synteny-based orthologs from both clades acquired similar organ-specific expression. This similar expression pattern of the genes is evidence of comparable evolutionary constraints (parallel evolution) rather than of functional orthology. The observation that functional orthology cannot be identified through analysis of expression similarity highlights the caution that needs to be exercised in extrapolating developmental networks from a model organism.

Extracellular invertase: key metabolic enzyme and PR protein

Extracellular invertase is the key enzyme of an apoplasmic phloem unloading pathway and catalyses the hydrolytic cleavage of the transport sugar sucrose released into the apoplast. This mechanism contributes to long-distance assimilate transport, provides the substrate to sustain heterotrophic growth and generates metabolic signals known to effect various processes of primary metabolism and defence responses. The essential function of extracellular invertase for supplying carbohydrates to sink organs was demonstrated by the finding that antisense repression of an anther-specific isoenzyme provides an efficient method for metabolic engineering of male sterility. The regulation of extracellular invertase by all classes of phytohormones indicates an essential link between the molecular mechanism of phytohormone action and primary metabolism. The up-regulation of extracellular invertase appears to be a common response to various biotic and abiotic stress-related stimuli such as pathogen infection and salt stress, in addition to specific stress-related reactions. Based on the observed co-ordinated regulation of source/sink relations and defence responses by sugars and stress-related stimuli, the identified activation of distinct subsets of MAP kinases provides a mechanism for signal integration and distribution within such complex networks. Sucrose derivatives not synthesized by higher plants, such as turanose, were shown to elicit responses distinctly different from metabolizable sugars and are rather perceived as stress-related stimuli.

AtSTP6, a new pollen-specific H+-monosaccharide symporter from Arabidopsis

This paper describes the molecular, kinetic, and physiological characterization of AtSTP6, a new member of the Arabidopsis H(+)/monosaccharide transporter family. The AtSTP6 gene (At3g05960) is interrupted by two introns and encodes a protein of 507 amino acids containing 12 putative transmembrane helices. Expression in yeast (Saccharomyces cerevisiae) shows that AtSTP6 is a high-affinity (K(m) = 20 microM), broad-spectrum, and uncoupler-sensitive monosaccharide transporter that is targeted to the plasma membrane and that can complement a growth deficiency resulting from the disruption of most yeast hexose transporter genes. Analyses of AtSTP6-promoter::GUS plants and in situ hybridization experiments detected AtSTP6 expression only during the late stages of pollen development. A transposon-tagged Arabidopsis mutant was isolated and homozygous plants were analyzed for potential effects of the Atstp6 mutation on pollen viability, pollen germination, fertilization, and seed production. However, differences between wild-type and mutant plants could not be observed.

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.

Differential expression of invertase genes in internal and external phloem tissues of potato (Solanum tuberosum L.)

The cloning of promoter sequences of two invertase genes from potato (Solanum tuberosum L.) is described. Histochemical analysis of series of reporter transgenic lines reveals phloem-specific expression from both promoters, with one expressed preferentially in internal phloem and the other in external phloem of stem vascular bundles.