Recruitment of distinct UDP-glycosyltransferase families demonstrates dynamic evolution of chemical defense within Eucalyptus L'Hér.

The economic and ecologically important genus Eucalyptus is rich in structurally diverse specialized metabolites. While some specialized metabolite classes are highly prevalent across the genus, the cyanogenic glucoside prunasin is only produced by c. 3% of species. To investigate the evolutionary mechanisms behind prunasin biosynthesis in Eucalyptus, we compared de novo assembled transcriptomes, together with online resources between cyanogenic and acyanogenic species. Identified genes were characterized in vivo and in vitro. Pathway characterization of cyanogenic Eucalyptus camphora and Eucalyptus yarraensis showed for the first time that the final glucosylation step from mandelonitrile to prunasin is catalyzed by a novel UDP-glucosyltransferase UGT87. This step is typically catalyzed by a member of the UGT85 family, including in Eucalyptus cladocalyx. The upstream conversion of phenylalanine to mandelonitrile is catalyzed by three cytochrome P450 (CYP) enzymes from the CYP79, CYP706, and CYP71 families, as previously shown. Analysis of acyanogenic Eucalyptus species revealed the loss of different ortholog prunasin biosynthetic genes. The recruitment of UGTs from different families for prunasin biosynthesis in Eucalyptus demonstrates important pathway heterogeneities and unprecedented dynamic pathway evolution of chemical defense within a single genus. Overall, this study provides relevant insights into the tremendous adaptability of these long-lived trees.

Molecular Characterisation of Flavanone O-methylation in Eucalyptus.

Flavonoids are ubiquitous polyphenolic compounds in plants, long recognised for their health-promoting properties in humans. Methylated flavonoids have received increasing attention due to the potential of methylation to enhance medicinal efficacy. Recently, Eucalyptus species with high levels of the O-methylated flavanone pinostrobin have been identified. Pinostrobin has potential commercial value due to its numerous pharmacological and functional food benefits. Little is known about the identity or mode of action of the enzymes involved in methylating flavanones. This study aimed to identify and characterise the methyltransferase(s) involved in the regiospecific methylation of pinostrobin in Eucalyptus and thereby add to our limited understanding of flavanone biosynthesis in plants. RNA-seq analysis of leaf tips enabled the isolation of a gene encoding a flavanone 7-O-methyltransferase (EnOMT1) in Eucalyptus. Biochemical characterisation of its in vitro activity revealed a range of substrates upon which EnOMT1 acts in a regiospecific manner. Comparison to a homologous sequence from a Eucalyptus species lacking O-methylated flavonoids identified critical catalytic amino acid residues within EnOMT1 responsible for its activity. This detailed molecular characterisation identified a methyltransferase responsible for chemical ornamentation of the core flavanone structure of pinocembrin and helps shed light on the mechanism of flavanone biosynthesis in Eucalyptus.

The efficacy and safety of pinocembrin in a sheep model of bleomycin-induced pulmonary fibrosis.

The primary flavonoid, pinocembrin, is thought to have a variety of medical uses which relate to its reported anti-oxidant, anti-inflammatory, anti-microbial and anti-cancer properties. Some studies have reported that this flavonoid has anti-fibrotic activities. In this study, we investigated whether pinocembrin would impede fibrosis, dampen inflammation and improve lung function in a large animal model of pulmonary fibrosis. Fibrosis was induced in two localized lung segments in each of the 10 sheep participating in the study. This was achieved via two infusions of bleomycin delivered bronchoscopically at a two-week interval. Another lung segment in the same sheep was left untreated, and was used as a healthy control. The animals were kept for a little over 5 weeks after the final infusion of bleomycin. Pinocembrin, isolated from Eucalyptus leaves, was administered to one of the two bleomycin damaged lung segments at a dose of 7 mg. This dose was given once-weekly over 4-weeks, starting one week after the final bleomycin infusion. Lung compliance (as a measure of stiffness) was significantly improved after four weekly administrations of pinocembrin to bleomycin-damaged lung segments. There were significantly lower numbers of neutrophils and inflammatory cells in the bronchoalveolar lavage of bleomycin-infused lung segments that were treated with pinocembrin. Compared to bleomycin damaged lung segments without drug treatment, pinocembrin administration was associated with significantly lower numbers of immuno-positive CD8+ and CD4+ T cells in the lung parenchyma. Histopathology scoring data showed that pinocembrin treatment was associated with significant improvement in inflammation and overall pathology scores. Hydroxy proline analysis showed that the administration of pinocembrin did not reduce the increased collagen content that was induced by bleomycin in this model. Analyses of Masson's Trichrome stained sections showed that pinocembrin treatment significantly reduced the connective tissue content in lung segments exposed to bleomycin when compared to bleomycin-infused lungs that did not receive pinocembrin. The striking anti-inflammatory and modest anti-fibrotic remodelling effects of pinocembrin administration were likely linked to the compound's ability to improve lung pathology and functional compliance in this animal model of pulmonary fibrosis.

Monoterpene synthases responsible for the terpene profile of anther glands in Eucalyptus polybractea R.T. Baker (Myrtaceae).

Research on terpene biosynthesis in the genus Eucalyptus (Myrtaceae) is poorly developed, but recently large numbers of terpene synthase (TPS) genes have been identified. Few of these have been characterized or their expression localized to specific tissues. A prime candidate for detailed examination of TPS gene expression is the bisexual eucalypt flower-composed of male and female reproductive organs, and vegetative tissues that may express different TPS genes. We aimed to characterize and compare the terpene profile and TPS genes expressed in anthers and gynoecia in the high oil-yielding Eucalyptus polybractea R.T. Baker. We hypothesized that gynoecia will produce greater amounts of defensive terpenes, whereas anthers will have a terpene profile that is biased towards a role in pollination. Microscopy of isolated anthers showed them to possess a single, prominent oil gland. Chemical analysis of whole floral structures at different stages of development showed total oil per unit dry mass increased as flower buds expanded, with highest concentrations in mature flower buds just prior to flower opening. The oil profile of gynoecia was dominated by the monoterpene 1,8-cineole, whereas that of isolated anthers were enriched with the monoterpene α-pinene. Through transcriptomic analysis and recombinant protein expression, we were able to identify monoterpene synthases responsible for the different profiles. Synthases for α-pinene and 1,8-cineole were expressed in each tissue type, but the relative expression of the former was higher in anthers. Sequence comparison and site-directed mutagenesis of the α-pinene synthase allowed us to identify amino acids that influence the α-pinene to ß-pinene ratio of the product profile. We suggest the terpene constituents of anthers may have multiple roles including attracting pollinators through emission of volatile α-pinene, deterrence of palynivores through emission of volatile 1,8-cineole and adhesion of pollen to pollinators via the release of sticky α-pinene onto the anther surface.

Oil Body Formation in Marchantia polymorpha Is Controlled by MpC1HDZ and Serves as a Defense against Arthropod Herbivores.

The origin of a terrestrial flora in the Ordovician required adaptation to novel biotic and abiotic stressors. Oil bodies, a synapomorphy of liverworts, accumulate secondary metabolites, but their function and development are poorly understood. Oil bodies of Marchantia polymorpha develop within specialized cells as one single large organelle. Here, we show that a class I homeodomain leucine-zipper (C1HDZ) transcription factor controls the differentiation of oil body cells in two different ecotypes of the liverwort M. polymorpha, a model genetic system for early divergent land plants. In flowering plants, these transcription factors primarily modulate responses to abiotic stress, including drought. However, loss-of-function alleles of the single ortholog gene, MpC1HDZ, in M. polymorpha did not exhibit phenotypes associated with abiotic stress. Rather, Mpc1hdz mutant plants were more susceptible to herbivory, and total plant extracts of the mutant exhibited reduced antibacterial activity. Transcriptomic analysis of the mutant revealed a reduction in expression of genes related to secondary metabolism that was accompanied by a specific depletion of oil body terpenoid compounds. Through time-lapse imaging, we observed that MpC1HDZ expression maxima precede oil body formation, indicating that MpC1HDZ mediates differentiation of oil body cells. Our results indicate that M. polymorpha oil bodies, and MpC1HDZ, are critical for defense against herbivory, but not for abiotic stress tolerance. Thus, C1HDZ genes were co-opted to regulate separate responses to biotic and abiotic stressors in two distinct land plant lineages.

Plant traits linked to field-scale flammability metrics in prescribed burns in Eucalyptus forest.

Vegetation is a key determinant of wildfire behaviour at field scales as it functions as fuel. Past studies in the laboratory show that plant flammability, the ability of plants to ignite and maintain combustion, is a function of their traits. However, the way the traits of individual plants combine in a vegetation community to affect field flammability has received little attention. This study aims to bridge the gap between the laboratory and field by linking plant traits to metrics of field-scale flammability. Across three prescribed burns, in Eucalyptus-dominated damp and dry forest, we measured pre-burn plant species abundance and post-burn field flammability metrics (percentage area burnt, char and scorch height). For understory species with dominant cover-abundance, we measured nine traits that had been demonstrated to influence flammability in the laboratory. We used fourth-corner ordination to evaluate covariation between the plant traits, species abundance and flammability. We found that several traits covaried at the species level. In some instances, these traits (e.g. specific leaf area and bulk density) could have cumulative effects on the flammability of a species while in other instances (e.g. moisture and specific leaf area) they may have counteractive effects, assuming trait effects on flammability are akin to previous research. At field scales, species with similar traits tended to co-occur, suggesting that the effects of individual traits accumulate within a plant community. Fourth-corner analyses found the trait-field flammability relationship to be statistically significant. Traits significantly associated with increasing field flammability metrics were: bulk density (negatively associated) and hydrocarbon quantity, specific leaf area and surface area to volume ratio (all positively associated). Our study demonstrates that some traits known to influence flammability in the laboratory can be associated with field-scale flammability metrics. Further research is needed to isolate the contributions of individual traits to understand how species composition drives forest flammability.

Differential metabolic specialization of foliar oil glands in Eucalyptus brevistylis Brooker (Myrtaceae).

Trees and shrubs from the genus Eucalyptus are characterized by the presence of numerous foliar oil glands that generally house mono- and sesquiterpenes. In some species, glands are also known to house substantial quantities of unrelated secondary metabolites such as volatile, aromatic ß-triketones. It is not known if these compounds are co-housed with terpenes or if they are produced in distinct, metabolically specialized glands. We showed that Eucalyptus brevistylis-a species with appreciable foliar quantities of both ß-triketones and terpenes-contains two visually distinct gland types in leaves, one that is translucent and the other golden-brown. Gas chromatographic analyses of solvent extracts of the two gland types showed that the translucent glands contain sesquiterpene alcohol cubenols and cubebols (termed 'sesquiterpene glands'), whereas the golden-brown glands contain predominantly the ß-triketone conglomerone with lesser amounts of sesquiterpene hydrocarbon caryophyllenes (termed 'triketone glands'). Analysis of leaves from trees of different ages, from young saplings through to advanced age trees, showed a gradual increase in the abundance of sesquiterpene glands relative to triketone glands as plants aged. Such ontogenetic regulation of foliar secondary metabolite concentration appears to be a common feature of Eucalyptus species, albeit at different temporal scales. A similar ontogenetic pattern was observed in ageing leaves, with mature leaves having a higher proportion of sesquiterpene glands than young leaf tips. It is concluded that regulation of the relative abundances of the two gland types with ontogeny likely reflects the different herbivores present at the different life stages of leaves and whole plants. In particular, leaf tips and young plants may be advantaged by deploying higher amounts of insecticidal ß-triketones. The concurrent deployment of two metabolically distinct gland types in leaves is a rare phenomenon and a novel finding for myrtaceous trees.

Variation in the web-based chemical cues of Argiope keyserlingi.

Pheromones are chemical compounds used to transmit information between individuals of the same species. Pheromone composition is influenced by both genetic and environmental factors. Numerous studies, predominately of insects, have demonstrated a role for diet in pheromone expression. The chemical composition of spider web-silk varies with diet and in many species these chemicals are crucial to mate choice processes. Here, we investigated individual variation in the chemical compounds found on the surface of web-silk of female Argiope keyserlingi, and further explored the degree to which they are influenced by diet, investment in egg sac production and site of collection. We observed variation in the web-based chemical cues both between and within individuals. Additionally, we found that some of this variation could be explained by diet and gravid status but not by collection site. We discuss our findings in relation to mate choice processes and the costs and benefits of the observed variation in these web-based chemicals.

Correction: Foliar Essential Oil Glands of Eucalyptus Subgenus Eucalyptus (Myrtaceae) Are a Rich Source of Flavonoids and Related Non-Volatile Constituents.

[This corrects the article DOI: 10.1371/journal.pone.0151432.].

Location-specific cuticular hydrocarbon signals in a social insect.

Social insects use cuticular hydrocarbons (CHCs) to convey different social signals, including colony or nest identity. Despite extensive investigations, the exact source and identity of CHCs that act as nest-specific identification signals remain largely unknown. Perhaps this is because studies that identify CHC signals typically use organic solvents to extract a single sample from the entire animal, thereby analysing a cocktail of chemicals that may serve several signal functions. We took a novel approach by first identifying CHC profiles from different body parts of ants (Iridomyrmex purpureus), then used behavioural bioassays to reveal the location of specific social signals. The CHC profiles of both workers and alates varied between different body parts, and workers paid more attention to the antennae of non-nest-mate and the legs of nest-mate workers. Workers responded less aggressively to non-nest-mate workers if the CHCs on the antennae of their opponents were removed with a solvent. These data indicate that CHCs located on the antennae reveal nest-mate identity and, remarkably, that antennae both convey and receive social signals. Our approach and findings could be valuably applied to chemical signalling in other behavioural contexts, and provide insights that were otherwise obscured by including chemicals that either have no signal function or may be used in other contexts.

Foliar Essential Oil Glands of Eucalyptus Subgenus Eucalyptus (Myrtaceae) Are a Rich Source of Flavonoids and Related Non-Volatile Constituents.

The sub-dermal secretory cavities (glands) embedded within the leaves of Eucalyptus (Myrtaceae) were once thought to be the exclusive repositories of monoterpene and sesquiterpene oils. Recent research has debunked this theory and shown that abundant non-volatile compounds also occur within foliar glands. In particular, glands of four species in subgenus Eucalyptus contain the biologically active flavanone pinocembrin. Pinocembrin shows great promise as a pharmaceutical and is predominantly plant-sourced, so Eucalyptus could be a potential commercial source of such compounds. To explore this we quantified and assessed the purity of pinocembrin in glands of 11 species of E. subg. Eucalyptus using Electro-Spray Ionisation Liquid Chromatography Mass Spectrometry of acetonitrile extracts and Gas Chromatography Mass Spectrometry analyses of hexane extracts of isolated glands which were free from other leaf tissues. Our results showed that the glands of subgenus Eucalyptus contain numerous flavanones that are structurally related to pinocembrin and often present in much greater abundance. The maximum concentration of pinocembrin was 2 mg g-1 dry leaf found in E. stellulata, whereas that of dimethylpinocembrin (5,7-dimethoxyflavanone) was 10 mg g-1 in E. oreades and that of pinostrobin (5-hydroxy-7-methoxyflavanone) was 12 mg g-1 in E. nitida. We also found that the flavanones are exclusively located within the foliar glands rather than distributed throughout leaf tissues. The flavanones differ from the non-methylated pinocembrin in the degree and positions of methylation. This finding is particularly important given the attractiveness of methylated flavonoids as pharmaceuticals and therapeutics. Another important finding was that glands of some members of the subgenus also contain flavanone O-glucosides and flavanone-ß-triketone conjugates. In addition, glands contain free ß-triketones, ß-triketone heterodimers and chromone C-glucosides. Therefore, the foliar glands of this taxonomically distinct group of plants are a rich source of a range of flavonoids and other biologically active compounds with great commercial potential.

Contrasting ontogenetic trajectories for phenolic and terpenoid defences in Eucalyptus froggattii.

BACKGROUND AND AIMS: Plant defence metabolites are considered costly due to diversion of energy and nutrients away from growth. These costs combined with changes in resource availability and herbivory throughout plant ontogeny are likely to promote changes in defence metabolites. A comprehensive understanding of plant defence strategy requires measurement of lifetime ontogenetic trajectories--a dynamic component largely overlooked in plant defence theories. This study aimed to compare ontogenetic trajectories of foliar phenolics and terpenoids. Phenolics are predicted to be inexpensive to biosynthesize, whereas expensive terpenoids also require specialized, non-photosynthetic secretory structures to avoid autotoxicity. Based on these predicted costs, it is hypothesized that phenolics would be maximally deployed early in ontogeny, whereas terpenoids would be maximally deployed later, once the costs of biosynthesis and foregone photosynthesis could be overcome by enhanced resource acquisition. METHODS: Leaves were harvested from a family of glasshouse-grown Eucalyptus froggattii seedlings, field-grown saplings and the maternal parent tree, and analysed for total terpenoids and phenolics. KEY RESULTS: Foliar phenolics were highest in young seedlings and lowest in the adult tree. Indeed the ratio of total phenolics to total terpenoids decreased in a significantly exponential manner with plant ontogeny. Most individual terpene constituents increased with plant ontogeny, but some mono- and sesquiterpenes remained relatively constant or even decreased in concentration as plants aged. CONCLUSIONS: Plant ontogeny can influence different foliar defence metabolites in directionally opposite ways, and the contrasting trajectories support our hypothesis that phenolics would be maximally deployed earlier than terpenoids. The results highlight the importance of examining ontogenetic trajectories of defence traits when developing and testing theories of plant defence, and illustrate an advantage of concurrently studying multiple defences.

Plant chemical defense: at what cost?

Plants are sessile organisms and dependent on deployment of secondary metabolites for their response to biotic and abiotic challenges. A trade-off is envisioned between resources allocated to growth, development, and reproduction and to the biosynthesis, storage, and maintenance of secondary metabolites. However, increasing evidence suggests that secondary metabolites serve auxiliary roles, including functions associated with primary metabolism. In this opinion article, we examine how the costs of plant chemical defense can be offset by multifunctional biosynthesis and the optimization of primary metabolism. These additional benefits may negate the trade-off between primary and secondary metabolism, and provide plants with an innate plasticity required for growth, development, and interactions with their environment.

Localization of oleuropeyl glucose esters and a flavanone to secretory cavities of Myrtaceae.

We report the widespread occurrence of structurally diverse oleuropeyl glucose esters, including the new diester eucaglobulin B, localized specifically to the essential oil secretory cavities of myrtaceous species. Clear taxonomic patterns in the composition of cavity extracts within the genus Eucalyptus are shown with species from subgenus Symphyomyrtus dominated by oleuropeyl glucose esters and species from subgenus Eucalyptus dominated instead by the flavanone, pinocembrin. We also examined the intra-species occurrence of oleuropeyl glucose esters by quantifying the abundant constituents cuniloside B and froggattiside A in trees from two populations of Eucalyptus polybractea R.T. Baker. All trees contained both compounds, which were positively correlated with total essential oil concentration. This apparent ubiquity of oleuropeyl glucose esters at both intra- and inter-specific levels in Eucalyptus is indicative of important physiological or ecological functions. The significance of their prevalence and the sequestration of these esters and also pinocembrin to the extracellular domain of secretory cavities is discussed in light of their potential biological activities and our findings that they are spatially segregated to the exterior of cavity lumina. The localization of oleuropeyl glucose esters to a specific and isolatable tissue type has the potential to aid in future elucidation of function and biosynthesis.

α,ß-Unsaturated monoterpene acid glucose esters: structural diversity, bioactivities and functional roles.

The glycosylation of lipophilic small molecules produces many important plant secondary metabolites. The majority of these are O-glycosides with relatively fewer occurring as glucose esters of aromatic or aliphatic acids. In particular, monoterpene acid glucose esters have much lower structural diversity and distribution compared to monoterpene glycosides. Nevertheless, there have been over 20 monoterpene acid glucose esters described from trees in the genus Eucalyptus (Myrtaceae) in recent years, all based on oleuropeic acid, menthiafolic acid or both. Here we review all of the glucose esters containing these monoterpenoids identified in plants to date. Many of the compounds contain phenolic aglycones and all contain at least one α,ß-unsaturated carbonyl, affording a number of important potential therapeutic reactivities such as anti-tumor promotion, carcinogenesis suppression, and anti-oxidant and anti-inflammatory activities. Additional properties such as cytotoxicity, bitterness, and repellency are suggestive of a role in plant defence, but we also discuss their localization to the exterior of foliar secretory cavity lumina, and suggest they may also protect secretory cells from toxic terpenes housed within these structures. Finally we discuss how the use of a recently developed protocol to isolate secretory cavities in a functional state could be used in conjunction with systems biology approaches to help characterize their biosynthesis and roles in plants.

Phenylalanine derived cyanogenic diglucosides from Eucalyptus camphora and their abundances in relation to ontogeny and tissue type.

The cyanogenic glucoside profile of Eucalyptus camphora was investigated in the course of plant ontogeny. In addition to amygdalin, three phenylalanine-derived cyanogenic diglucosides characterized by unique linkage positions between the two glucose moieties were identified in E. camphora tissues. This is the first time that multiple cyanogenic diglucosides have been shown to co-occur in any plant species. Two of these cyanogenic glucosides have not previously been reported and are named eucalyptosin B and eucalyptosin C. Quantitative and qualitative differences in total cyanogenic glucoside content were observed across different stages of whole plant and tissue ontogeny, as well as within different tissue types. Seedlings of E. camphora produce only the cyanogenic monoglucoside prunasin, and genetically based variation was observed in the age at which seedlings initiate prunasin biosynthesis. Once initiated, total cyanogenic glucoside concentration increased throughout plant ontogeny with cyanogenic diglucoside production initiated in saplings and reaching a maximum in flower buds of adult trees. The role of multiple cyanogenic glucosides in E. camphora is unknown, but may include enhanced plant defense and/or a primary role in nitrogen storage and transport.

Re-examining the role of ABA as the primary long-distance signal produced by water-stressed roots.

The role of ABA as the primary long-distance signal produced by water-stressed roots and transported to stomata continues to be challenged. We have recently reported that expression of ABA biosynthetic genes in roots only increases in the later stage of water stress. Our results support the hypothesis that in early water stress, increased levels of ABA in xylem sap are due to leaf biosynthesis and translocation to roots and from there to xylem. If so, other xylem-borne chemicals may be the primary stress signal(s) inducing ABA biosynthesis in leaves. We found that apart from ABA, sulfate was the only xylem-borne chemical that consistently showed higher concentrations from early to later water stress. We also found increased expression of a sulfate transporter gene in roots from early water stress onwards. Moreover, using bioassays we found an interactive effect of ABA and sulfate in decreasing maize transpiration rate, as compared to ABA alone. While ABA is undoubtedly the key mediator of water stress responses such as stomatal closure, it may not be the primary signal produced by roots perceiving water stress.

Huperzine alkaloids from Australasian and southeast Asian Huperzia.

CONTEXT: The pharmaceutical alkaloid huperzine A (HupA), currently used in herbal supplements and medicines worldwide, is predominantly sourced from the Chinese lycopod Huperzia serrata (Thunb. ex Murray) Trev. (Lycopodiaceae), which on average contains only 0.08 mg HupA g(-1) dry weight, and is experiencing a rapid decline in China due to over-harvesting. OBJECTIVE: To find a high-yielding, natural source of HupA and/or the related huperzine B (HupB) that could potentially be used as the starting material in a commercial propagation program. MATERIALS AND METHODS: We surveyed 17 Huperzia species (15 indigenous to Australia and southeast Asia) for their foliar HupA and HupB concentrations. We also studied intra-specific variation for the huperzines in four species that were available in sufficient numbers, and determined tissue-specific accumulation in larger specimens. RESULTS: HupA was detected in 11 Australasian and southeast Asian species, with eight also containing HupB, albeit at much lower concentrations. A H. elmeri (Herter) Holub plant from the Philippines had one of the highest HupA concentrations recorded (1.01 mg g(-1) dry wt) and it also had the highest HupB content of all plants surveyed (0.34 mg g(-1) dry wt). Intra-specific HupA and HupB concentrations were extremely variable, and at the intra-plant level, reproductive strobili were found to accumulate the highest HupA concentrations. DISCUSSION AND CONCLUSION: Select Huperzia species from Australia and southeast Asia have potential as the starting material for establishing commercial HupA plantations, but the high intra-specific variability observed suggests that detailed screening is needed to isolate high huperzine-yielding individuals.

Synthesis of the monoterpenoid esters cypellocarpin C and cuniloside B and evidence for their widespread occurrence in Eucalyptus.

Short syntheses of cuniloside B and cypellocarpin C, (+)-(R)-oleuropeic acid-containing carbohydrates, are reported. Also disclosed are syntheses of the noreugenin glycosides, undulatoside A and corymbosins K(1) and K(2). Leaf extracts of 28 diverse eucalypts revealed cuniloside B to be present in all, and cypellocarpin C to be present in most, of the species examined. The widespread occurrence of these carbohydrate monoterpenoid esters supports their roles in essential oil biosynthesis or mobilization from sites of synthesis to secretory cavity lumena.

Sulphate as a xylem-borne chemical signal precedes the expression of ABA biosynthetic genes in maize roots.

Recent reports suggest that early sensing of soil water stress by plant roots and the concomitant reduction in stomatal conductance may not be mediated by root-sourced abscisic acid (ABA), but that other xylem-borne chemicals may be the primary stress signal(s). To gain more insight into the role of root-sourced ABA, the timing and location of the expression of genes for key enzymes involved in ABA biosynthesis in Zea mays roots was measured and a comprehensive analysis of root xylem sap constituents from the early to the later stages of water stress was conducted. Xylem sap and roots were sampled from plants at an early stage of water stress when only a reduction in leaf conductance was measured, as well as at later stages when leaf xylem pressure potential decreased. It was found that the majority of ABA biosynthetic genes examined were only significantly expressed in the elongation region of roots at a later stage of water stress. Apart from ABA, sulphate was the only xylem-borne chemical that consistently showed significantly higher concentrations from the early to the later stages of stress. Moreover, there was an interactive effect of ABA and sulphate in decreasing maize transpiration rate and Vicia faba stomatal aperture, as compared to ABA alone. The expression of a sulphate transporter gene was also analysed and it was found that it had increased in the elongation region of roots from the early to the later stages of water stress. Our results support the suggestion that in the early stage of water stress, increased levels of ABA in xylem sap may not be due to root biosynthesis, ABA glucose ester catabolism or pH-mediated redistribution, but may be due to shoot biosynthesis and translocation to the roots. The analysis of xylem sap mineral content and bioassays indicate that the anti-transpirant effect of the ABA reaching the stomata at the early stages of water stress may be enhanced by the increased concentrations of sulphate in the xylem which is also transported from the roots to the leaves.