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

Plant age-dependent dynamics of annatto pigment (bixin) biosynthesis in Bixa orellana

Age affects the production of secondary metabolites, but how developmental cues regulate secondary metabolism remains poorly understood. The achiote tree (Bixa orellana L.) is a source of bixin, an apocarotenoid used in diverse industries worldwide. Understanding how age-dependent mechanisms control bixin biosynthesis is of great interest for plant biology and for economic reasons. Here we overexpressed miRNA156 (miR156) in B. orellana to comprehensively study the effects of the miR156-SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) module on age-dependent bixin biosynthesis in leaves. Overexpression of miR156 in annatto plants (miR156ox) reduced BoSPL transcript levels, impacted leaf ontogeny, lessened bixin production, and increased abscisic acid levels. Modulation of expression of BoCCD4-4 and BoCCD1, key genes in carotenoid biosynthesis, was associated with diverting the carbon flux from bixin to abscisic acid in miR156ox leaves. Proteomic analyses revealed an overall low accumulation of most secondary metabolite-related enzymes in miR156ox leaves, suggesting that miR156-targeted BoSPLs may be required to activate several secondary metabolic pathways. Our findings suggest that the conserved BomiR156-BoSPL module is deployed to regulate leaf dynamics of bixin biosynthesis, and may create novel opportunities to fine-tune bixin output in B. orellana breeding programs.

Exploring the co-operativity of secretory structures for defense and pollination in flowering plants

MAIN CONCLUSION

In flowers multiple secretory systems cooperate to deliver specialized metabolites to support specific roles in defence and pollination. The collective roles of cell types, enzymes, and transporters are discussed. The interplay between reproductive strategies and defense mechanisms in flowering plants has long been recognized, with trade-offs between investment in defense and reproduction predicted. Glandular trichomes and secretory cavities or ducts, which are epidermal and internal structures, play a pivotal role in the secretion, accumulation, and transport of specialized secondary metabolites, and contribute significantly to defense and pollination. Recent investigations have revealed an intricate connection between these two structures, whereby specialized volatile and non-volatile metabolites are exchanged, collectively shaping their respective ecological functions. However, a comprehensive understanding of this profound integration remains largely elusive. In this review, we explore the secretory systems and associated secondary metabolism primarily in Asteraceous species to propose potential shared mechanisms facilitating the directional translocation of these metabolites to diverse destinations. We summarize recent advances in our understanding of the cooperativity between epidermal and internal secretory structures in the biosynthesis, secretion, accumulation, and emission of terpenes, providing specific well-documented examples from pyrethrum (Tanacetum cinerariifolium). Pyrethrum is renowned for its natural pyrethrin insecticides, which accumulate in the flower head, and more recently, for emitting an aphid alarm pheromone. These examples highlight the diverse specializations of secondary metabolism in pyrethrum and raise intriguing questions regarding the regulation of production and translocation of these compounds within and between its various epidermal and internal secretory systems, spanning multiple tissues, to serve distinct ecological purposes. By discussing the cooperative nature of secretory structures in flowering plants, this review sheds light on the intricate mechanisms underlying the ecological roles of terpenes in defense and pollination.

Neither lysigenous nor just oil: Demystifying myrtaceous secretory cavities

PREMISE

Leaf subepidermal secretory cavities are a notable trait in Myrtaceae, but their formation is still controversial because of the lack of consensus on their ontogeny among authors. Knowledge about the compounds present in these cavities has grown over the last few years, demonstrating that terpenoid-rich oils are not their unique content. These two points are the focus of this study on the ontogeny, structure, and contents of secretory cavities in neotropical Myrtaceae.

METHODS

We used histochemical tests and Raman analysis to verify the basic chemical composition of the cavity contents of nine species. We studied the ontogeny of glands in one species, comparing aldehyde-fixed tissues and fresh sections mounted in an inert medium.

RESULTS

We observed schizogenous development and appearance of the secretory cavities and found that sample processing may induce cell breakdown, which can be misinterpreted as lysigeny. The content of these cavities contains putative terpenes, resins, carbonyl groups, and flavonoids.

CONCLUSIONS

Our findings support the hypothesis that the lysigenous appearance of the oil glands is a technical artifact. These tissue distortions must be considered when interpreting the development of this type of secretory structure. Moreover, the basic analyses of chemical constituents show for the first time that the glands of neotropical Myrtaceae are potential reservoirs of some compounds such as flavonoids previously reported as novelties for a few other myrtaceous species. Because some of them are non-lipid compounds, the idea that the glands are just oil repositories is no longer applicable.

Age-dependent modulation of oleoresin production in the stem of Sindora glabra

Plants produce specialized metabolites in various organs which serve important functions in defense and development. However, the molecular regulatory mechanisms of oleoresin production in stems from broadleaved tree species are not fully understood. To determine whether endogenous developmental cues play a role in the regulation of oleoresin biosynthesis in tree stems, anatomy, multi-omics and molecular experiments were utilized to investigate the change of secretory structures, chemical profiles and gene expression in different ontogenetic stages of Sindora glabra tree, which accumulates copious amount of sesquiterpene-rich oleoresin in stems. The size of secretory canals and the concentration of five sesquiterpenes in Sindora stems exhibited obvious increase with plant age, from 0.5- to 20-year-old plants. Moreover, α-copaene and β-copaene were found to be stem-specific sesquiterpenes. Metabolomic analysis revealed that salicylic acid highly accumulated in mature stems, but the content of triterpenes was greatly decreased. The expression of three repressors AUX/IAA, DELLA and JAZ involved in hormone signaling transduction pathways was significantly downregulated in stems of 10- and 20-year-old plants. Two key genes SgTPS3 and SgTPS5 were identified, whose expression was highly correlated with the accumulation patterns of specific sesquiterpenes and their enzymatic products were consistent with the chemical profiles in the stem. The promoters of three SgTPSs exhibiting high activity were isolated. Furthermore, we demonstrated that SgSPL15 directly interacts with SgTPS3 and SgTPS5 promoters and activates SgTPS5 expression but SgSPL15 inhibits SgTPS3 expression. In addition, SgSPL15 enhanced sesquiterpene levels by upregulating AtTPSs expression in Arabidopsis. These results suggested that sesquiterpene biosynthesis in S. glabra stem was dependent on the regulation of endogenous hormones as well as plant age, and SgSPL15 might act as a buffering factor to regulate sesquiterpene biosynthesis by targeting SgTPS genes.

Acute methyl jasmonate exposure results in major bursts of stress volatiles, but in surprisingly low impact on specialized volatile emissions in the fragrant grass Cymbopogon flexuosus

Methyl jasmonate (MeJA) is an airborne hormonal elicitor that induces a fast rise of emissions of characteristic stress marker compounds methanol and green leaf volatiles (GLV), and a longer-term release of volatile terpenoids, but there is limited information of how terpene emissions respond to MeJA in terpene-storing species. East-Indian lemongrass (Cymbopogon flexuosus), an aromatic herb with a large terpenoid storage pool in idioblasts, was used to investigate the short- (0-1 h) and long-term (1-16 h) responses of leaf net assimilation rate (A), stomatal conductance (G_s) and volatile emissions to MeJA concentrations ranging from moderate to lethal. Both A and G_s were increasingly inhibited with increasing MeJA concentration in both short and long term. MeJA exposure resulted in a rapid elicitation, within 1 h after exposure, of methanol and GLV emissions. Subsequently, a secondary rise of GLV emissions was observed, peaking at 2 h after MeJA exposure for the highest and at 8 h for the lowest application concentration. The total amount and maximum emission rate of methanol and the first and second GLV emission bursts were positively correlated with MeJA concentration. Unexpectedly, no de novo elicitation of terpene emissions was observed through the experiment. Although high MeJA application concentrations led to visible lesions and desiccation in extensive leaf regions, this did not result in breakage of terpene-storing idioblasts. The study highlights an overall insensitivity of lemongrass to MeJA and indicates that differently from mechanical wounding, MeJA-driven cellular death does not break terpene-storing cells. Further studies are needed to characterize the sensitivity of induced defense responses in species with strongly developed constitutive defenses.

Insights into crucial odourants dominating the characteristic flavour of citrus-white teas prepared from citrus reticulata Blanco 'Chachiensis' and Camellia sinensis 'Fudingdabai'

Citrus-white teas (CWs), which possess a balanced flavour of tea and citrus, are becoming more popular worldwide; however, their characteristic flavour and odourants received limited research. Volatile components of two types of CWs prepared from Citrus reticulata Blanco 'Chachiensis' and Camellia sinensis 'Fudingdabai' were comprehensively investigated using a combination of stir bar sorptive extraction and gas chromatography-mass spectrometry (GC-MS). Ninety-nine crucial odourants in the CWs were quantified by applying GC-olfactometry/MS, significant differences were compared, and their odour activity values (OAVs) were calculated. Twenty-two odourants (in total 2628.09 and 1131.18 mg/kg respectively) were further confirmed as traditional CW (CW-A) and innovated CW (CW-B) characteristic flavour crucial contributors which all possessed > 1 OAVs, particularly limonene (72919 in CW-A) and trans-β-ionone (138953 in CW-B). The unravelling of CWs aroma composition will greatly expanding our understanding of tea aroma chemistry and the potential aroma interactions will offer insights into tea blending technologies.

Short-term changes related to autotetraploidy in essential oil composition of Eucalyptus benthamii Maiden & Cambage and its applications in different bioassays

Some forest trees have been polyploidized to improve their traits and to supply new germplasms for breeding programs. As trees have a long juvenile stage, the early characterization of the chromosome set doubling effects is crucial for previous selection. Thus, we aimed to characterize the chemical variability of essential oils from diploid and autotetraploid germplasms (autotetraploid A and B) of Eucalyptus benthamii, as well as to evaluate their larvicidal and allelopathic effects. Autotetraploid A showed a higher essential oil yield than diploid and autotetraploid B, which did not differ quantitatively. Aromadendrene, viridiflorol and α-pinene were the major compounds in the diploid essential oil. In contrast, compounds were present in autotetraploids, such as 1,8-cineole, limonene, α-terpineol, and α-terpinyl-acetate. Essential oils from the diploid at 50-200 ppm were twice as larvicidal than those from autotetraploids against Aedes aegypti larvae. Considering the phytotoxicity bioassays using Lactuca sativa, essential oils from both ploidy levels affected root growth. Moreover, the essential oils inhibited shoot growth at all concentrations tested (187.5; 375; 750; 1500; and 3000 ppm). Autotetraploid A and B had the same effect on shoot growth as glyphosate. The essential oils had no cytogenotoxic effect on root meristematic cells of L. sativa, whereas phytotoxic potential was identified mainly in shoot growth. This work demonstrated a dramatic change in secondary metabolism (terpene composition) related to an increase in the ploidy level in Eucalyptus germplasms. In addition, we report the novelty of the chemical composition of essential oils among germplasms and their potential use as larvicidal and post-emergence weed control agents.

Plant biochemistry influences tropospheric ozone formation, destruction, deposition, and response

Tropospheric ozone (O₃) is among the most damaging air pollutant to plants. Plants alter the atmospheric O₃ concentration in two distinct ways: (i) by the emission of volatile organic compounds (VOCs) that are precursors of O_3; and (ii) by dry deposition, which includes diffusion of O₃ into vegetation through stomata and destruction by nonstomatal pathways. Isoprene, monoterpenes, and higher terpenoids are emitted by plants in quantities that alter tropospheric O₃. Deposition of O₃ into vegetation is related to stomatal conductance, leaf structural traits, and the detoxification capacity of the apoplast. The biochemical fate of O₃ once it enters leaves and reacts with aqueous surfaces is largely unknown, but new techniques for the tracking and identification of initial products have the potential to open the black box.

Tissue specificity of (E)-β-farnesene and germacrene D accumulation in pyrethrum flowers

Plant defensive mimicry based on the aphid alarm pheromone (E)-β-farnesene (EβF) was previously shown to operate in Tanacetum cinerariifolium (Asteraceae) flowers. Germacrene D (GD), is another dominant volatile of T. cinerariifolium flowers and may modulate both defense and pollination. Here, we find that the increase in GD/EβF ratio at later developmental stages is correlated with the tissue distribution in the flower head: the total content of EβF and GD is similar, but GD accumulates comparatively more in the upper disk florets. Naphthol and N, N-dimethyl-p-phenylenediamine dihydrochloride (NADI)-stained purple ducts containing EβF and GD, were observed in the five petal lips of the corolla and two-lobed stigma of disk florets. By contrast in the peduncle, EβF accounts for nearly 80% of total terpenes, compared to 5% for GD. EβF is accumulated inside inner cortex cells and parenchyma cells of the pith in young peduncle. This is followed by the formation of terpene-filled axial secretory cavities parallel to the vascular bundles. In conclusion, the observed developmental and diurnal emissions of different EβF/GD ratios appear to be regulated by their tissue distribution.

Dynamic trajectories of volatile and non-volatile specialised metabolites in 'overnight' fragrant flowers of Murraya paniculata

Ephemeral flowers, especially nocturnal ones, usually emit characteristic scent profiles within their post-anthesis lifespans of a few hours. Whether these flowers exhibit temporal variability in the composition and profile of volatile and non-volatile specialised metabolites has received little attention. Flowers of Murraya paniculata bloom in the evenings during the summer and monsoon, and their sweet, intense fragrance enhances the plant's value as an ornamental. We aimed to investigate profiles of both volatile and non-volatile endogenous specialised metabolites (ESM) in nocturnal ephemeral flowers of M. paniculata to examine whether any biochemically diverse groups of ESM follow distinct patterns of accumulation while maintaining synchrony with defensive physiological functions. Targeted ESM contents of M. paniculata flowers were profiled at ten time points at 2-h intervals, starting from late bud stage (afternoon) up to the start of petal senescence (mid-morning). Emitted volatiles were monitored continuously within the whole 20-h period using headspace sampling. The ESM contents were mapped by time point to obtain a highly dynamic and biochemically diverse profile. Relative temporal patterns of ESM accumulation indicated that the active fragrance-emitting period might be divided into 'early bloom', 'mid-bloom' and 'late bloom' phases. Early and late bloom phases were characterised by high free radical generation, with immediate enhancement of antioxidant enzymes and phenolic compounds. The mid-bloom phase was relatively stable and dedicated to maximum fragrance emission, with provision for strong terpenoid-mediated defence against herbivores. The late bloom phase merged into senescence with the start of daylight; however, even the senescent petals continued to emit fragrance to attract diurnal pollinators. Our study suggests that dynamic relations between the different ESM groups regulate the short-term requirements of floral advertisement and phytochemical defence in this ephemeral flower. This study also provided fundamental information on the temporal occurrence of emitted volatiles and internal pools of specialised metabolites in M. paniculata flowers, which could serve as an important model for pollination biology of Rutaceae, which includes many important fruit crops.

Not Led by the Nose: Volatiles from Undamaged Eucalyptus Hosts Do Not Influence Psyllid Orientation

Psyllids (Hemiptera: Psylloidea) are small sucking insects with high host plant specificity. Despite the primitive olfactory system of psyllids, some species have been suggested to rely on host plant volatiles (HPVs) for seasonal migration between summer deciduous hosts and winter coniferous hosts. Similarly, enhanced attraction of psyllid vectors has been observed as a result of the manipulation of host odors by plant pathogens. As yet, there are no studies of olfaction in psyllids that utilize evergreen eucalypt hosts. We investigated the behavioral responses of adults of four Eucalyptus-feeding psyllids—Ctenarytaina eucalypti, C. bipartita, Anoeconeossa bundoorensis and Glycaspis brimblecombei—to their respective HPVs in Y-tube olfactometer bioassays. We also used existing physiological data for C. eucalypti to investigate potential olfactory tuning that may modulate the preference for morphologically juvenile leaves over morphologically adult leaves. Although adult C. eucalypti were consistently repelled by HPVs from damaged host leaves, none of the species exhibited positive chemotaxis to HPVs from undamaged leaves. Surprisingly, G. brimblecombei was repelled by HPVs from undamaged host leaves. Our findings provide little support for a significant role of olfaction in host location by Eucalyptus-feeding psyllids. We propose a number of ecological hypotheses to explain these unexpected findings.