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.

The occurrence and structure of radial sieve tubes in the secondary xylem of Aquilaria and Gyrinops

New observations of radial sieve tubes in the secondary xylem of two genera and four species of agarwood — Aquilaria sinensis, A. crasna, A. malaccensis and Gyrinops versteeghii (Thymelaeaceae) — are presented in this study. The earliest radial sieve tubes in Gyrinops are formed in the secondary xylem adjacent to the pith. The radial sieve tubes originate from the vascular cambium and develop in both uniseriate and multiseriate ray tissue. In addition to sieve plates in lateral and end walls, scattered or clustered minute sieve pores are localized in the lateral wall of radial sieve tubes. There is a direct connection between radial sieve tubes in ray tissue and axial sieve tubes in interxylary phloem strands (IP), such as (i) connection by bending of radial sieve tube strands, (ii) connection of two IP strands by an oblique bridge, and (iii) connection of two IP strands at a right angle. The average number of radial sieve tubes and interxylary phloem was found to be 1.7 per mm3 and 9.1 per mm2 in the secondary xylem. Considering the higher frequency of radial sieve tubes with the increasing thickness of the secondary xylem, the direct connections between radial and axial sieve tubes could play a significant role in assisting the translocation of metabolites in Aquilaria and Gyrinops.

The occurrence and development of intraxylary phloem in young Aquilaria sinensis shoots

ABSTRACT Agarwoods such as Aquilaria spp. and Gyrinops spp. (Thymelaeaceae) produce interxylary phloem in their secondary xylem and intraxylary phloem at the periphery of the pith, facing the primary xylem. We studied young shoots of Aquilaria sinensis and characterized the development of its intraxylary phloem. It was initiated by the division of parenchyma cells localized in the outer parts of the ground meristem immediately following the maturation of first-formed primary xylem. Its nascent sieve plates bore donut-like structures, the individual pores of which were so small (less than 0.1 μm) that they were hardly visible under FE-SEM. Intraxylary phloem developed into mature tissue by means of the division and proliferation of parenchyma cells. During the shoots’ active growth period, the sieve pore sizes were 0.1–0.5 μm, with tubular elements passing through them. In the maturation stage, large clusters of sieve tubes continued to be differentiated in the intraxylary phloem. In the partial senescence stage observed in a three-centimeter-diameter branch, intraxylary phloem cells in the adaxial part became crushed, and sieve plates had pores over 1–2 μm in diameter without any callose deposition. Before and after the differentiation of interxylary phloem in the first and second internodes, callose staining detected more than twice as many sieve tubes in intraxylary phloem than in external phloem. However, after differentiation of interxylary phloem in the eleventh internode, more sieve tubes were found in interxylary phloem than in intraxylary and external phloem. This suggests that prior to the initiation of interxylary phloem intraxylary phloem acts as the principal phloem. After its differentiation, however, interxylary phloem takes over the role of principal phloem. Interxylary phloem thus acts as the predominant phloem in the translocation of photosynthates in Aquilaria sinensis.

Redescription of Echinoderes levanderi Karling, 1954 (Kinorhyncha: Cyclorhagida) – a kinorhynch tolerant to very low salinities

The kinorhynch species Echinoderes levanderi Karling, 1954 is redescribed. The species can now be recognized by the presence of spines in middorsal positions on segments 4–8, and in lateroventral positions on segments 6–9, with lateroventral spines on segment 9 showing sexual dimorphism; tubes in subdorsal and ventrolateral positions on segment 2, in sublateral positions on segments 4 and 8, in lateroventral positions on segment 5, and in laterodorsal positions on segment 10. Furthermore, the enlarged sieve plates on segment 9 make the species highly characteristic. New records of the species extend its distributional range into the Bothnian Bay where the bottom water salinity drops below 5 ppt, which is the lowest salinity recorded for a habitat with kinorhynchs.