Impact of Susceptibility on Plant Hormonal Composition during Clubroot Disease Development in Canola (Brassica napus)

Clubroot, caused by Plasmodiophora brassicae, is a soilborne disease of crucifers associated with the formation of large root galls. This root enlargement suggests modulation of plant hormonal networks by the pathogen, stimulating cell division and elongation and influencing host defense. We studied physiological changes in two Brassica napus cultivars, including plant hormone profiles-salicylic acid (SA), jasmonic acid (JA), abscisic acid (ABA), the auxin indole-3-acetic acid (IAA), and the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC)-along with their selected derivatives following inoculation with virulent and avirulent P. brassicae pathotypes. In susceptible plants, water uptake declined from the initial appearance of root galls by 21 days after inoculation, but did not have a significant effect on photosynthetic rate, stomatal conductance, or leaf chlorophyll levels. Nonetheless, a strong increase in ABA levels indicated that hormonal mechanisms were triggered to cope with water stress due to the declining water uptake. The free SA level in the roots increased strongly in resistant interactions, compared with a relatively minor increase during susceptible interactions. The ratio of conjugated SA to free SA was higher in susceptible interactions, indicating that resistant interactions are linked to the plant's ability to maintain higher levels of bioactive free SA. In contrast, JA and its biologically active form JA-Ile declined up to 7-fold in susceptible interactions, while they were maintained during resistant interactions. The ACC level increased in the roots of inoculated plants by 21 days, irrespective of clubroot susceptibility, indicating a role of ethylene in response to pathogen interactions that is independent of disease severity. IAA levels at early and later infection stages were lower only in susceptible plants, suggesting a modulation of auxin homeostasis by the pathogen relative to the host defense system.

Auxin receptors as integrators of developmental and hormonal signals during reproductive development in pea

Auxins regulate many aspects of plant growth and development. In pea, three of the five TIR1/AFB members (PsTIR1a, PsTIR1b, and PsAFB2) have been implicated in auxin-related responses during fruit/seed development; however, the roles of PsAFB4 and PsAFB6 in these processes are unknown. Using yeast two-hybrid assays, we found that all five pea TIR1/AFB receptor proteins interacted with the pea AUX/IAAs PsIAA6 and/or PsIAA7 in an auxin-dependent manner, a requirement for functional auxin receptors. All five auxin receptors are expressed in young ovaries (pericarps) and rapidly developing seeds, with overlapping and unique developmental and hormone-regulated gene expression patterns. Pericarp PsAFB6 expression was suppressed by seeds and increased in response to deseeding, and exogenous hormone treatments suggest that seed-derived auxin and deseeding-induced ethylene are involved in these responses, respectively. Ethylene-induced elevation of pericarp PsAFB6 expression was associated with 4-Cl-IAA-specific reduction in ethylene responsiveness. In developing seeds, expression of PsTAR2 and PsYUC10 auxin biosynthesis genes was associated with high auxin levels in seed coat and cotyledon tissues, and PsAFB2 dominated the seed tissue transcript pool. Overall, auxin receptors had overlapping and unique developmental and hormone-regulated gene expression patterns during fruit/seed development, suggesting mediation of diverse responses to auxin, with PsAFB6 linking auxin and ethylene signaling.

Balancing of hormonal biosynthesis and catabolism pathways, a strategy to ameliorate the negative effects of heat stress on reproductive growth

In pea (Pisum sativum L.), moderate heat stress during early flowering/fruit set increased seed/ovule abortion, and concomitantly produced fruits with reduced ovary (pericarp) length, and fewer seeds at maturity. Plant hormonal networks coordinate seed and pericarp growth and development. To determine if these hormonal networks are modulated in response to heat stress, we analyzed the gene expression patterns and associated these patterns with precursors, and bioactive and inactive metabolites of the auxin, gibberellin (GA), abscisic acid (ABA), and ethylene biosynthesis/catabolism pathways in young developing seeds and pericarps of non-stressed and 4-day heat-stressed fruits. Our data suggest that within the developing seeds heat stress decreased bioactive GA levels reducing GA growth-related processes, and that increased ethylene levels may have promoted this inhibitory response. In contrast, heat stress increased auxin biosynthesis gene expression and auxin levels in the seeds and pericarps, and seed ABA levels, both effects can increase seed sink strength. We hypothesize that seeds with higher auxin- and ABA-induced sink strength and adequate bioactive GA levels will set and continue to grow, while the seeds with lower sink strength (low auxin, ABA, and GA levels) will become more sensitive to heat stress-induced ethylene leading to ovule/seed abortion.

Seed-specific down-regulation of Arabidopsis CELLULOSE SYNTHASE 1 or 9 reduces seed cellulose content and differentially affects carbon partitioning

Seed-specific down-regulation of AtCESA1 and AtCESA9, which encode cellulose synthase subunits, differentially affects seed storage compound accumulation in Arabidopsis. High amounts of cellulose can negatively affect crop seed quality, and, therefore, diverting carbon partitioning from cellulose to oil, protein and/or starch via molecular breeding may improve seed quality. To determine the effect of seed cellulose content reduction on levels of storage compounds, Arabidopsis thaliana CELLULOSE SYNTHASE1 (AtCESA1) and AtCESA9 genes, which both encode cellulose synthase subunits, were individually down-regulated using seed-specific intron-spliced hairpin RNA (hpRNAi) constructs. The selected seed-specific AtCESA1 and AtCESA9 Arabidopsis RNAi lines displayed reduced cellulose contents in seeds, and exhibited no obvious visual phenotypic growth defects with the exception of a minor effect on early root development in AtCESA1 RNAi seedlings and early hypocotyl elongation in the dark in both types of RNAi line. The seed-specific down-regulation of AtCESA9 resulted in a reduction in seed weight compared to empty vector controls, which was not observed in AtCESA1 RNAi lines. In terms of effects on carbon partitioning, AtCESA1 and AtCESA9 RNAi lines exhibited distinct effects. The down-regulation of AtCESA1 led to a ~ 3% relative increase in seed protein content (P = 0.04) and a ~ 3% relative decrease in oil content (P = 0.02), but caused no alteration in soluble glucose levels. On the contrary, AtCESA9 RNAi lines did not display a significant reduction in seed oil, protein or soluble glucose content. Taken together, our results indicate that the seed-specific down-regulation of AtCESA1 causes alterations in seed storage compound accumulation, while the effect of AtCESA9 on carbon partitioning is absent or minor in Arabidopsis.

A transferase interactome that may facilitate channeling of polyunsaturated fatty acid moieties from phosphatidylcholine to triacylglycerol

Polyunsaturated fatty acids (PUFAs) such as α-linolenic acid (ALA, 18:3Δ^{9 cis ,12 cis ,15 cis} ) have high nutritional and industrial values. In oilseed crops, PUFAs are synthesized on phosphatidylcholine (PC) and accumulated in triacylglycerol (TAG). Therefore, exploring the mechanisms that route PC-derived PUFA to TAG is essential for understanding and improving PUFA production. The seed oil of flax (Linum usitatissimum) is enriched in ALA, and this plant has many lipid biosynthetic enzymes that prefer ALA-containing substrates. In this study, using membrane yeast two-hybrid and bimolecular fluorescence complementation assays, we probed recombinant flax transferase enzymes, previously shown to contribute to PUFA enrichment of TAG, for physical interactions with each other under in vivo conditions. We found that diacylglycerol acyltransferases, which catalyze the final reaction in acyl-CoA-dependent TAG biosynthesis, interact with the acyl-editing enzymes phosphatidylcholine: diacylglycerol cholinephosphotransferase, and lysophosphatidylcholine acyltransferase. Physical interactions among the acyl-editing enzymes were also identified. These findings reveal the presence of an assembly of interacting transferases that may facilitate the channeling of PUFA from PC to TAG in flax and possibly also in other oleaginous plants that produce seeds enriched in PC-modified fatty acids.

Pea polyphenolics and hydrolysis processing alter microbial community structure and early pathogen colonization in mice

Health benefits associated with pea consumption have been attributed to the fiber and polyphenolic content concentrated within the pea seed coat. However, the amount of pea polyphenols can vary between cultivars, and it has yet to be studied whether pea polyphenols impact the intestinal microbiota. We hypothesized that pea polyphenols promote a healthy microbiome that supports intestinal integrity and pathogen colonization resistance. To investigate the effects of pea polyphenols, pea cultivars rich and poor in proanthocyanidins were supplemented in raw or acid hydrolyzed form to an isocaloric diet in mice. Acid hydrolysis increases the absorption of pea polyphenols by cleaving polymeric proanthocyanidins to their readily absorbable anthocyanidin monomers. After 3 weeks of diet, mice were challenged with Citrobacter rodentium and pathogen colonization and inflammation were assessed. Counter to our hypothesis, pea seed coat fraction supplementation, especially the non-hydrolyzed proanthocyanidin-rich fraction diet adversely increased C. rodentium pathogen load and inflammation. Ileal, cecal and colon microbial communities were notably distinct between pea seed cultivar and hydrolysis processing. The consumption of intact proanthocyanidins decreased microbial diversity indicating that proanthocyanidins have antimicrobial properties. Together our results indicate supplementation of raw pea seed coat rich in proanthocyanidins adversely affect intestinal integrity. However, acid hydrolysis processing restored community structure and colonization resistance, and the anthocyanidin-rich fractions reduced weight gain on a high fat diet. Establishing a clear understanding of the effects of pea fiber and polyphenolic form on health will help to develop research-based pea products and dietary recommendations.

TIR1 auxin receptors are implicated in the differential response to 4-Cl-IAA and IAA in developing pea fruit

The auxins indole-3-acetic acid (IAA) and 4-chloroindole-3-acetic acid (4-Cl-IAA) occur naturally in pea (Pisum sativum); however, only 4-Cl-IAA mimics the presence of seeds in stimulating pericarp growth. To examine if this differential auxin effect is mediated through TIR1/AFB auxin receptors, pea TIR1 and AFB2 homologs were functionally characterized in Arabidopsis, and receptor expression, and auxin distribution and action were profiled in developing pea fruits. PsTIR1a, PsTIR1b, and PsAFB2 restored the auxin-sensitive root growth response to the mutant Arabidopsis seedlings Attir1-10 and/or Attir1-10 afb2-3. Expression of PsTIR1 or AtTIR1 in Attir1-10 afb2-3 mutants also restored the greater root inhibitory response of 4-Cl-IAA compared to that of IAA, implicating TIR1 receptors in this response. The ability of 4-Cl-IAA to stimulate a stronger DR5::GUS auxin response than IAA at the same concentration in pea pericarps was associated with its ability to enrich the auxin-receptor transcript pool with PsTIR1a and PsAFB2 by decreasing the transcript abundance of PsTIR1b (mimicking results in pericarps with developing seeds). Therefore, the markedly different effect of IAA and 4-Cl-IAA on pea fruit growth may at least partially involve TIR1/AFB receptors and the differential modulation of their population, resulting in specific Aux/IAA protein degradation that leads to an auxin-specific tissue response.

Heat stress differentially modifies ethylene biosynthesis and signaling in pea floral and fruit tissues

KEY MESSAGE

Ethylene biosynthesis is regulated in reproductive tissues in response to heat stress in a manner to optimize resource allocation to pollinated fruits with developing seeds. High temperatures during reproductive development are particularly detrimental to crop fruit/seed production. Ethylene plays vital roles in plant development and abiotic stress responses; however, little is known about ethylene's role in reproductive tissues during development under heat stress. We assessed ethylene biosynthesis and signaling regulation within the reproductive and associated tissues of pea during the developmental phase that sets the stage for fruit-set and seed development under normal and heat-stress conditions. The transcript abundance profiles of PsACS [encode enzymes that convert S-adenosyl-L-methionine to 1-aminocyclopropane-1-carboxylic acid (ACC)] and PsACO (encode enzymes that convert ACC to ethylene), and ethylene evolution were developmentally, environmentally, and tissue-specifically regulated in the floral/fruit/pedicel tissues of pea. Higher transcript abundance of PsACS and PsACO in the ovaries, and PsACO in the pedicels was correlated with higher ethylene evolution and ovary senescence and pedicel abscission in fruits that were not pollinated under control temperature conditions. Under heat-stress conditions, up-regulation of ethylene biosynthesis gene expression in pre-pollinated ovaries was also associated with higher ethylene evolution and lower retention of these fruits. Following successful pollination and ovule fertilization, heat-stress modified PsACS and PsACO transcript profiles in a manner that suppressed ovary ethylene evolution. The normal ethylene burst in the stigma/style and petals following pollination was also suppressed by heat-stress. Transcript abundance profiles of ethylene receptor and signaling-related genes acted as qualitative markers of tissue ethylene signaling events. These data support the hypothesis that ethylene biosynthesis is regulated in reproductive tissues in response to heat stress to modulate resource allocation dynamics.

Regulation of ethylene-related gene expression by indole-3-acetic acid and 4-chloroindole-3-acetic acid in relation to pea fruit and seed development

In pea, the auxins 4-chloroindole-3-acetic acid (4-Cl-IAA) and indole-3-acetic acid (IAA) occur naturally; however, only 4-Cl-IAA stimulates pericarp growth and gibberellin (GA) biosynthesis, and inhibits the ethylene response in deseeded ovaries (pericarps), mimicking the presence of seeds. Expression of ovary ethylene biosynthesis genes was regulated similarly in most cases by the presence of 4-Cl-IAA or seeds. PsACS1 [which encodes an enzyme that synthesizes 1-aminocyclopropane-1-carboxylic acid (ACC)] transcript abundance was high in pericarp tissue adjacent to developing seeds following pollination. ACC accumulation in 4-Cl-IAA-treated deseeded pericarps was driven by high PsASC1 expression (1800-fold). 4-Cl-IAA, but not IAA, also suppressed the pericarp transcript levels of PsACS4. 4-Cl-IAA increased PsACO1 and decreased PsACO2 and PsACO3 expression (enzymes that convert ACC to ethylene) but did not change ACO enzyme activity. Increased ethylene was countered by a 4-Cl-IAA-specific decrease in ethylene responsiveness potentially via modulation of pericarp ethylene receptor and signaling gene expression. This pattern did not occur in IAA-treated pericarps. Overall, the effect of 4-Cl-IAA and IAA on ethylene biosynthesis gene expression generally explains the ethylene evolution patterns, and their effects on GA biosynthesis and ethylene signaling gene expression explain the tissue response patterns in young pea ovaries.

Hormonal regulation of reproductive growth under normal and heat-stress conditions in legume and other model crop species

Legume crops are grown throughout the world and provide an excellent food source of digestible protein and starch, as well as dietary fibre, vitamins, minerals, and flavonoids. Fruit and seeds from legumes are also an important source of vegetables for a well-balanced diet. A trend in elevated temperature as a result of climate change increases the risk of a heat stress-induced reduction in legume crop yield. High temperatures during the crop reproductive development phase are particularly detrimental to fruit/seed production because the growth and development of the reproductive tissues are sensitive to small changes in temperature. Hormones are signalling molecules that play important roles in a plant's ability to integrate different environmental inputs and modify their developmental processes to optimize growth, survival, and reproduction. This review focuses on the hormonal regulation of reproductive development and heat stress-induced alteration of this regulation during (i) pollination, (ii) early fruit set, and (iii) seed development that affects fruit/seed yield in legume and other model crops. Further understanding of hormone-regulated reproductive growth under non-stress and heat-stress conditions can aid in trait selection and the development of gene modification strategies and cultural practices to improve heat tolerance in legume crops contributing to improved food security.

Neglecting legumes has compromised human health and sustainable food production

The United Nations declared 2016 as the International Year of Pulses (grain legumes) under the banner 'nutritious seeds for a sustainable future'. A second green revolution is required to ensure food and nutritional security in the face of global climate change. Grain legumes provide an unparalleled solution to this problem because of their inherent capacity for symbiotic atmospheric nitrogen fixation, which provides economically sustainable advantages for farming. In addition, a legume-rich diet has health benefits for humans and livestock alike. However, grain legumes form only a minor part of most current human diets, and legume crops are greatly under-used. Food security and soil fertility could be significantly improved by greater grain legume usage and increased improvement of a range of grain legumes. The current lack of coordinated focus on grain legumes has compromised human health, nutritional security and sustainable food production.

Neglecting legumes has compromised human health and sustainable food production

The United Nations declared 2016 as the International Year of Pulses (grain legumes) under the banner 'nutritious seeds for a sustainable future'. A second green revolution is required to ensure food and nutritional security in the face of global climate change. Grain legumes provide an unparalleled solution to this problem because of their inherent capacity for symbiotic atmospheric nitrogen fixation, which provides economically sustainable advantages for farming. In addition, a legume-rich diet has health benefits for humans and livestock alike. However, grain legumes form only a minor part of most current human diets, and legume crops are greatly under-used. Food security and soil fertility could be significantly improved by greater grain legume usage and increased improvement of a range of grain legumes. The current lack of coordinated focus on grain legumes has compromised human health, nutritional security and sustainable food production.

Developmental profile of anthocyanin, flavonol, and proanthocyanidin type, content, and localization in saskatoon fruits (Amelanchier alnifolia Nutt.)

Saskatoons (Amelanchier alnifolia Nutt.) are small fruits that contain substantial quantities of flavonoids. To further characterize and understand the role of these flavonoids during fruit development, anthocyanins, flavonols, and proanthocyanidins were identified, quantified, and localized over development in cultivars that produce blue-purple or white fruit at maturity. Anthocyanin content was low in young fruit and then dramatically increased as the fruit transitioned into ripening only in the pigmented-fruit (blue-purple) cultivars. Proanthocyanidins with both A-type and B-type linkages were detected in fruit, with (-)-epicatechin as the most abundant proanthocyanidin subunit. Flavonol and proanthocyanidin content was high in, and localized throughout, the tissues of young fruit and in the developing seed coats, with levels decreasing as the fruit expanded. Our data show that flavonoid type, content, and tissue localization vary throughout development in saskatoon fruit. These data can be used to target specific fruit developmental stages and flavonoid classes for optimization of health-beneficial flavonoid content.

Characterization of proanthocyanidin metabolism in pea (Pisum sativum) seeds

BACKGROUND

Proanthocyanidins (PAs) accumulate in the seeds, fruits and leaves of various plant species including the seed coats of pea (Pisum sativum), an important food crop. PAs have been implicated in human health, but molecular and biochemical characterization of pea PA biosynthesis has not been established to date, and detailed pea PA chemical composition has not been extensively studied.

RESULTS

PAs were localized to the ground parenchyma and epidermal cells of pea seed coats. Chemical analyses of PAs from seeds of three pea cultivars demonstrated cultivar variation in PA composition. 'Courier' and 'Solido' PAs were primarily prodelphinidin-types, whereas the PAs from 'LAN3017' were mainly the procyanidin-type. The mean degree of polymerization of 'LAN3017' PAs was also higher than those from 'Courier' and 'Solido'. Next-generation sequencing of 'Courier' seed coat cDNA produced a seed coat-specific transcriptome. Three cDNAs encoding anthocyanidin reductase (PsANR), leucoanthocyanidin reductase (PsLAR), and dihydroflavonol reductase (PsDFR) were isolated. PsANR and PsLAR transcripts were most abundant earlier in seed coat development. This was followed by maximum PA accumulation in the seed coat. Recombinant PsANR enzyme efficiently synthesized all three cis-flavan-3-ols (gallocatechin, catechin, and afzalechin) with satisfactory kinetic properties. The synthesis rate of trans-flavan-3-ol by co-incubation of PsLAR and PsDFR was comparable to cis-flavan-3-ol synthesis rate by PsANR. Despite the competent PsLAR activity in vitro, expression of PsLAR driven by the Arabidopsis ANR promoter in wild-type and anr knock-out Arabidopsis backgrounds did not result in PA synthesis.

CONCLUSION

Significant variation in seed coat PA composition was found within the pea cultivars, making pea an ideal system to explore PA biosynthesis. PsANR and PsLAR transcript profiles, PA localization, and PA accumulation patterns suggest that a pool of PA subunits are produced in specific seed coat cells early in development to be used as substrates for polymerization into PAs. Biochemically competent recombinant PsANR and PsLAR activities were consistent with the pea seed coat PA profile composed of both cis- and trans-flavan-3-ols. Since the expression of PsLAR in Arabidopsis did not alter the PA subunit profile (which is only comprised of cis-flavan-3-ols), it necessitates further investigation of in planta metabolic flux through PsLAR.

Gibberellin 3-oxidase gene expression patterns influence gibberellin biosynthesis, growth, and development in pea

Gibberellins (GAs) are key modulators of plant growth and development. PsGA3ox1 (LE) encodes a GA 3β-hydroxylase that catalyzes the conversion of GA20 to biologically active GA1. To further clarify the role of GA3ox expression during pea (Pisum sativum) plant growth and development, we generated transgenic pea lines (in a lele background) with cauliflower mosaic virus-35S-driven expression of PsGA3ox1 (LE). PsGA3ox1 transgene expression led to higher GA1 concentrations in a tissue-specific and development-specific manner, altering GA biosynthesis and catabolism gene expression and plant phenotype. PsGA3ox1 transgenic plants had longer internodes, tendrils, and fruits, larger stipules, and displayed delayed flowering, increased apical meristem life, and altered vascular development relative to the null controls. Transgenic PsGA3ox1 overexpression lines were then compared with lines where endogenous PsGA3ox1 (LE) was introduced, by a series of backcrosses, into the same genetic background (BC LEle). Most notably, the BC LEle plants had substantially longer internodes containing much greater GA1 levels than the transgenic PsGA3ox1 plants. Induction of expression of the GA deactivation gene PsGA2ox1 appears to make an important contribution to limiting the increase of internode GA1 to modest levels for the transgenic lines. In contrast, PsGA3ox1 (LE) expression driven by its endogenous promoter was coordinated within the internode tissue to avoid feed-forward regulation of PsGA2ox1, resulting in much greater GA1 accumulation. These studies further our fundamental understanding of the regulation of GA biosynthesis and catabolism at the tissue and organ level and demonstrate that the timing/localization of GA3ox expression within an organ affects both GA homeostasis and GA1 levels, and thereby growth.

Developmental and seed aging mediated regulation of antioxidative genes and differential expression of proteins during pre- and post-germinative phases in pea

Enzymatic antioxidant system plays an important role in maintaining seed vigor and regulating plant growth and development. It involves a number of enzymes that scavenge excessive reactive oxygen species (ROS) produced during seed aging and also modulate the level of these compounds during plant developmental processes. This study investigated the transcriptional regulation of enzymatic antioxidative capacity in pea during the pre- and post-germinative phases and in response to seed aging by analyzing the spatio-temporal expression of five antioxidative genes: PsAPX, PsSOD, PsGRcyt, PsGRcm and PsCAT. Transcripts of all these genes were found in mature dry seeds, embryo axes and cotyledons of germinating seeds, and cotyledons, roots and shoots of young seedlings. However, PsAPX and PsSOD were predominant and exhibited developmental regulation, suggesting that these genes play important roles in controlling the intracellular homeostasis of ROS for promoting cell elongation, and thereby embryo axis expansion and early seedling growth in pea. Accelerated aging of pea seeds led to reduction in seed viability and seedling growth, and this effect was correlated with substantial decrease in the transcriptional activation of the prominent antioxidative genes. Furthermore, our proteomic analysis indicated the association of seed aging with changes in the abundance of specific proteins, revealing additional mechanisms underlying seed aging in pea.

Assessment of the mechanisms exerting glucose-lowering effects of dried peas in glucose-intolerant rats

The present study compared the effects of feeding uncooked pea fractions (embryo v. seed coat) on glucose homeostasis in glucose-intolerant rats and examined potential mechanisms influencing glucose homeostasis. Rats were made glucose intolerant by high-fat feeding, after which diets containing both high-fat and pea fractions were fed for 4 weeks. Rats fed diets containing uncooked pea seed coats low (non-coloured seed coat; NSC) or high (coloured seed coat; CSC) in proanthocyanidins but not embryos had improved oral glucose tolerance (P < 0·05). NSC also lowered fasting and glucose-stimulated insulin secretion (P < 0·05), decreased β-cell mass by 50 % (P < 0·05) and lowered levels of malondialdehyde, a marker of oxidative stress. Furthermore, NSC decreased the mucosal thickness of the colon by 25 % (P < 0·05), which might affect fibre fermentation and other gut functions. Small but statistically significant (P < 0·05) effects consistent with enhanced glucose transport or metabolism were observed in the skeletal muscle of rats fed NSC or CSC, for example, increased levels of AMP-dependent kinase or akt. We conclude that pea seed coats are the fraction exerting beneficial effects on glucose tolerance. Most of the changes were small in amplitude, suggesting that additive effects on multiple tissues may be important. NSC content appeared to have the most beneficial effects in improving glucose homeostasis but our ability to detect the effect of flavonoids may have been limited by their low concentration in the diet.

Gene expression and metabolite profiling of developing highbush blueberry fruit indicates transcriptional regulation of flavonoid metabolism and activation of abscisic acid metabolism

Highbush blueberry (Vaccinium corymbosum) fruits contain substantial quantities of flavonoids, which are implicated in a wide range of health benefits. Although the flavonoid constituents of ripe blueberries are known, the molecular genetics underlying their biosynthesis, localization, and changes that occur during development have not been investigated. Two expressed sequence tag libraries from ripening blueberry fruit were constructed as a resource for gene identification and quantitative real-time reverse transcription-polymerase chain reaction primer design. Gene expression profiling by quantitative real-time reverse transcription-polymerase chain reaction showed that flavonoid biosynthetic transcript abundance followed a tightly regulated biphasic pattern, and transcript profiles were consistent with the abundance of the three major classes of flavonoids. Proanthocyanidins (PAs) and corresponding biosynthetic transcripts encoding anthocyanidin reductase and leucoanthocyanidin reductase were most concentrated in young fruit and localized predominantly to the inner fruit tissue containing the seeds and placentae. Mean PA polymer length was seven to 8.5 subunits, linked predominantly via B-type linkages, and was relatively constant throughout development. Flavonol accumulation and localization patterns were similar to those of the PAs, and the B-ring hydroxylation pattern of both was correlated with flavonoid-3'-hydroxylase transcript abundance. By contrast, anthocyanins accumulated late in maturation, which coincided with a peak in flavonoid-3-O-glycosyltransferase and flavonoid-3'5'-hydroxylase transcripts. Transcripts of VcMYBPA1, which likely encodes an R2R3-MYB transcriptional regulator of PA synthesis, were prominent in both phases of development. Furthermore, the initiation of ripening was accompanied by a substantial rise in abscisic acid, a growth regulator that may be an important component of the ripening process and contribute to the regulation of blueberry flavonoid biosynthesis.

Tissue-specific regulation of gibberellin biosynthesis in developing pea seeds

Previous work suggests that gibberellins (GAs) play an important role in early seed development. To more fully understand the roles of GAs throughout seed development, tissue-specific transcription profiles of GA metabolism genes and quantitative profiles of key GAs were determined in pea (Pisum sativum) seeds during the seed-filling development period (8-20 d after anthesis [DAA]). These profiles were correlated with seed photoassimilate acquisition and storage as well as morphological development. Seed coat growth (8-12 DAA) and the subsequent dramatic expansion of branched parenchyma cells were correlated with both transcript abundance of GA biosynthesis genes and the concentration of the growth effector GA, GA(1). These results suggest GA(1) involvement in determining the rate of seed coat growth and sink strength. The endosperm's PsGA20ox transcript abundance and the concentration of GA(20) increased markedly as the endosperm reached its maximum volume (12 DAA), thus providing ample GA(20) substrate for the GA 3-oxidases present in both the embryo and seed coat. Furthermore, PsGA3ox transcript profiles and trends in GA(1) levels in embryos at 10 to 16 DAA and also in embryo axes at 18 DAA suggest localized GA(1)-induced growth in these tissues. A shift from synthesis of GA(1) to that of GA(8) occurred after 18 DAA in the embryo axis, suggesting that deactivation of GA(1) to GA(8) is a likely mechanism to limit embryo axis growth and allow embryo maturation to proceed. We hypothesize that GA biosynthesis and catabolism are tightly regulated to bring about the unique developmental events that occur during seed growth, development, and maturation.

Developmental and hormonal regulation of gibberellin biosynthesis and catabolism in pea fruit

In pea (Pisum sativum), normal fruit growth requires the presence of the seeds. The coordination of growth between the seed and ovary tissues involves phytohormones; however, the specific mechanisms remain speculative. This study further explores the roles of the gibberellin (GA) biosynthesis and catabolism genes during pollination and fruit development and in seed and auxin regulation of pericarp growth. Pollination and fertilization events not only increase pericarp PsGA3ox1 message levels (codes for GA 3-oxidase that converts GA(20) to bioactive GA(1)) but also reduce pericarp PsGA2ox1 mRNA levels (codes for GA 2-oxidase that mainly catabolizes GA(20) to GA(29)), suggesting a concerted regulation to increase levels of bioactive GA(1) following these events. 4-Chloroindole-3-acetic acid (4-Cl-IAA) was found to mimic the seeds in the stimulation of PsGA3ox1 and the repression of PsGA2ox1 mRNA levels as well as the stimulation of PsGA2ox2 mRNA levels (codes for GA 2-oxidase that mainly catabolizes GA(1) to GA(8)) in pericarp at 2 to 3 d after anthesis, while the other endogenous pea auxin, IAA, did not. This GA gene expression profile suggests that both seeds and 4-Cl-IAA can stimulate the production, as well as modulate the half-life, of bioactive GA(1), leading to initial fruit set and subsequent growth and development of the ovary. Consistent with these gene expression profiles, deseeded pericarps converted [(14)C]GA(12) to [(14)C]GA(1) only if treated with 4-Cl-IAA. These data further support the hypothesis that 4-Cl-IAA produced in the seeds is transported to the pericarp, where it differentially regulates the expression of pericarp GA biosynthesis and catabolism genes to modulate the level of bioactive GA(1) required for initial fruit set and growth.

Characterization of cyanidin- and quercetin-derived flavonoids and other phenolics in mature saskatoon fruits (Amelanchier alnifolia Nutt.)

In order to further characterize the anthocyanins, flavonols, and other phenolics present in mature saskatoon ( Amelanchier alnifolia Nutt.) fruit, extracts were characterized using high-performance liquid chromatography, gas chromatography, and liquid chromatography-mass spectrometry. Cyanidin 3-O-galactoside, cyanidin 3-O-glucoside, cyanidin 3-O-arabinoside, and cyanidin 3-O-xyloside were identified as the four major anthocyanins in the mature fruit. The quercetin-derived flavonols, quercetin 3-O-glucoside, quercetin 3-O-galactoside, quercetin 3-O-arabinoside, quercetin 3-O-xyloside, quercetin 3-O-arabinoglucoside, quercetin 3-O-robinobioside, and quercetin 3-O-rutinoside were also identified in mature fruit extracts. In addition, two chlorogenic acid isomers (hydroxycinnamates), 3-O-caffeoylquinic acid and 5-O-caffeoylquinic acid were detected. The total content of the anthocyanin-, flavonol-, and hydroxycinnamate-type phenolics detected in mature 'Smoky' saskatoon fruit was 140, 25, and 96 mg/100 g fresh weight, respectively. These data further our knowledge of the phenolic composition of mature saskatoon fruit, and as anthocyanins, flavonols, and hydroxycinnamates exhibit antioxidant activities, the presence and levels of these classes of phenolics will aid in the understanding of the potential health-beneficial effects of saskatoon fruits in the human diet.

Developmental and embryo axis regulation of gibberellin biosynthesis during germination and young seedling growth of pea

The expression patterns of five genes (PsGA20ox1, PsGA20ox2, PsGA3ox1, PsGA2ox1, and PsGA2ox2) encoding five regulatory gibberellin (GA) biosynthesis enzymes (two GA 20-oxidases, a GA 3beta-hydroxylase, and two GA 2beta-hydroxylases) were examined to gain insight into how these genes coordinate GA biosynthesis during germination and early postgermination stages of the large-seeded dicotyledonous plant pea (Pisum sativum). At the time the developing embryo fills the seed coat, high mRNA levels of PsGA20ox2 (primarily responsible for conversion of C20-GAs to GA(20)), PsGA2ox1 (primarily responsible for conversion of GA(20) to GA(29)), and PsGA2ox2 (primarily responsible for conversion of GA(1) to GA(8)) were detected in the seeds, along with high GA(20) and GA(29) levels, the enzymatic products of these genes. Embryo maturation was accompanied by a large reduction in PsGA20ox2 and PsGA2ox1 mRNA and lower GA(20) and GA(29) levels. However, PsGA2ox2 transcripts remained high. Following seed imbibition, GA(20) levels in the cotyledons decreased, while PsGA3ox1 mRNA and GA(1) levels increased, implying that GA(20) was being used for de novo synthesis of GA(1). The presence of the embryo axis was required for stimulation of cotyledonary GA(1) synthesis at the mRNA and enzyme activity levels. As the embryo axis doubled in size, PsGA20ox1 and PsGA3ox1 transcripts increased, both GA(1) and GA(8) were detectable, PsGA2ox2 transcripts decreased, and PsGA2ox1 transcripts remained low. Cotyledonary-, root-, and shoot-specific expression of these GA biosynthesis genes and the resultant endogenous GA profiles support a key role for de novo GA biosynthesis in each organ during germination and early seedling growth of pea.

Pollination-, development-, and auxin-specific regulation of gibberellin 3beta-hydroxylase gene expression in pea fruit and seeds

To understand further how pollination, seeds, auxin (4-chloroindole-3-acetic acid [4-Cl-IAA]), and gibberellins (GAs) regulate GA biosynthesis in pea (Pisum sativum) fruit, we studied expression of the gene PsGA3ox1 that codes for the enzyme that converts GA(20) to biologically active GA(1) using real-time reverse transcription-polymerase chain reaction analysis. PsGA3ox1 mRNA levels were minimally detectable in prepollinated pericarps and ovules (-2 d after anthesis [DAA]), increased dramatically after pollination (0 DAA), then decreased by 1 DAA. Seed PsGA3ox1 mRNA levels increased at 4 DAA and again 8 to 12 DAA, when seed development was rapid. Pericarp PsGA3ox1 mRNA levels peaked coincidentally with rapid pod diameter expansion (6-10 DAA) to accommodate the growing seeds. The effects of seeds and hormones on the expression of pericarp PsGA3ox1 were investigated over a 24-h treatment period. Pericarp PsGA3ox1 mRNA levels gradually increased from 2 to 3 DAA when seeds were present; however, when the seeds were removed, the pericarp transcript levels dramatically declined. When 2-DAA deseeded pericarps were treated with 4-Cl-IAA, PsGA3ox1 mRNA levels peaked 4 h after hormone treatment (270-fold increase), then decreased. PsGA3ox1 mRNA levels in deseeded pericarps treated with indole-3-acetic acid or GA(3) were the same or lower than deseeded controls. These data show that PsGA3ox1 is expressed and developmentally regulated in pea pericarps and seeds. These data also show that pericarp PsGA3ox1 expression is hormonally regulated and suggest that the conversion of GA(20) to GA(1) occurs in the pericarp and is regulated by the presence of seeds and 4-Cl-IAA for fruit growth.

Specificity of auxin regulation of gibberellin 20-oxidase gene expression in pea pericarp

During early pea fruit growth, the physiological roles of 4-chloroindole-3-acetic acid (4-Cl-IAA) and IAA, natural pea auxins, in regulating gibberellin (GA) 20-oxidase gene expression (PsGA20ox1) were tested with 4-position, ring-substituted auxins that have a range of biological activities (fruit growth). The effect of seeds, and natural and synthetic auxins (4-Cl-IAA, and IAA; 4-Me-IAA, 4-Et-IAA and 4-F-IAA, respectively), and auxin concentration (4-Cl-IAA) on PsGA20ox1 mRNA levels in pea pericarp were investigated over a 24 h treatment period. The ability of the 4-substituted auxins to increase PsGA20ox1 mRNA levels in deseeded pericarp was correlated with their ability to stimulate pericarp growth. The greatest increase in pericarp PsGA20ox1 mRNA levels and growth was observed when deseeded pericarps were treated with the naturally occurring pea auxin, 4-Cl-IAA; however, IAA was not effective. Silver thiosulfate, an ethylene action antagonist, did not reverse IAA's lack of stimulation of PsGA20ox1 over the control treatment. 4-Me-IAA was the second most active auxin in stimulating PsGA20ox1 and was the second most biologically active auxin. Application of the 4-substituted IAA analogs, 4-Et-IAA and 4-F-IAA, to deseeded pericarps resulted in minimal or no increase in PsGA20ox1 transcript levels or pericarp growth. Pericarp PsGA20ox1 mRNA levels increased with increasing 4-Cl-IAA concentration and showed transitory increases at low 4-Cl-IAA treatments (30 to 300 pmol). These results support a unique physiological role for the auxin 4-Cl-IAA in the regulation of GA metabolism by effecting PsGA20ox1 expression during early pea fruit growth.

Hormone and seed-specific regulation of pea fruit growth

Growth of young pea (Pisum sativum) fruit (pericarp) requires developing seeds or, in the absence of seeds, treatment with gibberellin (GA) or auxin (4-chloroindole-3-acetic acid). This study examined the role of seeds and hormones in the regulation of cell division and elongation in early pea fruit development. Profiling histone H2A and gamma-tonoplast intrinsic protein (TIP) gene expression during early fruit development identified the relative contributions of cell division and elongation to fruit growth, whereas histological studies identified specific zones of cell division and elongation in exocarp, mesocarp, and endocarp tissues. Molecular and histological studies showed that maximal cell division was from -2 to 2 d after anthesis (DAA) and elongation from 2 to 5 DAA in pea pericarp. Maximal increase in pericarp gamma-TIP message level preceded the maximal rate of fruit growth and, in general, gamma-TIP mRNA level was useful as a qualitative marker for expanding tissue, but not as a quantitative marker for cell expansion. Seed removal resulted in rapid decreases in pericarp growth and in gamma-TIP and histone H2A message levels. In general, GA and 4-chloroindole-3-acetic acid maintained these processes in deseeded pericarp similarly to pericarps with seeds, and both hormones were required to obtain mesocarp cell sizes equivalent to intact fruit. However, GA treatment to deseeded pericarps resulted in elevated levels of gamma-TIP mRNA (6 and 7 DAA) when pericarp growth and cell enlargement were minimal. Our data support the theory that cell division and elongation are developmentally regulated during early pea fruit growth and are maintained by the hormonal interaction of GA and auxin.

Seed and 4-chloroindole-3-acetic acid regulation of gibberellin metabolism in pea pericarp

In this study, we investigated seed and auxin regulation of gibberellin (GA) biosynthesis in pea (Pisum sativum L.) pericarp tissue in situ, specifically the conversion of [14C]GA19 to [14C]GA20. [14C]GA19 metabolism was monitored in pericarp with seeds, deseeded pericarp, and deseeded pericarp treated with 4-chloroindole-3-acetic acid (4-CI-IAA). Pericarp with seeds and deseeded pericarp treated with 4-CI-IAA continued to convert [14C]GA19 to [14C]GA20 throughout the incubation period (2-24 h). However, seed removal resulted in minimal or no accumulation of [14C]GA20 in pericarp tissue. [14C]GA29 was also identified as a product of [14C]GA19 metabolism in pea pericarp. The ratio of [14C]GA29 to [14C]GA20 was significantly higher in deseeded pericarp (with or without exogenous 4-CI-IAA) than in pericarp with seeds. Therefore, conversion of [14C]GA20 to [14C]GA29 may also be seed regulated in pea fruit. These data support the hypothesis that the conversion of GA19 to GA20 in pea pericarp is seed regulated and that the auxin 4-CI-IAA can substitute for the seeds in the stimulation of pericarp growth and the conversion of GA19 to GA20.

Seed effects on gibberellin metabolism in pea pericarp

Pea fruit (Pisum sativum L.) is a model system for studying the effect of seeds on fruit growth in order to understand coordination of organ development. The metabolism of (14)C-labeled gibberellin A(12) (GA(12)) by pea pericarp was followed using a method that allows access to the seeds while maintaining pericarp growth in situ. Identification and quantitation of GAs in pea pericarp was accomplished by combined gas chromatography-mass spectrometry following extensive purification of the putative GAs. Here we report for the first time that the metabolism of [(14)C]GA(12) to [(14)C]GA(19) and [(14)C]GA(20) occurs in pericarp of seeded pea fruit. Removal of seeds from the pericarp inhibited the conversion of radiolabeled GA(19) to GA(20) and caused the accumulation of radiolabeled and endogenous GA(19). Deseeded pericarp contained no detectable GA(20), GA(1), or GA(8), whereas pericarp with seeds contained endogenous and radiolabeled GA(20) and endogenous GA(1). These data strongly suggest that seeds are required for normal GA biosynthesis in the pericarp, specifically the conversion of GA(19) to GA(20).