The Role of Pectobacterium atrosepticum Exopolysaccharides in Plant–Pathogen Interactions

The phytopathogenic bacterium Pectobacterium atrosepticum (Pba), one of the members of the soft rot Pectobacteriaceae, forms biofilm-like structures known as bacterial emboli when colonizing the primary xylem vessels of the host plants. The initial extracellular matrix of the bacterial emboli is composed of the host plant’s pectic polysaccharides, which are gradually substituted by the Pba-produced exopolysaccharides (Pba EPS) as the bacterial emboli “mature”. No information about the properties of Pba EPS and their possible roles in Pba-plant interactions has so far been obtained. We have shown that Pba EPS possess physical properties that can promote the maintenance of the structural integrity of bacterial emboli. These polymers increase the viscosity of liquids and form large supramolecular aggregates. The formation of Pba EPS aggregates is provided (at least partly) by the acetyl groups of the Pba EPS molecules. Besides, Pba EPS scavenge reactive oxygen species (ROS), the accumulation of which is known to be associated with the formation of bacterial emboli. In addition, Pba EPS act as suppressors of the quantitative immunity of plants, repressing PAMP-induced reactions; this property is partly lost in the deacetylated form of Pba EPS. Overall, our study shows that Pba EPS play structural, protective, and immunosuppressive roles during Pba–plant interactions and thus should be considered as virulence factors of these bacteria.

Genome-wide Analysis of the WUSCHEL-related Homeobox Gene Family and Functional Characterization of VcWOX4b Regarding the Inhibition of Adventitious Root Formation in Blueberry (Vaccinium Spp.)

Background: Blueberry (Vaccinium corymbosum L.) is one of the most important commercial fruit tree species. The development of high-quality seedlings is a prerequisite for fruit production. Stem cutting and tissue culture methods are widely applied for propagating blueberry seedlings. Both methods require adventitious roots (ARs), indicating ARs are critical for vegetative propagation. However, the underlying factors and molecular mechanisms regulating blueberry AR formation remain relatively unknown. Results: In this study, the rooting abilities of differentially lignified cuttings from various cultivars or the same cultivars cultured differently were evaluated following an indole-3-butyric acid (IBA) treatment. Field-grown semi-lignified and tissue culture-grown cuttings formed ARs, but the latter had more pericycle and secondary xylem cells and formed ARs more easily and faster. WUSCHEL-related homeobox genes are commonly involved in vascular tissue development and early root meristem maintenance. On the basis of the available Vaccinium corymbosum genome data, 29 putative WOX genes with conserved homeodomains were identified and divided into three major clades (modern/WUS, intermediate, and ancient). These 29 WOX genes were differentially expressed in the root, shoot, leaf, flower bud, and fruit. Additionally, a qRT-PCR analysis revealed that five selected VcWOX genes were responsive to an IBA treatment during AR formation. Accordingly, VcWOX4b was functionally characterized. The overexpression of VcWOX4b in transgenic tobacco inhibited AR formation by altering vascular cell division and differentiation and the indole-3-acetic acid (IAA):cytokinin (CTK) ratio. These observations suggest that VcWOX4b regulates the IAA:CTK ratio to promote primary xylem cell differentiation, thereby inhibiting AR formation. However, an IBA treatment can induce AR formation by inhibiting VcWOX4b expression. Conclusions: Current study elucidates the rooting abilities of various cultivars and the cytological characters of influence on AR formation of blueberry cuttings, which may provide novel insights into the selection of high-quality blueberry cuttings. VcWOX4b, VcWOX8/9a, VcWOX11/12c, and VcWOX13b might regulate blueberry AR formation in an IBA-dependent manner. Ectopic expression of VcWOX4b modulated the IAA:CTK ratio to promotes primary xylem cell differentiation, but inhibit secondary xylem cell differentiation, ultimately leading to decreased AR formation.

Study on the Regulatory Effects of GA3 on Soybean Internode Elongation

Excessive plant height is an important factor that can lead to lodging, which is closely related to soybean yield. Gibberellins are widely used as plant growth regulators in agricultural production. Gibberellic acid (GA3), one of the most effective active gibberellins, has been used to regulate plant height and increase yields. The mechanism through which GA3 regulates internode elongation has been extensively investigated. In 2019 and 2020, we applied GA3 to the stems, leaves, and roots of two soybean cultivars, Heinong 48 (a high-stalk cultivar) and Henong 60 (a dwarf cultivar), and GA3 was also applied to plants whose apical meristem was removed or to girded plants to compare the internode length and stem GA3 content of soybean plants under different treatments. These results suggested that the application of GA3 to the stems, leaves, and roots of soybean increased the internode length and GA3 content in the stems. Application of GA3 decreased the proportion of the pith in the soybean stems and primary xylem while increasing the proportion of secondary xylem. The apical meristem is an important site of GA3 synthesis in soybean stems and is involved in the regulation of stem elongation. GA3 was shown to be transported acropetally through the xylem and laterally between the xylem and phloem in soybean stems. We conclude that the GA3 level in stems is an important factor affecting internode elongation.

Vincetoxicum indicum (Burm.f.) Mabb. - a case of transient abnormal secondary thickening in stem

Vincetoxicum indicum (Burm.f.) Mabb. (Syn. Tylophora indica (Burm.f.) Merr., T. asthmatica W. &A.), an important twining medicinal plant of India, was found to show a transient anomalous secondary thickening to facilitate the twisting of stem around a support. The very young stem had a continuous primary xylem cylinder, the protoxylem of which differentiated to two large bundles on the opposing sides and many smaller bundles in between. In the initial stages of maturation, the cambium outside the two large protoxylem groups behaved abnormally in producing more phloem outwards and little or no secondary xylem inwards to produce two wedge shaped phloem groups on opposite sides. The production of crescent shaped secondary xylem on the other two sides which fell at a right-angled plane resulted in bulging of these sides away from the anomalous cambium to create a rectangular stem having two broad sides which were used as the facing sides of stem against a support. As soon as the twisting nature was established, the anomaly was reversed and the anomalous cambial patches started behaving normally to produce a complete ring of secondary xylem. As this abnormal behaviour happened only during the twisting of stem, this is considered as an adaptational anomalous secondary thickening.

The stem anatomy of the Clematis species (Ranunculaceae) in Taiwan

Background Studies on the stem anatomical characteristics of Taiwanese species from the Clematis genus (Ranunculaceae) are scarce. This study aimed to investigate and compare the patterns of secondary growth in stems of 22 Clematis species. Results The rhytidome is composed of periderm and non-conducting phloem and formed either cogwheel-like or continuous segment bark. Key features of the genus were stem with an irregular conformation, wedge-like phloem and rays, indentations in the axial parenchyma, ray dilatation, and narrow rays. Approximately eight Clematis species formed bark arc shape, which developed the cogwheel- like rhytidome. There were with approximately 27% of the Clematis species in Taiwan having 12 vascular bundles. The vessels dispersed throughout the stem were semi-ring-porous in most species but were ring-porous in others. No species had diffuse-porous vessels. The vessel restriction pattern was only found in the two shrubs, C. psilandra and C. tsugetorum. The primary xylem ring was located around the pith in C. uncinata var. uncinata, making its pith cavity hexagon in shape. Four species had the pith cavity feature. Narrow rays that occurred in the secondary xylem increased with increasing stem diameter. Conclusions The cambial variants described in this study provide a foundation for further morphological studies of the Clematis genus.

Microcutting Redox Profile and Anatomy in Eucalyptus spp. With Distinct Adventitious Rooting Competence

Adventitious root (AR) development takes place in an intricate cellular environment. Reactive oxygen species (ROS) and antioxidant defenses, triggered by wounding in cuttings, can modulate this process. A comparative assessment of biochemical and anatomical parameters at critical rooting stages in hard- (Eucalyptus globulus Labill.) and easy- (Eucalyptus grandis W.Hill ex Maiden) to-root species was carried out. Microcuttings from seedlings were inoculated in auxin-free AR induction medium and, after 96 h, transferred to AR formation medium for a period of 24 h. Samples were collected upon excision (Texc) and at the 5th day post excision (Tform). Delayed xylem development, with less lignification, was recorded in E. globulus, when compared to E. grandis, suggesting lower activity of the cambium layer, an important site for AR development. Superoxide was more densely present around the vascular cylinder at both sampled times, and in greater quantity in E. globulus than E. grandis, declining with time in the former. Hydrogen peroxide was localized primarily along cell walls, more intensely in the primary xylem and phloem, and increased significantly at Tform in E. globulus. Ascorbate peroxidase (APX), superoxide dismutase (SOD), and catalase (CAT) activities were generally higher in E. grandis and varied as a function of time in E. globulus. Soluble guaiacol peroxidase (GPRX) activity increased from Texc to Tform in both species, whereas cell wall-bound GPRX activity increased with time in E. grandis, surpassing E. globulus. Flavonoid content increased with time in E. grandis and was higher than E. globulus at Tform. Principal component analysis showed that species- and time-derived differences contributed to almost 80% of the variance. Overall, data indicate that E. grandis shows higher cambium activity and tighter modulation of redox conditions than E. globulus. These features may influence ROS-based signaling and phytohormone homeostasis of cuttings, thereby impacting on AR development. Besides being players in the realm of AR developmental differences, the specific features herein identified could become potential tools for early clone selection and AR modulation aiming at improved clonal propagation of this forest crop.

Bark and Cambial Variation in the Genus Clematis (Ranunculaceae) in Taiwan

BackgroundStudies on the anatomical characteristics of stems of Taiwanese species from the Clematis genus (Ranunculaceae) are scarce. The aim of this study was to investigate and compare cambial variation in stems of 22 Clematis species. ResultsThe rhytidome (outer bark) was either cogwheel-like or continuous, except for in the species Clematis tashiroi. Key features of the genus were eccentric to elliptical or polygonous-lobed stems, wedge-like phloem, wedge-like rays, indentations in the axial parenchyma, and ray dilatation. The cortical sclerenchyma fibers were embedded in the phloem rays with approximately 23% of the Clematis species. Both C. psilandra and C. tsugetorum had restricted vessels. There were three vascular bundle patterns, with approximately 27% of the Clematis species in Taiwan having 12 vascular bundles. The vessels dispersed throughout the stem were semi-ring-porous in most species, but were ring-porous in others. No species had diffuse-porous vessels. Only two species had a primary xylem ring located around the pith. Secondary xylem rays split the secondary xylem into parts, increasing stem diameter. The developmental stage of each sample was determined, with the initial ring-like periderm being produced in the primary phloem during the second stage. ConclusionsThe cambial variations described in this study provide a foundation for further morphological studies of the Clematis genus.

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.