Anatomical features of conductive tissues of soybeans that prevent the penetration of pathogenic agents of bacterial blight into seeds

The ways of penetration of pathogenic bacteria from the infected vegetative parts of plants into soybean seeds remain practically unexplored. It is widely believed that soybean seeds are infected through the vascular system from already infected areas of the vegetative parts. The aim of the present research was to study the possibility of penetration of pathogens of bacterial blight into soybean seeds through the conductive tissues of plants. The studies were carried out in 2019–2021 in V.S. Pustovoit All-Russian Research Institute of Oil Crops on plants and seeds of soybean variety Vilana. It was found that the size of stomatal slots in soybean leaves is 8–12 µm. This ensures free penetration of bacteria with a diameter of 1.3–1.7 µm into the leaf mesophyll. The pore sizes of the sieve plates of the phloem range from 0.4–0.7 to 0.8–1.6 µm, depending on the age of the plants. The largest pores of the phloem sieve plates are comparable to the diameters of pathogenic bacteria. However, a large number of transverse sieve plates located in the vessels of the phloem every 0.05–0.1 mm will filter and partially retain bacteria in each sieve tube along the path of cell sap in the phloem. Therefore, the pathogenic bacteria passing through the entire phloem from leaves infected with bacteriosis up to pods is physically unlikely. In pods, the vascular system ends in the area of attachment of the placenta to the seed hilum. In the hilum, there are no conductive tissues, and the further flow of water and nutrients into the seed is carried out diffusely through the plasmodesma of cell walls. It was found that the anatomical structure of the soybean phloem prevents the free movement of pathogenic bacteria along the conductive system directly into the inner tissues of the seeds. Therefore, the hypothesis of infection of soybean seeds with pathogens of bacterial blight through the conducting system of the plant should be considered untenable

Sieve tube structural variation in Austrobaileya scandens and its significance for lianescence

Lianas combine large leaf areas with slender stems, features that require an efficient vascular system. The only extant member of the Austrobaileyaceae is an endemic twining liana of the tropical Australian forests with well-known xylem hydraulics, but the vascular phloem continuum aboveground remains understudied. Microscopy analysis across leaf veins and stems of A. scandens revealed a low foliar xylem to phloem ratio, with isodiametric vascular elements along the midrib, but tapered across vein orders. Small sieve plate pore radii increased from 0.08 µm in minor veins to 0.12 µm in the petiole, but only to 0.20 µm at the stem base, tens of meters away. In searcher branches, phloem conduits contained a pectin-rich wall and simple plates, whereas in twinning stems, conduits connected through highly-angled-densely populated sieve plates. Twisted and elongated stems of A. scandens display a high hydraulic resistance of phloem conduits, which decreases from leaves to stems, efficiently delivering photoassimilate from sources under Münch predictions. Sink strength of a continuously growing canopy might be stronger than in self-supporting understory plants, favoring resource allocation to aerial organs in angiosperms that colonized the vertical niche.

Sieve tube structural variation in Austrobaileya scandens and its significance for lianescence

Lianas are characterized by large leaf areas and slender stems, a combination of features that require an efficient vascular system. The only extant member of the Austrobaileyaceae, is an endemic twining liana of the tropical Australian forests with well-known xylem hydraulic traits. However, the vascular phloem continuum through aerial organs remains understudied. We analyzed the structure of phloem conduits across leaf veins and stems of A. scandens, combining topological data obtained through light and electron microscopy, with current models of phloem transport. Leaves displayed a low xylem to phloem ratio compared with leaves of other angiosperms, with vascular elements invariant in diameter along the midrib, but tapered across vein hierarchies. Sieve plate pore radii were extremely small: 0.08μm in minor veins, increasing to 0.12μm in the petiole and only to 0.20μm at the base of the stem, tens of meters away. Searcher branches contained tube shaped phloem conduits with a pectin-rich wall, whereas twining stems displayed sieve elements with tangential connections that displayed a greater fraction of the tubes populated with an astonishing number of sieve plates. Hydraulic segmentation of the leaves in Austrobaileyaceae correlate with vesseless leaves that benefit photoassimilate export through volumetric scaling of the sieve tube elements. Yet, compared with canopy dominant trees, the geometrical properties of the sieve tube in twining stems, restrict considerably energy distribution in the sub-canopy layers, potentially favoring the allocation of assimilates toward the elongating branches. Thus, the conductive xylem of twining stems contrasts with a poorly conductive phloem that meets the mechanical constraints of lianescence.

Conductivity of the phloem in mango (Mangifera indica L.)

Mango (Mangifera indica L., Anacardiaceae), the fifth most consumed fruit worldwide, is one of the most important fruit crops in tropical regions, but its vascular anatomy is quite unexplored. Previous studies examined the xylem structure in the stems of mango, but the anatomy of the phloem has remained elusive, leaving the long-distance transport of photoassimilates understudied. We combined fluorescence and electron microscopy to evaluate the structure of the phloem tissue in the tapering branches of mango trees, and used this information to describe the hydraulic conductivity of its sieve tube elements following current models of fluid transport in trees. We revealed that the anatomy of the phloem changes from current year branches, where it was protected by pericyclic fibres, to older ones, where the lack of fibres was concomitant with laticiferous canals embedded in the phloem tissue. Callose was present in the sieve plates, but also in the walls of the phloem sieve cells, making them discernible from other phloem cells. A scaling geometry of the sieve tube elements—including the number of sieve areas and the pore size across tapering branches—resulted in an exponential conductivity towards the base of the tree. These evaluations in mango fit with previous measurements of the phloem architecture in the stems of forest trees, suggesting that, despite agronomic management, the phloem sieve cells scale with the tapering branches. The pipe model theory applied to the continuous tubing system of the phloem appears as a good approach to understand the hydraulic transport of photoassimilates in fruit trees.

Evaluation of dsRNA delivery methods for targeting macrophage migration inhibitory factor MIF in RNAi-based aphid control

Macrophage migration inhibitory factors (MIFs) are multifunctional proteins regulating major processes in mammals, including activation of innate immune responses. In invertebrates, MIF proteins participate in the modulation of host immune responses when secreted by parasitic organisms, such as aphids. In this study, we assessed the possibility to use MIF genes as targets for RNA interference (RNAi)-based control of the grain aphid Sitobion avenae (Sa) on barley (Hordeum vulgare). When nymphs were fed on artificial diet containing double-stranded (ds)RNAs (SaMIF-dsRNAs) that target sequences of the three MIF genes SaMIF1, SaMIF2 and SaMIF3, they showed higher mortality rates and these rates correlated with reduced MIF transcript levels as compared to the aphids feeding on artificial diet containing a control dsRNA (GFP-dsRNA). Comparison of different feeding strategies showed that nymphs’ survival was not altered when they fed from barley seedlings sprayed with naked SaMIF-dsRNAs, suggesting they did not effectively take up dsRNA from the sieve tubes of these plants. Furthermore, aphids’ survival was also not affected when the nymphs fed on leaves supplied with dsRNA via basal cut ends of barley leaves. Consistent with this finding, the use of sieve tube-specific YFP-labeled Arabidopsis reporter lines confirmed that fluorescent 21 nt dsRNACy3, when supplied via petioles or spraying, co-localized with xylem structures, but not with phloem tissue. Our results suggest that MIF genes are a potential target for insect control and also imply that application of naked dsRNA to plants for aphid control is inefficient. More efforts should be put into the development of effective dsRNA formulations.

Bark characterization of Tachigali guianensis and Tachigali glauca from the Amazon under a valorization perspective

Barks of Tachigali guianensis and Tachigali glauca, from the Brazilian Amazon rainforest, were studied regarding anatomy and chemical composition. The barks were similar, with a narrow rhytidome, a ring of sclerified cells below the periderm, a widely dilated and sclerified nonconducting phloem, septate crystal strands, and extensive phenolic deposits in cells. Differences between the species were mainly in the sclerenchyma. Proportions of cell types in the T. guianensis and T. glauca barks were, respectively: 27.8% and 28.3% axial parenchyma, 15.6% and 15.1% sieve tube elements, 11.6% and 13.4% radial parenchyma, 15.6% and 8.7% sclereids, and 30.5% and 34.5% fibers. Chemical analysis showed that the T. guianensis and T. glauca barks included, respectively: 18.0% and 15.3% extractives, 1.8% and 1.0% suberin, 26.8% and 27.9% lignin, and 3.5% and 4.5% ash. The predominant polysaccharides were glucose (72.8% and 82.8% of total neutral sugars) and xylose (17.9% and 11.6%). Ethanol-water extracts were high in phenolics (total phenolics of 441.0 mg gallic acid equivalents (GAE) / g extract and 641.7 mg GAE / g extract), with moderate antioxidant activities (IC50 values of 7.3 µg extract / mL and 5.6 µg extract / mL). Tachigali guianensis bark and, particularly, T. glauca bark may be sources of phenolic compounds.

Evaluation of dsRNA delivery methods for targeting macrophage migration inhibitory factor MIF in RNAi-based aphid control

Macrophage migration inhibitory factors (MIF) are multifunctional proteins regulating major processes in mammals, including activation of innate immune responses. In invertebrates, MIF proteins participate in the modulation of host immune responses when secreted by parasitic organisms, such as aphids. In this study, we assessed the possibility to use MIF genes as targets for RNA interference (RNAi)-based control of the grain aphid Sitobion avenae (Sa) on barley (Hordeum vulgare). When nymphs were fed on artificial diet containing double-stranded (ds)RNAs (SaMIF-dsRNAs) that target sequences of the three MIF genes SaMIF1, SaMIF2 and SaMIF3, they showed higher mortality rates and these rates correlated with reduced MIF transcript levels as compared to the aphids feeding on artificial diet containing a control dsRNA (GFP-dsRNA). Comparison of different feeding strategies showed that nymphs’ survival was not altered when they fed from barley seedlings sprayed with SaMIF-dsRNAs, suggesting they did not effectively take up dsRNA from the sieve tubes of these plants. Furthermore, aphids’ survival was also not affected when the nymphs fed on leaves supplied with dsRNA via basal cut ends of barley leaves. Consistent with this finding, the use of sieve-tube-specific YFP-labeled Arabidopsis reporter lines confirmed that fluorescent 21 nt dsRNACy3 supplied via petioles co-localized with xylem structures, but not with phloem tissue. Our results suggest that MIF genes are a potential target for insect control and also imply that application of naked dsRNA to plants for aphid control is inefficient. More efforts should be put into the development of effective dsRNA formulations.

Conductivity of the phloem in Mangifera indica L.

ABSTRACTMangifera indica is the fifth most consumed fruit worldwide, and the most important in tropical regions, but its anatomy is quite unexplored. Previous studies examined the effect of chemicals on the xylem structure in the stems of mango, but the anatomy of the phloem has remained elusive, leaving the long distance transport of photo assimilates understudied.In this work, we used a combination of fluorescence and electron microscopy to evaluate in detail the structure of the sieve tube elements composing the phloem tissue in the tapering branches of mango trees. We then used this information to better understand the hydraulic conductivity of the sieve tubes following current models of fluid transport in trees.Our results revealed that the anatomy of the phloem in the stems changes from current year branches, where it was protected by pericyclic fibers, to older ones, where the lack of fibers was concomitant with laticiferous canals embedded in the phloem tissue. Callose was present in the sieve plates, but also in the walls of the phloem conduits, making them discernible from other phloem cells in fresh sections. A scaling geometry of the sieve tube elements, including the number of sieve areas and the pore size across tapering branches resulted in an exponential conductivity from current year branches to the base of the tree.Our measurements of the phloem in mango fit with measurements of the phloem architecture in the stems of forest woody species, and imply that, despite agronomic pruning practices, the sieve conduits of the phloem scale with the tapering branches. As a result, the pipe model theory applied to the continuous tubing system of the phloem appears as a good approach to understand the “long distance” hydraulic transport of photoassimilates in fruit trees.

Association of the 16SrII-D Phytoplasma with African Marigold (Tagetes erecta) Phyllody in Oman

The African marigold (Tagetes erecta L.) is an ornamental, herbaceous plant commonly found in Oman. In 2019, African marigold plants showing phyllody and virescence symptoms, which are typical symptoms of phytoplasmas disease, were found in at Sultan Qaboos University in Oman. Transmission electron microscopy of marigold leaf midrib from phyllody disease plants showed the presence of numerous phytoplasma bodies in the sieve tube of all of the symptomatic samples. DNA was extracted from asymptomatic and symptomatic marigold plant samples, followed by PCR of the 16S ribosomal RNA (rRNA) and imp genes. The PCR assays showed that the symptomatic plants are positive for phytoplasma. The DNA sequence analysis and phylogenetic trees showed that the 16S rDNA and imp gene sequences from all marigold phyllody strains shared 100% sequence identity to 16SrII-D subgroup sequences in the GenBank. This is the first report of a phytoplasma of the 16SrII-D subgroup associated with the African marigold (T. erecta) worldwide.

Phloem development, growth markers, and sieve-tube longevity in two Neotropical trees

This study provides a detailed analysis of phloem anatomy, development, the formation of cell types, differentiation, and sieve-tube element’s longevity in two tropical arboreal species, Cedrela fissilis (Meliaceae, Rosid) and Citharexylum myrianthum (Verbenaceae, Asterid), growing in natural populations in the semi-deciduous Atlantic Rainforest. We periodically collected samples from the main stem at breast height (1.3 m), during both the dry and the wet seasons. Differences in the cells produced at these different seasons suggest that annual growth increments in the phloem are present in both species, marked by files of terminal narrow sieve-tube elements radially grouped in Cedrela fissilis, and in assemblages of narrow sieve tubes and axial parenchyma in Cytharexylum myrianthum, both appearing at the end of the wet season. In Cedrela fissilis, where fiber bands are present, each fiber band marks the end of the early phloem, acting as an indirect annual growth ring marker. Sieve-tube element longevity varied for both species from 4–26 months, a result similar to that obtained in temperate species.