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Laticifers - among the most common defensive reservoirs in plants - are hypothesized to benefit plant fitness by preventing microbes from entering wounds. I argue that while latex seals wounds, and can suppress microbial growth, direct evidence that these processes benefit plant fitness is scarce. I outline a roadmap for filling this knowledge gap.

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Secretory structures in plants play a crucial role in producing bioactive compounds. Despite the potential of the Swartzia genus, comprehensive studies in this context are still scarce. Swartzia is a legume tree (Fabaceae) that occurs in the Brazilian Atlantic Forest, a biodiversity hotspot, and includes species such as Swartzia flaemingii. Therefore, we aim to achieve: (1) identify and characterize the key secretory sites responsible for saponin production in S. flaemingii leaflets; (2) confirm the presence of saponins in S. flaemingii leaves by comparing them with known chemical profiles of other Swartzia species; (3) assess the potential hemolytic and cytotoxic effects of crude leaf extracts. Our investigation unveils the presence of phenolic idioblasts, mucilage cells, and articulate laticifers, which play pivotal roles in defense and adaptation. Notably, we report the first-ever ultrastructural details of laticifers in a legume species. Additionally, oleanane-type saponins were identified in the leaves, giving insights into the chemotaxonomic profile of Swartzia. The crude extracts show low cytotoxicity levels, showcasing as a promising alternative source of saponins. This investigation reinforces the importance of conserving plants in threatened regions like the Atlantic Forest, a global biodiversity hotspot facing substantial anthropogenic pressures.

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Jatropha, a traditional medicinal plant known for its anti-inflammatory, antimicrobial, anticancer, antiviral, antidiabetic, and anticoagulant properties, was the subject of our study. We examined the morphology and chemical composition of three Jatropha species using cross- and longitudinal sections of fresh samples, observed with light microscopy. Histochemical analysis was conducted using various reagents to reveal the metabolites present. Anatomically, the distinguishing feature among the three Jatropha species was the presence of secretory cavities. These structures were identified in the petiole and stem bark of J. multifida, while in J. gossypiifolia and J. curcas they were present in roots. The stem bark cells of J. gossypiifolia were roundish in shape, whereas the others were rectangular. Laticifers were detected in the leaves, petioles, and stem bark of all three Jatropha species, while idioblasts were present in almost all organs. Histochemical tests revealed that excretory idioblasts and laticifers in Jatropha species contained alkaloids, phenolics, lipophilic compounds, and terpenoids. The cuticle of non-glandular trichomes contained terpenoids, while phenolic compounds were found within the secretory cavities. These findings contribute to the identification of Jatropha species and provide valuable insights for the selection and collection of specific plant organs containing bioactive compounds.

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[This corrects the article DOI: 10.3389/fpls.2022.971235.].

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The Sapotaceae are a significant component of the humid forests of the Neotropics and have many species of economic interest. Chrysophyllum gonocarpum is one of them and its edible fruits have currently acquired a high commercial value. Since there are no studies that cover its floral anatomy and elucidate its sexual system, the objective of the present study is to describe these aspects based on field observations and a detailed anatomical analysis of their flowers. Conventional techniques of plant anatomy are implemented. The results indicate that the species presents cryptic dioecy, showing specimens with morphologically and functionally pistillate flowers (with reduced staminodes), and trees with morphologically hermaphrodite and functionally staminate flowers. In addition, data on floral nectaries and laticiferous are provided.

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Taraxacum kok-saghyz Rodin (Tk) is a promising alternative rubber-producing grass. However, low biomass and rubber-producing capability limit its commercial application. As a carbon source transporter in plants, sugar will eventually be exported transporters (SWEETs) have been reported to play pivotal roles in diverse physiological events in the context of carbon assimilate transport and utilization. Theoretically, SWEETs would participate in Tk growth, development and response to environmental cues with relation to the accumulation of rubber and biomass, both of which rely on the input of carbon assimilates. Here, we identified 22 TkSWEETs through homology searching of the Tk genomes and bioinformatics analyses. RNA-seq and qRT-PCR analysis revealed these TkSWEETs to have overlapping yet distinct tissue expression patterns. Two TkSWEET isofroms, TkSWEET1 and TkSWEET12 expressed substantially in the latex, the cytoplasm of rubber-producing laticifers as well as the rubber source. As revealed by the transient expression analysis using Tk mesophyll protoplasts, both TkSWEET1 and TkSWEET12 were located in the plasma membrane. Heterologous expressions of the two TkSWEETs in a yeast mutant revealed that only TkSWEET1 exhibited apparent sugar transport activities, with a preference for monosaccharides. Interestingly, TkSWEET12, the latex-predominant TkSWEET isoform, seemed to have evolved from a tandem duplication event that results in a cluster of six TkSWEET genes with the TkSWEET12 therein, suggesting its specialized roles in the laticifers.

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Intestinal mucositis is characterized by inflammation and ulceration of the mucosa that affects the gastrointestinal tract and is associated with administering some drugs, such as 5- Fluorouracil (5-FU), conventional chemotherapy used in clinics for cancer therapy. Inside intestinal mucosa, the 5-FU acts, leading to oxidative stress, stimulating the production/release of proinflammatory cytokines, local accumulation of neutrophils and consequent tissue damage. These alterations favor bacterial proliferation, triggering secondary infections, and are responsible for undesired effects such as myelosuppression and diarrhea. These factors negatively impact oncological patients' quality of life and explain why they commonly interrupt their treatment prematurely. Currently, there is no specific drug with the ability to completely avoid this condition, so the search for new molecules with pharmacological properties that can be used for preventing or ameliorating intestinal mucositis is important. Plumeria pudica is a plant that produces latexcontaining molecules with therapeutic potential. A protein fraction obtained from this latex (LPPp), which comprises a well-defined mixture of chitinases, proteinases proteinase inhibitors, was demonstrated to have antioxidant and anti-inflammatory activities, preserving tissue glutathione and malondialdehyde concentration, reducing superoxide dismutase and myeloperoxidase activity, and reducing the level of proinflammatory cytokines in different experimental models. Given this scenario, inflammation and oxidative stress are directly involved in the pathogenesis of intestinal mucositis promoted by 5-FU. So, the hypothesis is that LPPp could inhibit these factors to attenuate the cytotoxicity of this pathology associated with 5-FU-treatment. This article brings new insights into the potential of the laticifer proteins extracted from the latex of P. pudica and opens new perspectives for the treatment of this type of intestinal mucositis with LPPp.

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Trehalose 6-phosphate (T6P), the intermediate of trehalose biosynthesis and a signaling molecule, affects crop yield via targeting sucrose allocation and utilization. As there have been no reports of T6P signaling affecting secondary metabolism in a crop plant, the rubber tree Hevea brasiliensis serves as an ideal model in this regard. Sucrose metabolism critically influences the productivity of natural rubber, a secondary metabolite of industrial importance. Here, we report on the characterization of the T6P synthase (TPS) gene family and the T6P/SNF1-related protein kinase1 (T6P/SnRK1) signaling components in Hevea laticifers under tapping (rubber harvesting), an agronomic manipulation that itself stimulates rubber production. A total of fourteen TPS genes were identified, among which a class II TPS gene, HbTPS5, seemed to have evolved with a function specialized in laticifers. T6P and trehalose increased when the trees were tapped, this being consistent with the observed enhanced activities of TPS and T6P phosphatase (TPP) and expression of an active TPS-encoding gene, HbTPS1. On the other hand, SnRK1 activities decreased, suggesting the inhibition of elevated T6P on SnRK1. Expression profiles of the SnRK1 marker genes coincided with elevated T6P and depressed SnRK1. Interestingly, HbTPS5 expression decreased significantly with the onset of tapping, suggesting a regulatory function in the T6P pathway associated with latex production in laticifers. In brief, transcriptional, enzymatic, and metabolic evidence supports the participation of T6P/SnRK1 signaling in rubber formation, thus providing a possible avenue to increasing the yield of a valuable secondary metabolite by targeting T6P in specific cells.

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Laticifers are secretory structures that produce latex, forming a specialized defense system against herbivory. Studies using anatomical approaches to investigate laticifer growth patterns have described their origin; however, their mode of growth, i.e., whether growth is intrusive or diffuse, remains unclear. Studies investigating how cytoskeleton filaments may influence laticifer shape establishment and growth patterns are lacking. In this study, we combined microtubule immunostaining and developmental anatomy to investigate the growth patterns in different types of laticifers. Standard anatomical methods were used to study laticifer development. Microtubules were labelled through immunolocalization of α-tubulin in three types of laticifers from three different plant species: nonanastomosing (Urvillea ulmacea), anastomosing unbranched with partial degradation of terminal cell walls (Ipomoea nil), and anastomosing branched laticifers with early and complete degradation of terminal cell walls (Asclepias curassavica). In both nonanastomosing and anastomosing laticifers, as well as in differentiating meristematic cells, parenchyma cells and idioblasts, microtubules were perpendicularly aligned to the cell growth axis. The analyses of laticifer microtubule orientation revealed an arrangement that corresponds to those cells that grow diffusely within the plant body. Nonanastomosing and anastomosing laticifers, branched or not, have a pattern which indicates diffuse growth. This innovative study on secretory structures represents a major advance in the knowledge of laticifers and their growth mode.

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Sesquiterpene lactone (STL) and natural rubber (NR) are characteristic isoprenoids in lettuce (Lactuca sativa). Both STL and NR co-accumulate in laticifers, pipe-like structures located along the vasculature. NR-biosynthetic genes are exclusively expressed in laticifers, but cell-type specific expression of STL-biosynthetic genes has not been studied. Here, we examined the expression pattern of germacrene A synthase (LsGAS), which catalyzes the first step in STL biosynthesis in lettuce. Quantitative PCR and Illumina read mapping revealed that the transcripts of two GAS isoforms (LsGAS1/LsGAS2) are expressed two orders of magnitude (~100-200) higher in stems than laticifers. This result implies that the cellular site for LsGAS1/2 expression is not in laticifers. To gain more insights, promoters of LsGAS1/2 were cloned and fused to ß-glucuronidase (GUS), followed by transformations of lettuce with these promoter-GUS constructs. In in situ GUS assays, the GUS expression driven by the LsGAS1/2 promoters was tightly associated with vascular bundles. High-resolution microsections showed that GUS signals are not present in laticifers but are detected in the vascular parenchyma cells neighboring the laticifers. These results suggest that expression of LsGAS1/2 occurs in the parenchyma cells neighboring laticifers, while the resulting STL metabolites accumulate in laticifers. It can be inferred that active metabolite-trafficking occurs from the parenchyma cells to laticifers in lettuce.

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Jatropha mollissima is endemic to Brazil and is used for traditional medicinal purposes, including the treatment of snakebite. In this study, latex obtained from this plant was fractioned using reversed-phase chromatography, and the fractions were then screened for peptides. A 755 g/mol peptide was obtained, and MS/MS analyses indicated it had a cyclic sequence (Pro-Leu-Gly-Val-Leu-Leu-Tyr). This peptide sequence was present in the Jatropha genome database, and an identity value of 90.71%, an E-value of 0.0, and a score of 883 with NO-associated protein 1/chloroplastic/mitochondria of Jatropha curcas were obtained from the NCBI nonredundant protein sequence (nr) database. Molecular docking analyses performed with the peptide against a metalloendopeptidase belonging to Crotalus adamanteus snake venom suggested the cyclic peptide establishes favorable interactions with the catalytic site of the enzyme. Therefore, it could inhibit enzyme catalysis. This belief was corroborated by the formation of 6 hydrogen bonds with the linear form of the peptide. Tighter complexation of the cyclic form (41 kcal/mol more energetic) revealed better spatial blocking. The linear form outperformed the cyclic form in complexing the required energy, recruiting more catalytic residues (6/2), and in establishing more hydrogen bonds (6/3). However, cyclic folding provided a more significant spatial block within the catalytic site. The set of results suggests that the cycle peptide, here called Jatromollistatin, which was previously described as jatrophidin and pohlianin A in two other species of Jatropha, is a promising candidate to inhibit venom proteases. This belief is corroborated by the topical use of the latex for initial treatment of snakebites.

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Araujia odorata is a sub-shrub native from Argentina, Brazil, Paraguay and Uruguay, whose latex, roots and leaves are used in traditional medicine. The objective of this work is to study the foliar morpho-anatomy of six populations in an altitudinal gradient (359-2155 m.a.s.l.) of Northwestern Argentine and to determine the nature of the compounds present in the laticiferous of the stems and fruits using conventional techniques for plant anatomy. The populations under study did not show significant morpho-anatomical differences. They presented simple leaves, pinnated brochydodromous venation, amphiestomatic isolateral lamina, brachy, anomo and amphicyclocytic stomata, eglandular trichomes, midvein with bicolateral vascular bundle and non-articulated laticifers continuous in the petiole, stem and fruits. Differences in the quantified foliar parameters are observed, however, only the density of trichomes, stomata and the thickness of the cuticle are positively correlated with the altitudinal gradient, indicating phenotypic plasticity. Histochemical analysis of laticifers and other stem idioblasts of A. odoratawas performed for the first time.

Araujia odorata, es un subarbusto nativo de Argentina, Brasil, Paraguay y Uruguay, cuyo látex, raíces y hojas son utilizados en medicina popular. Se plantea como objetivo realizar un estudio morfo-anatómico foliar de seis poblaciones del Noroeste Argentino en un gradiente altitudinal (359-2155 m.s.n.m) y determinar la naturaleza de los compuestos presentes en laticíferos de tallos y frutos mediante técnicas convencionales de anatomía vegetal. Las poblaciones estudiadas no evidenciaron diferencias morfo-anatómicas significativas. Presentan hojas simples, venación pinnada broquidódroma, lámina isolateral anfiestomática, estomas braqui, anomo y anficiclocíticos, tricomas eglandulares, nervio medio con haz bicolateral y laticíferos no-articulados continuos en pecíolo, tallo y frutos. Se observan diferencias en los parámetros foliares cuantificados, sin embargo, solo la de densidad de tricomas, estomas y el espesor de cutícula se correlacionan positivamente con el gradiente altitudinal indicando plasticidad fenotípica. Se realiza por primera vez un análisis histoquímico de los laticíferos y otros idioblastos del tallo A. odorata.

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[This corrects the article DOI: 10.3389/fpls.2020.612985.].

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The Swartzia species are commonly known as bloodwood due to the red exudate released from the stem after injury. This exudate has aroused great interest, and an integrative study is essential to describe it in detail. Thus, this work aimed to identify the red exudate's secreting-site in S. flaemingii and S. langsdorffii, and determine if it is a latex or a resin. Samples of the stem bark and the secondary xylem were prepared for histological analysis. Fresh exudates were dissolved in deuterated methanol and analyzed by 1H-NMR; other samples were resuspended in MeOH:H2O (9:1), partitioned with organic solvents and analyzed by direct infusion mass spectrometry. Total phenolic and total flavonoid contents were determined spectrophotometrically, and antioxidant capacity was determined using ferric reducing antioxidant power assay. The results showed that the exudate is a red latex produced by articulated laticifers located among the phloem cells. The latex is composed of sucrose, catechin glucosides, chlorophyll derivatives, and hederagenin-type saponins. Both samples of S. flaemingii and S. langsdorffii presented high amounts of phenolics and flavonoids, as well as a strong antioxidant capacity. The anatomical study showed that the secreting-site of the Swartzia red exudates were laticifers. This finding allows us to exclude other substances such as resin or oleoresin, generally produced by secretory cavities or ducts. Furthermore, since laticifers are rare in Fabaceae, this finding is significant, and represents an essential taxonomic feature. The showy red color is due to the large amounts of flavonoids. This latex probably has a protective role against microorganisms and photodamage. The bioactive potential of this exudate inspires further studies, which may boost the economic importance of Swartzia.

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MAIN CONCLUSION: Promoters of lettuce cis-prenyltransferase 3 (LsCPT3) and CPT-binding protein 2 (LsCBP2) specify gene expression in laticifers, as supported by in situ ß-glucuronidase stains and microsection analysis. Lettuce (Lactuca sativa) has articulated laticifers alongside vascular bundles. In the cytoplasm of laticifers, natural rubber (cis-1,4-polyisoprene) is synthesized by cis-prenyltransferase (LsCPT3) and CPT-binding protein (LsCBP2), both of which form an enzyme complex. Here we determined the gene structures of LsCPT3 and LsCBP2 and characterized their promoter activities using ß-glucuronidase (GUS) reporter assays in stable transgenic lines of lettuce. LsCPT3 has a single 7.4-kb intron while LsCBP2 has seven introns including a 940-bp intron in the 5'-untranslated region (UTR). Serially truncated LsCPT3 promoters (2.3 kb, 1.6 kb, and 1.1 kb) and the LsCBP2 promoter with (1.7 kb) or without (0.8 kb) the 940-bp introns were fused to GUS to examine their promoter activities. In situ GUS stains of the transgenic plants revealed that the 1.1-kb LsCPT3 and 0.8-kb LsCBP2 promoter without the 5'-UTR intron are sufficient to express GUS exclusively in laticifers. Fluorometric assays showed that the LsCBP2 promoter was several-fold stronger than the CaMV35S promoter and was ~ 400 times stronger than the LsCPT3 promoter in latex. Histochemical analyses confirmed that both promoters express GUS exclusively in laticifers, recognized by characteristic fused multicellular structures. We concluded that both the LsCPT3 and LsCBP2 promoters specify gene expression in laticifers, and the LsCBP2 promoter displays stronger expression than the CaMV35S promoter in laticifers. For the LsCPT3 promoter, it appears that unknown cis-elements outside of the currently examined LsCPT3 promoter are required to enhance LsCPT3 expression.

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The discovery of nickel hyperaccumulation, in Pycnandra acuminata, was the start of a global quest in this fascinating phenomenon. Despite recent advances in the physiology and molecular genetics of hyperaccumulation, the mechanisms and tolerance of Ni accumulation in the most extreme example reported to date, P. acuminata, remains enigmatic. We conducted a hydroponic experiment to establish Ni tolerance levels and translocation patterns in roots and shoots of P. acuminata, and analyzed elemental partitioning to gain insights into Ni regulation. We combined a phylogeny and foliar Ni concentrations to assess the incidence of hyperaccumulation within the genus Pycnandra. Hydroponic dosing experiments revealed that P. acuminata can resist extreme Ni concentrations in solution (up to 3,000 µM), and dosing at 100 µM Ni was beneficial to growth. All plant parts were highly enriched in Ni, but the latex had extreme Ni concentrations (124,000 µg g-1). Hyperaccumulation evolved independently in only two subgenera and five species of the genus Pycnandra. The extremely high level of Ni tolerance is posited to derive from the unique properties of laticifers. The evolutionary and ecological significance of Ni hyperaccumulation in Pycnandra is discussed in light of these findings. We suggest that Ni-rich laticifers might be more widespread in the plant kingdom and that more investigation is warranted.

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Due to the inconsistencies in the interpretation of laticifers within the Apocynaceae, the current study aimed to distinguish, for the first time, the type and distribution of the laticifers in the embryos, seedlings and adult plants of Tabernaemontana ventricosa (Forest Toad tree). The characterization and distribution of laticifers were determined using light and electron microscopy. The findings revealed the presence of articulated anastomosing laticifers. The laticifers were found to have originated from ground meristematic and procambium cells and were randomly distributed in all ground and vascular tissue, displaying complex branching conformations. The presence of chemical constituents within the laticifers and latex determined by histochemical analysis revealed the presence of alkaloids, phenolics, neutral lipids, terpenoids, mucilage, pectin, resin acids, carboxylated polysaccharides, lipophilic, and hydrophilic substances and proteins. These secondary metabolites perform an indispensable role in preventing herbivory, hindering and deterring micro-organisms and may possibly have medicinal importance. The outcomes of the present study outlined the first micromorphology, anatomy, ultrastructural and chemical analysis of the laticifers of T. ventricosa. In addition, this investigation similarly established the probable functions of latex and laticifers.

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Taraxacum koksaghyz has been identified as one of the most promising alternative rubber crops. Its high-quality rubber is produced in the latex of laticifers, a specialized cell type that is organized in a network of elongated tubules throughout the entire plant body. In order to gain insights into the physiological role(s) of latex and hence laticifer biology, we examine the effects of barnase-induced latex RNA degradation on the metabolite and protein compositions in the roots. We established high-quality datasets that enabled precise discrimination between cellular and physiological processes in laticifers and non-laticifer cell types of roots at different vegetative stages. We identified numerous latex-specific proteins, including a perilipin-like protein that has not been studied in plants yet. The barnase-expressing plants revealed a phenotype that did not exude latex, which may provide a valuable genetic basis for future studies of plant-environment interactions concerning latex and also help to clarify the evolution and arbitrary distribution of latex throughout the plant kingdom. The overview of temporal changes in composition and protein abundance provided by our data opens the way for a deeper understanding of the molecular interactions, reactions, and network relationships that underlie the different metabolic pathways in the roots of this potential rubber crop.

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Laticifer occurrence and structure are poorly known in Sapindaceae. Occurrence is likely underestimated owing to the low production of latex in most species. We investigated 67 species from 23 genera of Sapindaceae to verify laticifer occurrence and their structural, developmental and chemical features, as well as their evolutionary history in the family. Shoots were collected from herbarium and fresh specimens for histological analyses. Three characters derived from laticifer features were coded and their ancestral states reconstructed through Bayesian stochastic mapping and maximum likelihood estimation. Only articulated non-anastomosing laticifers were found in Sapindaceae. Laticifers differentiate early during shoot development and are found in the cortex, phloem, and pith. Latex is mostly composed of lipids. Callose and suberin were detected in laticifer cell walls in some genera. Reconstruction of laticifer ancestral states showed that laticifers are present in most clades of Sapindaceae with some reversals. Callose in the laticifer cell wall was found exclusively in Serjania and Paullinia (tribe Paullinieae), a character regarded as independently derived. Occurrence of laticifers in Sapindaceae is broader than previously reported. Articulated non-anastomosing laticifers had five independent origins in Sapindaceae with some secondary losses, occurring in five out of six genera of Paullinieae and 10 other genera outside Paullinieae. Particularly, callose in the laticifer cell wall evolved independently twice in the family, and its occurrence may be interpreted as a key-innovation that promoted the diversification of Paullinia and Serjania. Our study suggests that laticifer characters may be useful in understanding the generic relationships within the family.

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The cellular mechanisms of laticifer growth are of particular interest in plant biology but are commonly neglected. Using transmission electron microscopy and immunocytochemical methods, we recorded cytological differentiation and evaluated the cell wall involvement in the growth of articulated laticifers with intrusive growth in the mature embryo and plant shoot apex of Tabernaemontana catharinensis. The incorporation of adjacent meristematic cells into the laticifer system occurred in the embryo and plant shoot apex, and the incorporated cells acquired features of laticifer, confirming the laticifers' action-inducing mechanism. In the embryo, this was the main growth mechanism, and began with enlargement of the plasmodesmata and the formation of pores between laticifers and meristematic cells. In the plant shoot apex, it began with loose and disassembled walls and the reorientation of the cortical microtubules of the incorporated cell. Plasmodesmata were absent in these laticifers. There was stronger evidence of intrusive growth in undifferentiated portions of the plant shoot apex than in the embryo. The numerous plasmodesmata in laticifers of the embryo may have been related to the lower frequency of intrusive growth. Intrusive growth was associated with presence of arabinan (increasing wall flexibility and fluidity), and absence of galactan (avoiding wall stiffness), and callose (as a consequence of reduction in symplastic connections) in the laticifer walls. The abundance of low de-methyl-esterified homogalacturonan in the middle lamella and corners may reestablish cell-cell bonding in the laticifers. The cell wall features differed between embryo and plant shoot apex and are directly associated to laticifer growth mechanisms.

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