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Background and AimsUnderstanding the evolutionary patterns of ecologically relevant traits is a central goal in plant biology. However, for most important traits, we lack the comprehensive understanding of their taxonomic distribution needed to evaluate their evolutionary mode and tempo across the tree of life. Here we evaluate the broad phylogenetic patterns of a common plant-defence trait found across vascular plants: extrafloral nectaries (EFNs), plant glands that secrete nectar and are located outside the flower. EFNs typically defend plants indirectly by attracting invertebrate predators who reduce herbivory.MethodsRecords of EFNs published over the last 135 years were compiled. After accounting for changes in taxonomy, phylogenetic comparative methods were used to evaluate patterns of EFN evolution, using a phylogeny of over 55 000 species of vascular plants. Using comparisons of parametric and non-parametric models, the true number of species with EFNs likely to exist beyond the current list was estimated.Key ResultsTo date, EFNs have been reported in 3941 species representing 745 genera in 108 families, about 1–2 % of vascular plant species and approx. 21 % of families. They are found in 33 of 65 angiosperm orders. Foliar nectaries are known in four of 36 fern families. Extrafloral nectaries are unknown in early angiosperms, magnoliids and gymnosperms. They occur throughout monocotyledons, yet most EFNs are found within eudicots, with the bulk of species with EFNs being rosids. Phylogenetic analyses strongly support the repeated gain and loss of EFNs across plant clades, especially in more derived dicot families, and suggest that EFNs are found in a minimum of 457 independent lineages. However, model selection methods estimate that the number of unreported cases of EFNs may be as high as the number of species already reported.ConclusionsEFNs are widespread and evolutionarily labile traits that have repeatedly evolved a remarkable number of times in vascular plants. Our current understanding of the phylogenetic patterns of EFNs makes them powerful candidates for future work exploring the drivers of their evolutionary origins, shifts, and losses.

Ant–plant symbioses involve over 110 ant species in five subfamilies that are facultative or obligate occupants of stem, leaf or root domatia formed by hundreds of ant-plant species. The phylogenetic distribution and geological ages of these associations, and the frequency of gains or losses of domatium, are largely unknown. We compiled an up-to-date list of ant domatium-bearing plants, estimated their probable true number from model-based statistical inference, generated dated phylogenies that include c. 50% of ant-plant lineages, and traced the occurrence of domatia and extrafloral nectaries on a 1181-species tree, using likelihood and Bayesian methods. We found 681 vascular plants with domatia (159 genera in 50 families) resulting from minimally 158 inferred domatium origins and 43 secondary losses over the last 19 Myr. The oldest African ant–plant symbioses are younger than those in Australasia and the Neotropics. The best statistical model suggests that the true number of myrmecophytes may approach 1140 species. The phylogenetic distribution of ant-plants shows that domatia evolved from a range of pre-adapted morphological structures and have been lost frequently, suggesting that domatia have no generalizable effect on diversification. The Miocene origin of ant–plant symbioses is consistent with inferred changes in diet and behaviour during ant evolution.

Anticlinal cell wall impregnations are common in trichomatic nectaries and their functions as endodermis-like barriers have been discussed because of possible direct effects on the nectary physiology, mainly in the nectar secretion and resorption. However, the cytological events linked to nectary wall impregnations remain little explored. This study documents the ontogenesis and the fine structure of the EFN cells, and cytological events linked to the wall impregnations of multi-layered extrafloral nectaries (EFNs) in Citharexylum myrianthum Cham. (Verbenaceae). EFNs are patelliform, and differentiated into (a) a multicellular foot, which is compound in structure and vascularised with phloem strands, (b) a bi-layered intermediate region with thickened cell walls and (c) a single-layered secretory region with palisade-like cells. EFNs are protodermal in origin, starting with a single protodermal cell and ending with the complex, multi-layered structure. The cell wall impregnations first appear in the very young EFN and increase towards maturity. Lipid patches (assumed to be suberin) are deposited on the inner faces of the primary walls, first along the anticlinal walls and then extend to the periclinal walls. On both walls, plasmodesmata remain apparently intact during the maturation of the EFNs. In the peripheral cytoplasm there are abundant polymorphic plastids, well-developed Golgi bodies often close to rough endoplasmic reticulum profiles, mitochondria and polyribosomes. Cytological events linked to the wall impregnations are consistent with suberin synthesis, transport and deposition. Our findings offer new insights into the structure-properties of specialised nectary cell walls and so should contribute to our knowledge of the physiological and protective roles of this structure in nectar glands.

Plant-insect mutualisms often drive the evolution of adaptive morphological and physiological traits, enabling ecological specialization and diversification. Fig trees (Ficus spp., Moraceae) and their pollinating wasps (Agaonidae) are engaged in a brood-site pollination mutualism that exemplifies such adaptive specializations. This study investigates the morphological and ecological roles of maculae, characterized as distinct-pigmented regions on the fig surface, in the mutualistic interaction between Ficus citrifolia and fig wasps. Through morphological analyses using light and electron microscopy, we demonstrated that maculae concentrate numerous stomata and exhibit secretory activity. This activity is evidenced by the exudation of a sugary-like solution and by the presence of epidermal and subepidermal cells with features consistent with sugar- and terpene-secreting cells, such as abundant starch reserves, numerous mitochondria, plastids containing osmiophilic droplets, a Golgi complex with dilated cisternae, oil bodies, and extensive endoplasmic reticulum. Histochemical tests confirmed a terpenic-sugary secretion in the macula cells. We demonstrated that non-pollinating fig wasps avoid ovipositing through macular regions. This behavior may reflect a selective pressure to prevent structural damage to maculae caused by ovipositor insertion, thus preserving their functional integrity. Temperature measurements revealed that figs are up to 10% cooler on average than the ambient air. Therefore, our findings suggest that fig maculae are multifunctional structures, simultaneously performing the roles of extrafloral nectaries, gas exchange, and thermal regulation, which are crucial for maintaining suitable internal conditions for wasp larval development. These results provide novel insights into previously underexplored plant adaptations supporting specialized brood-site pollination mutualisms.

Discussing homology relationships among secretory structures remains a relatively underexplored area in botanical research. These structures are widely dispersed within Malpighiales, one of the largest orders of eudicots. Within Malpighiales, both extranuptial and nuptial nectaries are present, and they do not seem homoplastic or share evolutionary connections. Particularly in Malpighiaceae, extensive research has focused on the ecological interactions mediated by glands. Botanists largely agree that elaiophores in sepals of Neotropical Malpighiaceae have evolved from extrafloral nectaries on leaves. However, the evolutionary origin of elaiophores has yet to be thoroughly examined, particularly in comparison to outgroups. This study provides empirical evidence on the ontogeny of elaiophores and investigates their evolutionary origins and homology relationships across different lineages of Malpighiales using comparative anatomy. Our findings suggest that elaiophores are likely homologous to extranuptial nectaries found in sepals of other Malpighiales lineages, originating from nectaries on leaves. This discussion is a starting point for future studies exploring the evolution of nectaries found in flowers, whether extranuptial or nuptial, and their potential origins from nectaries in vegetative organs such as leaves. Understanding these relationships could shed light on the selective pressures influencing floral morphologies.

Thousands of plants produce both extrafloral nectaries (EFNs) on their leaves and nutrient‐rich appendages on their diaspores (elaiosomes). Although their individual ecology is well‐known, any possible functional link between these structures has almost always been ignored. Here, we recognized their co‐presence in the shrub, Adenanthos cygnorum (Proteaceae), and studied their function and interaction. We observed that the same ants frequently visit both structures, seeds are attractive to vertebrate granivores but are released into a leafy cup from where they are harvested by ants and taken to their nests, from which seeds, lacking elaiosomes, germinate after fire. We showed that juvenile plants do not produce EFNs and are not visited by ants. We conclude that EFNs are not just an indirect adaptation to minimize herbivory via aggressive ant visitors (the role of a minority) but specifically enhance reproductive success in two ways: First, by inducing ants to visit the plant as a reliable food source throughout the year. Second, by promoting discovery of the seasonally available, elaiosome‐bearing seeds for transport to their nests (the majority of visitors), so avoiding the risk of granivory should seeds instead fall to the ground. Parasitoid wasps play a supporting role in controlling the main insect herbivore whose larvae devour the reproductive apices. Thus, the EFN‐elaiosome relationship has three components that enhance species fitness: foliage protection, seed transport, and granivore escape. A similar system has been described only once before (in an unrelated biome) and, consistent with the objectives of ecology as an integrative science, deserves wider study. Thousands of flowering plants produce both extrafloral nectaries on their leaves and seeds with nutrient‐rich appendages. Their possible interaction was examined in an Australian shrub here and shown that their co‐presence serves to attract ants to the plant throughout the year that then take the seeds from the plant to their nests during the flowering season rather than allowing the seeds to fall to the ground where they are likely to be consumed by birds and rodents.

Herbivores are attracted to young shoots and leaves because of their tender tissues. However, in extrafloral nectaried plants, young leaves also attract patrolling ants, which may chase or prey on herbivores. We examined this scenario in extrafloral nectaried shrubs of Banisteriopsis malifolia resprouting after fire, which promoted both the aseasonal production of leaves and the activity of extrafloral nectaries (EFNs). Results were compared between resprouting (burned) and unburned control plants. The aggressive ant species Camponotus crassus and the herbivorous thrips Pseudophilothrips obscuricornis were respectively rapidly attracted to resprouting plants because of the active EFNs and their less sclerophyllous leaves. The abundance of these insects was almost negligible in the control (unburned) shrubs. Ants failed to protect B. malifolia, as no thrips were preyed upon or injured by ants in resprouting plants. Consequently, on average, 37 % of leaves from resprouting shrubs had necrosis marks. Upon contact with ants, thrips released small liquid droplets from their abdomen, which rapidly displaced ants from the surroundings. This study shows that P. obscuricornis disrupted the facultative mutualism between C. crassus and B. malifolia, since ants received extrafloral nectar from plants, but were unable to deter herbivore thrips.

In the Malveae tribe (Malvaceae), the axis supporting the flower has a joint at the upper third. This axis can be considered as an articulated pedicel, peduncle, peduncle-pedicel, or anthopodium. Such disparity in terminology reveals a duality in interpretation since this structure is classified as part of the inflorescence or part of the flower. In an effort to reach a consensus, this study aims to evaluate axes supporting the flowers of species from the Malveae tribe through ontogenetic, morphological, and histochemical analyses, using light microscopy and scanning electron microscopy. Ontogenetic analyses indicated that the axis supporting the flower is an articulated pedicel, which is divided into proximal and distal parts owing to the presence of the constriction (joint). Simultaneously, the articulated pedicel arises from the floral meristem, along with the establishment of the calyx and androecium. As development progresses, we observed frequent abscissions of the floral bud, along with the distal portion of the pedicel, at the joint. After this, the remaining proximal portion of the pedicel becomes secretory, as an extrafloral nectary, often foraged by ants of the genus Wasmannia . Thus, this ontogenetic analysis of the articulated pedicel helps in understanding its functionality and morphological variability, highlighting the importance of standardized terminology since it would lead to conceptual clarity in different studies. Additionally, this study, for the first time, reveals the presence of extrafloral nectaries on articulated pedicels in Malveae, a previously undocumented feature in Malveae and Malvaceae.

Plants with extrafloral nectaries attract a variety of ant species, in associations commonly considered mutualistic. However, the results of such interactions can be context dependent. Turnera subulata is a shrub widely distributed among disturbed areas which has extrafloral nectaries at the base of leaves. Here, we evaluated whether the ants associated with T. subulata (i) vary in space and/or time; (ii) respond to simulated herbivory, and (iii) reduce herbivory rates. For this, we quantified the abundance and species richness of ants associated with T. subulata throughout the day in six different sites and the defensive capability of these ants under simulated herbivory in the leaves and stems of T. subulata plants ( N  = 60). We also checked the proportion of the lost leaf area and quantified leaf damage by chewing herbivores in the host plant. We found that a total of 21 ant species associated with the host plant. Species composition showed significant variation across the sampled sites and throughout the day. Visitation rates and predation by ants were higher in plant stems than in leaves. In general, herbivory rates were not correlated with ant association or activity, with the exception of the proportion of leaf area consumed; there was a significant lower herbivory rate on plants in which ants defended the leaves. Our results suggest that the benefits of association may depend on the ecological context. This context dependence may mask the correlation between the defense of ants and herbivory rates.

BACKGROUND AND AIMS: Although most studies on plant defence strategies have focused on a particular defence trait, some plant species develop multiple defence traits. To clarify the effects of light on the development of multiple defence traits, the production of direct and indirect defence traits of young plants of Mallotus japonicus were examined experimentally under different light conditions. METHODS: The young plants were cultivated under three light conditions in the experimental field for 3 months from May to July. Numbers of ants and pearl bodies on leaves in July were examined. After cultivation, the plants were collected and the developments of trichomes and pellucid dots, and extrafloral nectaries (EFNs) on the leaves were examined. On plants without nectar-collecting insects, the size of EFNs and the volume of extrafloral nectar secreted from the EFNs were examined. KEY RESULTS: Densities of trichomes and pellucid dots did not differ significantly among the plants under the different light conditions, suggesting that the chemical and physical defences function under both high and low light availability. The number of EFNs on the leaves did not differ significantly among the plants under the different light conditions, but there appeared to be a trade-off between the size of EFNs and the number of pearl bodies; the largest EFNs and the smallest number of pearl bodies were found under high light availability. EFN size was significantly correlated with the volume of extrafloral nectar secreted for 24 h. The number of ants on the plants was smaller under low light availability than under high and moderate light availability. CONCLUSIONS: These results suggest that direct defence traits function regardless of light conditions, but light conditions affected the development of indirect defence traits.