Formation of medullary phloem in Argyreia nervosa (Burm. f.) Bojer

Histologically, family Convolvulaceae is characterised by the presence of successive cambia, medullary (internal/intraxylary) and interxylary phloem in majority of the species, whereas some of the members are devoid of successive cambia and medullary bundles. The present study on Argyreia nervosa (Burm. f.) Bojer showed the presence of medullary bundles while internal phloem was absent during the primary growth. As the secondary growth progressed, successive cambia initiated from the pericyclic parenchyma. Development of medullary bundles began along with the regular protoxylem and protophloem while formation of intraxylary phloem was observed only after the initiation of secondary growth. Medullary/intraxylary sieve elements began to develop from the marginal pith cells. In thick stems, small segments of internal cambium initiated between the protoxylem and internal phloem. This internal cambium was functionally unidirectional and produced internal phloem centripetally. Developmental particulars are described in details along with its significance.

Structure And Development Of Internal Phloem In Solanum Pseudocapsicum (Solanaceae)

The development of internal phloem in the Jerusalem cherry, Solanum pseudocapsicum L. (Solanaceae), was studied in young and mature stems. The early presence of primary internal phloem is succeeded by the development of secondary internal phloem from an internal cambium situated between the protoxylem and primary internal phloem. In the second and third visible internodes of the young stem, procambial derivatives begin to differentiate as discrete strands of internal protophloem in a perimedullary position prior to the differentiation of protoxylem and external protophloem. In 6–8 mm diameter stems, sieve elements of the internal phloem become non-conducting, begin to collapse, and undergo obliteration. In 15–20 mm diameter stems internal cambium is initiated from the parenchyma cells situated between the protoxylem and primary internal phloem. The development of internal phloem and an internal cambium in S. pseudocapsicum is compared with that in other taxa. There seems to be a gradual variation in the origin of an internal cambium from either remnants of the procambium or dedifferentiation of peripheral pith cells across dicotyledons with an internal cambium.

Morphoanatomy of Schultesia pachyphylla (Gentianaceae): a discordant pattern in the genus

Schultesia Mart. comprises 21 closely related taxa. The revision of the genus showed that Schultesia pachyphylla Griseb. and Xestaea lisianthoides Griseb. are different from other species. The aim of this study was to describe the morphoanatomy of S. pachyphylla, comparing it with other Schultesia species and X. lisianthoides, and to provide data on their geographical distribution and habitat. Morphological and anatomical data were subjected to multivariate analysis. Schultesia pachyphylla is endemic to the state of Bahia, Brazil, and is separated from the other species by having 5-merous flowers, 5-winged calyx, wings between the calyx lobes, loss of interveinal tissue on sepals during fruit maturation, dark red colored immature calyces, capsules opening from base to apex, 3-anaporate pollen grains, and epidermal papillae evident in the field. Schultesia pachyphylla has amphistomatic leaves with anisocytic stomata, uniseriate epidermis, dorsiventral mesophyll, and bicollateral vascular bundles. The stem has small wings, cortical and medullary parenchyma, and internal phloem. Foliar colleters and nectaries on leaves and stem were observed. We believe that morphological and anatomical data could support the creation of a monotypic genus or the relocation of this species into another genus. Phylogenetic analyses are in progress to define the species’ positioning.

Cucumber mosaic virus 2b protein compensates for restricted systemic spread of Potato virus Y in doubly infected tobacco

Tobacco plants (Nicotiana tabacum cv. Xanthi-nc) inoculated with a necrotic strain of Potato virus Y (PVY, T01 isolate) developed necrotic symptoms in some systemically infected leaves, but not in younger leaves. However, PVY expressed distinct symptoms not only in the older leaves, but also in the younger leaves, of plants that had been doubly inoculated with PVY and with Cucumber mosaic virus (CMV, strain Pepo). A tissue blot immunoassay of tissues from various positions of the stem detected PVY weakly in each stem, but not in the shoot apex, of singly infected plants, whereas PVY was detected at high levels in almost all sections of doubly infected plants. CMV was also detected at high levels in sections of singly and doubly infected plants. Immunohistochemistry of stem tissues showed that in singly infected plants, PVY was confined to external phloem cells and was not detected in internal phloem cells. However, in doubly infected plants, PVY was distributed uniformly throughout whole tissues, including the external phloem, xylem parenchyma and internal phloem cells. In plants that were doubly infected with PVY and PepoΔ2b, a modified CMV that cannot translate the 2b protein, the spread of PVY was restricted as in singly infected plants. These results suggested that the plant host has a counterdefence mechanism that restricts systemic spread of PVY T01, and that the 2b protein of CMV strain Pepo negates this restriction.

Cucumber mosaic virus Establishes Systemic Infection at Increased Temperature Following Viral Entrance Into the Phloem Pathway of Tetragonia expansa

A potential regulatory site for Cucumber mosaic virus (CMV, pepo strain) movement necessary to establish systemic infection was identified through immunological and hybridization studies on Tetragonia expansa, which was systemically infected by CMV at 36°C but not at 24°C. In inoculated leaves, cell-to-cell movement of CMV was enhanced at 36°C compared with that observed at 24°C. CMV was distributed in the phloem cells of minor veins as well as epidermal and mesophyll cells at both 36 and 24°C. CMV was detected in the petioles of inoculated leaves, stems, and petioles of uninoculated upper leaves at 36°C, whereas CMV was detected only in the petioles of inoculated leaves and in stems at 24°C. CMV moved into the phloem and was transported to the stem within 24 h postinoculation (hpi) at 36°C. However, it did not accumulate in the petioles of the upper leaves until 36 hpi. In petioles of inoculated leaves at 24°C, CMV was detected in the external phloem but not in the internal phloem. From these results, we conclude that systemic infection is established after viral entrance into the phloem pathway in T. expansa at 36°C.

Resistance of Capsicum annuum ‘Avelar’ to pepper mottle potyvirus and alleviation of this resistance by co-infection with cucumber mosaic cucumovirus are associated with virus movement

Capsicum annuum cv. Avelar plants resist systemic infection by the Florida isolate of pepper mottle potyvirus (PepMoV-FL). Immuno-tissue blot analysis for detection of PepMoV-FL infection in selected stem segments revealed that virus moved down the stem in external phloem, and, over time, accumulated to detectable levels throughout stem sections (appearing to accumulate in external and internal phloem) taken from below the inoculated leaf. At 21 days post-inoculation, PepMoV-FL was detected in stem segments one or two internodes above the inoculated leaf; however, no virus was observed in internal phloem in stem segments beyond these internodes. In contrast to these observations, PepMoV-FL was detected in the internal phloem of all internodes of the stem located above the inoculated leaf, with subsequent movement into non-inoculated leaves, in Avelar plants co-infected with PepMoV-FL and cucumber mosaic cucumovirus (CMV-KM). No apparent enhancement of PepMoV-FL accumulation occurred in protoplasts inoculated with PepMoV-FL alone versus a mixed inoculum of PepMoV-FL and CMV-KM. These findings confirm earlier observations that potyvirus movement up the stem of Capsicum species occurs via internal phloem. It is also shown that PepMoV-FL does not accumulate to detectable levels in internal phloem in the stem of Avelar plants, thereby limiting its movement to within the inoculated leaf and lower portions of the stem; however, co-infection of Avelar plants with CMV-KM alleviates this restricted movement, allowing PepMoV-FL to invade young tissues systemically.

Pathway for Phloem-Dependent Movement of Pepper Mottle Potyvirus in the Stem of Capsicum annuum

Phloem-dependent movement of pepper mottle potyvirus (PepMoV) through Capsicum annuum occurs in a defined pattern through the stem and into uninoculated leaves. The route of movement of PepMoV through the stem of C. annuum ‘Early Calwonder’ was tracked using immunotissue blot analysis and immunomicroscopy. Virus was shown to move from the inoculated leaf down the stem toward the roots via the external phloem. At some location between the cotyledonary node and the roots, PepMoV entered the internal phloem through which it rapidly spread upward the length of the stem to the young tissues. Translocation of PepMoV through the stem occurred in an asymmetric fashion, i.e., virus remained on the side of the stem to which the inoculated leaf was attached as it translocated the length of the stem. Spread and accumulation of PepMoV into uninoculated leaves appeared to occur in a source-to-sink pattern similar to that described for the flow of photoassimilates and similar to other virus and viroid-host systems.