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Passiflora species presents a coevolutive relationship with Heliconiini butterflies, their primary herbivores. The Heliconiini caterpillars are able to detoxify toxic compounds produced by Passiflora, thus morphological defense strategies stand out over chemical innovations. In this framework, we highlight the presence of mimetic structures and extrafloral nectaries (EFN) as morphological strategies. Heliconian butterflies oviposit only on leaves that do not possess previous eggs, so the presence of egg mimics could prevent the oviposition. EFN are glands that offer nectar to territorial and aggressive ants, establishing mutualistic relationships. Here, we present a structural and chemical analysis of petiolar EFN and nectar from Passiflora alata and P. edulis in order to have insights about the implications of these features in deterring heliconian caterpillars. P. alata have one to four stipitate-crateriform EFN while P. edulis possess a pair of convex glands. Butterflies lay their eggs isolatedly or in up to three on leaves of both species. Our morphological results suggest that EFN from P. alata may act as egg mimics. Ontogenetic data suggest that the variation in the number of glands observed in this species is a serial homology, wherein the selection pressure for this variation is possibly the oviposition pattern. P. alata retain alkaloids, flavonoids and terpenoids inside nectariferous cells; sugars and flavonoids are found in the nectar of both species, while alkaloids are also detected in P. edulis. There is a selective retention/release of secondary metabolites from the EFN tissues to nectar. Knowing that these compounds can be dissuasive to some herbivores and inoffensive to others, we plotted this relationship in a consumer growth versus secondary metabolite concentration diagram. Our results suggest a more active role in the modulation of the gland defense from plants besides the establishment of a mutualistic relationship with ants, an important response in a coevolutive scenario.

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Here, we proposed that volatile organic compounds (VOC), specifically methyl salicylate (MeSA), mediate the formation of calcium oxalate crystals (COC) in the defence against ozone (O3) oxidative damage. We performed experiments using Croton floribundus, a pioneer tree species that is tolerant to O3 and widely distributed in the Brazilian forest. This species constitutively produces COC. We exposed plants to a controlled fumigation experiment and assessed biochemical, physiological, and morphological parameters. O3 induced a significant increase in the concentrations of constitutive oxygenated compounds, MeSA and terpenoids as well as in COC number. Our analysis supported the hypothesis that ozone-induced VOC (mainly MeSA) regulate ROS formation in a way that promotes the opening of calcium channels and the subsequent formation of COC in a fast and stable manner to stop the consequences of the reactive oxygen species in the tissue, indeed immobilising the excess calcium (caused by acute exposition to O3) that can be dangerous to the plant. To test this hypothesis, we performed an independent experiment spraying MeSA over C. floribundus plants and observed an increase in the number of COC, indicating that this compound has a potential to directly induce their formation. Thus, the tolerance of C. floribundus to O3 oxidative stress could be a consequence of a higher capacity for the production of VOC and COC rather than the modulation of antioxidant balance. We also present some insights into constitutive morphological features that may be related to the tolerance that this species exhibits to O3.

The emission profile of volatile organic compounds (VOCs) and nitric oxide (NO) in young and mature leaves of Croton floribundus was assessed in plants exposed to filtered air (FA) and ozone-enriched filtered air (FA+O). After the period of exposure, leaves were enclosed in polyethylene terephthalate bags and VOCs were collected in young and mature leaves. Both young and mature leaves constitutively emitted the same VOC, but the concentrations were higher in young leaves. O exposure induced the emission of sesquiterpenes (mainly β-caryophyllene) known as antioxidant compounds that may scavenge O. Young leaves were the highest emitters of sesquiterpenes. O induced a rapid accumulation of NO in different tissues and leaf developmental stages; this accumulation was marked in palisade and spongy parenchyma cells in young and mature leaves, respectively. O altered the levels of the signaling compound methyl salicylate (MeSA). Moreover, our data showed that NO together with VOC emissions, such as geranyl acetate, α-cadiene, trans-farnesol, cis-β-farnesene, and MeSA, participate of plant defense mechanisms against the oxidative damage caused by O.

BackgroundNectar gain and loss are important flower transitions observed in angiosperms, and are particularly common in orchids. To understand such transitions, the availability of detailed anatomical data and species-level phylogenies are crucial. We investigated the evolution of food deception in Epidendrum, one of the largest orchid genera, using genus phylogeny to map transitions between nectar gain and loss among different clades. Associations between anatomical and histochemical changes and nectar gain and loss were examined using fresh material available from 27 species. The evolution of nectar presence/absence in Epidendrum species was investigated in a phylogenetic framework of 47 species, using one nuclear and five plastid DNA regions available from GenBank and sequenced in this study.ResultsThe presence or absence of nectar was strongly associated with changes in the inner epidermal tissues of nectaries. Nectar-secreting species have unornamented epidermal tissue, in contrast to the unicellular trichomes found on the epidermis of food deceptive species. Bayesian tests confirmed that transitions occurred preferentially from nectar presence to nectar absence across the Epidendrum phylogeny. In addition, independent nectar loss events were found across the phylogeny, suggesting a lack of constraint for these transitions.ConclusionsOrnamented nectaries may play an important role in the deceptive pollination strategy by secreting volatile organic compounds and providing tactile stimuli to pollinators. The recurrent and apparently irreversible pattern of nectar loss in Epidendrum suggests that food deception may constitute an alternative evolutionarily stable strategy, as observed in other orchid groups.Electronic supplementary materialThe online version of this article (10.1186/s12870-018-1398-y) contains supplementary material, which is available to authorized users.

Trichome-like colleters seem to be a widespread character in Epidendroideae, and digitiform colleters are possibly the common type in this subfamily. Mucilage from IEOW colleters may aid in the establishment of symbiotic fungi necessary for seed germination. The presence of colleters in the IEOW may be a case of homeoheterotopy, in which extrafloral nectaries that produce simple sugar-based secretions (as in other orchid species) have changed to glands that produce secretions with complex polysaccharides, as in Pleurothallidinae.

In the tropical region, the greatest challenge of the biomonitoring approach is to establish linear relationships between biomarkers measured in plants and pollutant concentrations, since the bioindicator responses can be intensified or restricted by climatic variations. In southeastern Brazil, there are two regions affected by air pollution, where the Atlantic Forest remains and should be preserved. Consequently, both areas have been monitored by biomonitoring procedures using standardized and tropical plants. The industrial complex settled in Cubatão is one of the world's most famous examples of environmental pollution and degradation, with consequent decline of the Atlantic Forest. An oil refinery is among the most polluting industries in the Cubatão region. The other region is located in the Metropolitan Region of Campinas (MRC). The MRC has been affected by high levels of air pollutants originated from road traffic and is responsible for over 80% of CO, NOx, and hydrocarbon emissions and develops industrial activities that emit about 70% of the particulate matter present in the region. Both regions are distinguished by the climate, despite the fact that they are only about 130 km far from each other. Several studies carried out by our group in these regions aimed to establish the best native tree species and respective potential biomarkers for future assessment of pollution effects on tropical Forests. We present a critical review about the efficiency of native species compared to standardized bioindicator plants considering antioxidant defense system, nutrient accumulation, and microscopic aspects when exposed to atmospheric pollutants and climate.