Identification of 16SrII-V Phytoplasma Associated with Mungbean Phyllody Disease in Taiwan

Mungbean (Vigna radiata (L.) R. Wilczek), an important legume crop in Asia, is primarily cultivated in the central-southern region of western Taiwan. In 2020, mungbean exhibiting typical phytoplasma-induced disease symptoms, such as witches’ broom, phyllody, virescence, and proliferation, was observed in Yunlin County, Taiwan. Moreover, the seeds harvested from diseased plants displayed premature germination. Transmission electron microscopy examination of leaf veins prepared from symptomatic mungbeans demonstrated that the occlusion of sieve tubes resulted from the accumulation of phytoplasma-like bodies in sieve elements along with filament-like structures in sieve pores. The association of phytoplasma in symptomatic mungbean was confirmed by PCR analyses of the 16S rRNA and immunodominant membrane protein genes. Further analyses of the 16S rRNA-based phylogenetic tree and the iPhyClassifier-based virtual RFLP study demonstrated that the phytoplasma-associated mungbean phyllody disease identified in this study belongs to the 16SrII-V subgroup. BLAST analysis and the phylogenetic analysis indicated that the SAP11-like protein identified in mungbean phyllody disease is identical to PnWB phytoplasma SAP11, which explains the witches’ broom phenotype observed in symptomatic mungbean. The results described in this report confirm that the 16SrII-V phytoplasma, a widely distributed phytoplasma associated with peanut witches’ broom disease in Taiwan, has also infected mungbean. This is not only the first instance of mungbean phyllody disease found in Taiwan, but also the first instance of mungbean phyllody disease causing by 16SrII-V subgroup phytoplasma.

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.

Sieve Plate Pores in the Phloem and the Unknowns of Their Formation

Sieve pores of the sieve plates connect neighboring sieve elements to form the conducting sieve tubes of the phloem. Sieve pores are critical for phloem function. From the 1950s onwards, when electron microscopes became increasingly available, the study of their formation had been a pillar of phloem research. More recent work on sieve elements instead has largely focused on sieve tube hydraulics, phylogeny, and eco-physiology. Additionally, advanced molecular and genetic tools available for the model species Arabidopsis thaliana helped decipher several key regulatory mechanisms of early phloem development. Yet, the downstream differentiation processes which form the conductive sieve tube are still largely unknown, and our understanding of sieve pore formation has only moderately progressed. Here, we summarize our current knowledge on sieve pore formation and present relevant recent advances in related fields such as sieve element evolution, physiology, and plasmodesmata formation.

The structure and development of interxylary and external phloem in Aquilaria sinensis

The structure and development of interxylary phloem (IP) and external phloem in Aquilaria sinensis were investigated using light and scanning electron microscopy. Complete IP strands were isolated, measuring 14 ± 4 mm in length and 417 ± 124 μm in width. The outer margin of IP was composed of two to three layers of fusiform parenchyma cells. The development of IP can be divided into five stages: 1) Locally IP starts its differentiation within a small segment of a broad cambial zone, at the cost of xylem differentiation. 2) Inward growth of IP advances, and fibres and sieve tubes differentiate. 3) IP is constricted by the encroachment of immature xylem cells between cambium and immature IP. 4) IP is isolated from the cambium and surrounded by immature, non-lignified xylem tissue. 5) IP is surrounded by lignified xylem tissue, and the fibres within IP become lignified.In all the phloem islands in a ten-year-old stem, sieve elements showed positive staining of callose with aniline blue. However, no staining of callose was observed in the external secondary phloem of agarwood trees collected from two different sites. No sieve tubes or sieve pores were detected by SEM observation of numerous serial cross and radial sections of the external phloem. We therefore conclude that sieve tubes are absent from the external phloem or extremely rare and that the transport of photosynthetic products in the stem of A. sinensis takes place in the interxylary phloem.

Response of Sweet Orange (Citrus sinensis) to ‘Candidatus Liberibacter asiaticus’ Infection: Microscopy and Microarray Analyses

Citrus greening or huanglongbing (HLB) is a devastating disease of citrus. HLB is associated with the phloem-limited fastidious prokaryotic α-proteobacterium ‘Candidatus Liberibacter spp.’ In this report, we used sweet orange (Citrus sinensis) leaf tissue infected with ‘Ca. Liberibacter asiaticus’ and compared this with healthy controls. Investigation of the host response was examined with citrus microarray hybridization based on 33,879 expressed sequence tag sequences from several citrus species and hybrids. The microarray analysis indicated that HLB infection significantly affected expression of 624 genes whose encoded proteins were categorized according to function. The categories included genes associated with sugar metabolism, plant defense, phytohormone, and cell wall metabolism, as well as 14 other gene categories. The anatomical analyses indicated that HLB bacterium infection caused phloem disruption, sucrose accumulation, and plugged sieve pores. The up-regulation of three key starch biosynthetic genes including ADP-glucose pyrophosphorylase, starch synthase, granule-bound starch synthase and starch debranching enzyme likely contributed to accumulation of starch in HLB-affected leaves. The HLB-associated phloem blockage resulted from the plugged sieve pores rather than the HLB bacterial aggregates since ‘Ca. Liberibacter asiaticus’ does not form aggregate in citrus. The up-regulation of pp2 gene is related to callose deposition to plug the sieve pores in HLB-affected plants.

YIELD OF DIFFERENT HEAD POSITIONS OF SUNFLOWER (Helianthus annuus L.) AND ITS RELATIONSHIP WITH VASCULARIZATION / RENDIMIENTO EN DIFERENTES POSICIONES DEL CAPÍTULO DE GIRASOL (Helianthus annuus L.) Y SU RELACIÓN CON LA VASCULARIZACIÓN / LE RENDEMENT DES DIFFERENTS ZONES DU CAPITULES DU TOURNESOL (Helianthus annuus L.) Y SON RELATION AVEC LA VASCULARISATION

SUMMARY This work was aimed to study, under different levels of radiation intercepted by the plants during the seed filling stage, the relationship between yield and vascularization in three concentric positions of the capitulum. At the end of flowering, we applied shading (to reduce intercepted radiation) and thinning (to increase it) to three culture plots: a shaded plot, a thinned plot and a shaded and thinned plot. One additional untreated plot was used as control. We harvested heads at flowering and at physiological maturity. We delimited on them three positions: outer, middle and inner. Portions of each position were extracted from physiological maturity heads and their yield components were determined. The remaining heads were fixed in F.A.A., soaked in paraffin, and transversely cut at seed insertion to measure vascularization variables (phloem and sieve tubes area, number of tranverse and longitudinal bundles and sieve pores diameter). The head area unit was used as a base in all measurements. Shading reduced dry weight in the three positions. The middle position showed the highest yield and the inner showed the lowest in the four plots. The yield of the former was high because its lower individual seed weight (decreasing from periphery to center in all plots) was compensated by a higher number of filled seeds. However, average sieve pores radius was similar among positions, and phloem and sieve tubes areas were similar among positions and treatments, which could not account for the differences in yield per head area unit between positions. This enables us to conclude that this variation would not be produced by vascularization lack.

Robustness of Methane Diffusion in Geometrically Restricted Molecular Sieve Pores

ABSTRACTNMR measurements of sorbate mobility in zeolites are especially attractive because of their capability of measuring multicomponent and anisotropic self-diffusion. We have recently reported the application of the pulsed field gradient NMR technique using very large zeolite crystals to study how easily methane can diffuse when we attempt to slow its migration by crowding the pore space. Here we analyze the implications of these PFG NMR experiments involving (i) ethylene blocking of methane in zeolite NaY; and (ii) methane molecules trying to pass one another in the molecular sieve A1PO4-5.