A sepal-expressed ADP-glucose pyrophosphorylase gene (NtAGP) is required for petal expansion growth in 'Xanthi' tobacco

Outstanding questions in flower metabolism

The great diversity of flowers, their color, odor, taste, and shape, is mostly a result of the metabolic processes that occur in this reproductive organ when the flower and its tissues develop, grow, and finally die. Some of these metabolites serve to advertise flowers to animal pollinators, other confer protection towards abiotic stresses, and a large proportion of the molecules of the central metabolic pathways have bioenergetic and signaling functions that support growth and the transition to fruits and seeds. Although recent studies have advanced our general understanding of flower metabolism, several questions still await an answer. Here, we have compiled a list of open questions on flower metabolism encompassing molecular aspects, as well as topics of relevance for agriculture and the ecosystem. These questions include the study of flower metabolism through development, the biochemistry of nectar and its relevance to promoting plant-pollinator interaction, recycling of metabolic resources after flowers whiter and die, as well as the manipulation of flower metabolism by pathogens. We hope with this review to stimulate discussion on the topic of flower metabolism and set a reference point to return to in the future when assessing progress in the field.

The AGPase Family Proteins in Banana: Genome-Wide Identification, Phylogeny, and Expression Analyses Reveal Their Involvement in the Development, Ripening, and Abiotic/Biotic Stress Responses

ADP-glucose pyrophosphorylase (AGPase) is the first rate-limiting enzyme in starch biosynthesis and plays crucial roles in multiple biological processes. Despite its importance, AGPase is poorly studied in starchy fruit crop banana (Musa acuminata L.). In this study, eight MaAGPase genes have been identified genome-wide in M. acuminata, which could be clustered into the large (APL) and small (APS) subunits. Comprehensive transcriptomic analysis revealed temporal and spatial expression variations of MaAPLs and MaAPSs and their differential responses to abiotic/biotic stresses in two banana genotypes, Fen Jiao (FJ) and BaXi Jiao (BX). MaAPS1 showed generally high expression at various developmental and ripening stages and in response to abiotic/biotic stresses in both genotypes. MaAPL-3 and -2a were specifically induced by abiotic stresses including cold, salt, and drought, as well as by fungal infection in FJ, but not in BX. The presence of hormone-related and stress-relevant cis-acting elements in the promoters of MaAGPase genes suggests that MaAGPases may play an important role in multiple biological processes. Taken together, this study provides new insights into the complex transcriptional regulation of AGPases, underlying their key roles in promoting starch biosynthesis and enhancing stress tolerance in banana.

Proteomic insights into floral biology

The flower is the most important biological structure for ensuring angiosperms reproductive success. Not only does the flower contain critical reproductive organs, but the wide variation in morphology, color, and scent has evolved to entice specialized pollinators, and arguably mankind in many cases, to ensure the successful propagation of its species. Recent proteomic approaches have identified protein candidates related to these flower traits, which has shed light on a number of previously unknown mechanisms underlying these traits. This review article provides a comprehensive overview of the latest advances in proteomic research in floral biology according to the order of flower structure, from corolla to male and female reproductive organs. It summarizes mainstream proteomic methods for plant research and recent improvements on two dimensional gel electrophoresis and gel-free workflows for both peptide level and protein level analysis. The recent advances in sequencing technologies provide a new paradigm for the ever-increasing genome and transcriptome information on many organisms. It is now possible to integrate genomic and transcriptomic data with proteomic results for large-scale protein characterization, so that a global understanding of the complex molecular networks in flower biology can be readily achieved. This article is part of a Special Issue entitled: Plant Proteomics--a bridge between fundamental processes and crop production, edited by Dr. Hans-Peter Mock.

Function of leafy sepals in Paris polyphylla: photosynthate allocation and partitioning to the fruit and rhizome

Paris polyphylla Smith var. yunnanensis (Franch.) Hand.-Mazz. is a rhizomatous, herbaceous, perennial plant that is used as a medicinal plant with a variety of pharmacological activities. However, the functions of the green, leafy sepal of this plant are poorly understood. The main objectives of this study were to: (a) test the hypothesis that sepals make measurable contributions to fruit development and rhizome growth; and (b) investigate the allocation and partitioning of photosynthates produced by sepals and leaves to fruit and rhizome. Net photosynthetic rate, photosynthetic pigment composition and δ13C values were similar for sepals and leaves. Sepal-darkening and sepal-removal treatments resulted in smaller fruit size and decreased rhizome biomass, whereas fruit removal led to a decrease in calyx size and an increase in rhizome yield and saponin content. Fruit and seed mass were positively and linearly related to calyx size. These results indicate that photosynthates produced by sepals are involved in the fruit growth and seed development and that developing fruit and rhizomes compete for the photosynthates exported by leaves. We propose that the sepals of P. polyphylla function partly as leaves to compensate for reproductive costs. Fruit removal increased carbon partitioning to the rhizome and improved rhizome yield and quality, offering a useful strategy for the domestication of this valuable medicinal plant.

Metabolic and enzymatic changes associated with carbon mobilization, utilization and replenishment triggered in grain amaranth (Amaranthus cruentus) in response to partial defoliation by mechanical injury or insect herbivory

BACKGROUND

Amaranthus cruentus and A. hypochondriacus are crop plants grown for grain production in subtropical countries. Recently, the generation of large-scale transcriptomic data opened the possibility to study representative genes of primary metabolism to gain a better understanding of the biochemical mechanisms underlying tolerance to defoliation in these species. A multi-level approach was followed involving gene expression analysis, enzyme activity and metabolite measurements.

RESULTS

Defoliation by insect herbivory (HD) or mechanical damage (MD) led to a rapid and transient reduction of non-structural carbohydrates (NSC) in all tissues examined. This correlated with a short-term induction of foliar sucrolytic activity, differential gene expression of a vacuolar invertase and its inhibitor, and induction of a sucrose transporter gene. Leaf starch in defoliated plants correlated negatively with amylolytic activity and expression of a β-amylase-1 gene and positively with a soluble starch synthase gene. Fatty-acid accumulation in roots coincided with a high expression of a phosphoenolpyruvate/phosphate transporter gene. In all tissues there was a long-term replenishment of most metabolite pools, which allowed damaged plants to maintain unaltered growth and grain yield. Promoter analysis of ADP-glucose pyrophosphorylase and vacuolar invertase genes indicated the presence of cis-regulatory elements that supported their responsiveness to defoliation. HD and MD had differential effects on transcripts, enzyme activities and metabolites. However, the correlation between transcript abundance and enzymatic activities was very limited. A better correlation was found between enzymes, metabolite levels and growth and reproductive parameters.

CONCLUSIONS

It is concluded that a rapid reduction of NSC reserves in leaves, stems and roots followed by their long-term recovery underlies tolerance to defoliation in grain amaranth. This requires the coordinate action of genes/enzymes that are differentially affected by the way leaf damage is performed. Defoliation tolerance in grain is a complex process that can't be fully explained at the transcriptomic level only.

Knockdown of MYB305 disrupts nectary starch metabolism and floral nectar production

MYB transcription factors have important roles during floral organ development. In this study, we generated myb305 RNAi knockdown tobacco plants and studied the role of MYB305 in the growth of the floral nectary. We have previously shown the MYB305 regulates the expression of flavonoid metabolic genes as well as of nectar proteins (nectarins); however, the myb305 plants showed other floral phenotypes that we investigate in these studies. The nectaries of myb305 plants show juvenile character at late stages of development and secrete reduced levels of nectar. Because starch metabolism is intimately involved in nectar secretion and is strongly regulated during normal nectary development, we examined the accumulation of starch in the nectaries of the myb305 plants. The myb305 plants accumulated lower levels of starch in their nectaries than did wild-type plants. The reduced starch correlated with the reduced expression of the ATP-glucose pyrophosphorylase (small subunit) gene in nectaries of the myb305 plants during the starch biosynthetic phase. Expression of genes encoding several starch-degrading enzymes including β-amylase, isoamylase 3, and α-amylase was also reduced in the myb305 plants. In addition to regulating nectarin and flavonoid metabolic gene expression, these results suggest that MYB305 may also function in the tobacco nectary maturation program by controlling the expression of starch metabolic genes.

Differential expression of genes identified by suppression subtractive hybridization in petals of opening carnation flowers

Flower opening is an event accompanied by morphological changes in petals which include elongation, expansion, and outward-curving. Petal cell growth is a fundamental process that underlies such phenomena, but its molecular mechanism remains largely unknown. Suppression subtractive hybridization was performed between petals during the early elongation period (stage 1) and during the opening period (stage 5) in carnation flowers and a pair of subtraction libraries abundant in differentially expressed genes was constructed at each stage. 393 cDNA clones picked up by differential screening out of 1728 clones were sequenced and 235 different cDNA fragments were identified, among which 211 did not match any known nucleotide sequence of carnation genes in the databases. BLASTX search of nucleotide sequences revealed that putative functions of the translational products can be classified into several categories including transcription, signalling, cell wall modification, lipid metabolism, and transport. Open reading frames of 15 selected genes were successfully determined by rapid amplification of cDNA ends (RACE). Time-course analysis of these genes by real-time RT-PCR showed that transcript levels of several genes correlatively fluctuate in petals of opening carnation flowers, suggesting an association with the morphological changes by elongation or curving. Based on the results, it is suggested that the growth of carnation petals is controlled by co-ordinated gene expression during the progress of flower opening. In addition, the possible roles of some key genes in the initiation of cell growth, the construction of the cell wall and cuticle, and transport across membranes were discussed.

Characterization of an ADP-glucose pyrophosphorylase small subunit gene expressed in developing cotton (Gossypium hirsutum) fibers

ADP-glucose pyrophosphorylase (ADPGp, EC 2.7.7.27) is a tetrameric protein composed of two small and two large subunits that catalyzes the biosynthesis of ADP-glucose from glucose-phosphate which is used to provide the glucose subunits for starch biosynthesis. A second cotton gene encoding an ADPGp small subunit has been cloned and characterized. The gene contains eight introns similar to previously reported potato and cotton ADPGp small subunit genes. The deduced translation of the gene contained a poorly conserved transit peptide and well conserved catalytic and regulatory elements typical of other plant ADPGps. The 5' end of the mRNA was cloned and sequenced to identify the transcriptional start site (TSS). The promoter region upstream of the TSS did not contain the core promoter sequence in the typical positions indicating this gene may not use a standard core promoter. Other sequence motifs associated with tissue specific expression and phytohormone response were present. Reverse transcription (RT)-PCR with gene specific primers identified the sites of expression of this gene. Expression was most abundant in the meristem region, and immature stem and relatively lower in starch accumulating roots demonstrating that this gene has a different pattern of expression than the previously reported cotton ADPGp small subunit gene. Additionally this gene was differentially expressed in cotton fibers. The presence of starch was confirmed in developing cotton fibers suggesting that starch metabolism plays a role in cotton fiber development.

Comparative analyses of the proteomes of leaves and flowers at various stages of development reveal organ-specific functional differentiation of proteins in soybean

The functional differentiation of protein networks in individual organs and tissues of soybean at various developmental stages was investigated by proteomic approach. Protein extraction by Mg/NP-40 buffer followed by alkaline phenol-based method was optimized for proteomic analysis. Proteome analyses of leaves at various developmental stages showed 26 differentially expressed proteins, wherein proteins in translocon at the outer/inner envelope membrane of chloroplast protein-transport machineries increased significantly at the first trifoliate. Immunoblot analysis showed chaperonin-60 expressed abundantly in young leaves, whereas HSP 70 and ATP-synthase beta were constitutively expressed in all tissues. The net photosynthesis rate and chlorophyll content showed an age-dependent correlation in leaves. These results suggest that proteins involved in carbon assimilation, folding and assembly, and energy may work synchronously and show a linear correlation to photosynthesis at developmental stages of leaves. Comparison of flower bud and flower proteome reveals 29 differentially expressed proteins, wherein proteins involved in mitochondrial protein transport and assembly, secondary metabolism, and pollen-tube growth were up-regulated during flower development. Together, these results suggest that during developmental stages, each type of tissue is associated with a specific group of proteins; wherein proteins involved in energy, sugar metabolism, and folding, assembly, and destination may play pivotal roles in the maturation process of each organ or tissue.

Non-targeted analysis of spatial metabolite composition in strawberry (Fragariaxananassa) flowers

Formation of flower organs and the subsequent pollination process require a coordinated spatial and temporal regulation of particular metabolic pathways. In this study a comparison has been made between the metabolite composition of individual flower organs of strawberry (Fragariaxananassa) including the petal, sepal, stamen, pistil and the receptacle that gives rise to the strawberry fruit. Non-targeted metabolomics analysis of the semi-polar secondary metabolites by the use of UPLC-qTOF-MS was utilized in order to localize metabolites belonging to various chemical classes (e.g. ellagitannins, proanthocyanidins, flavonols, terpenoids, and spermidine derivatives) to the different flower organs. The vast majority of the tentatively identified metabolites were ellagitannins that accumulated in all five parts of the flower. Several metabolite classes were detected predominantly in certain flower organs, as for example spermidine derivatives were present uniquely in the stamen and pistil, and the proanthocyanidins were almost exclusively detected in the receptacle and sepals. The latter organ was also rich in terpenoids (i.e. triterpenoid and sesquiterpenoid derivatives) whereas phenolic acids and flavonols were the predominant classes of compounds detected in the petals. Furthermore, we observed extensive variation in the accumulation of metabolites from the same class in a single organ, particularly in the case of ellagitannins, and the flavonols quercetin, kaempferol and isorhamnetin. These results allude to spatially-restricted production of secondary metabolite classes and specialized derivatives in flowers that take part in implementing the unique program of individual organs in the floral life cycle.