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.

Integrated mRNA, MicroRNA Transcriptome and Degradome Analyses Provide Insights into Stamen Development in Moso Bamboo

A flower is an essential organ for sexual reproduction in flowering plants, which has been extensively studied in model plants. In this study, we used transcriptomic, small RNA and degradome analyses to characterize key microRNAs (miRNAs) and their targets in floral organs of moso bamboo. In total, we identified 13,051 differentially expressed genes and 109 known miRNAs from 26 miRNA families. We aligned the miRNAs to known miRNA databases and revealed some conserved as well as novel miRNAs. Sixteen conserved miRNAs were specifically and highly expressed in stamens, including miRNA159 and miRNA166. In situ hybridization shows that miRNA159 plays a key role in the regulation of stamen development, and the expression levels of its targets PheMYB98 and PheMYB42 were low. Furthermore, Phe-MIRNA159 partially recovers phenotypes of mir159ab double mutant. Overexpression of Phe-MIR159 could cause failure in anther dehisce, and the mature pollens could not be dispersed and further reduce fertility in Arabidopsis. Semi-thin section result shows that anther endothelial layer of Phe-MIRNA159 overexpressing lines is lack of secondary thickening, resulting in limited force for anther opening. Phe-miR159 may regulate the expression of genes related to secondary thickening through negative regulation of AtMYB33, affecting the anther dehiscence. Taken together, this study provides insights regarding molecular networks underlying floral organs development of moso bamboo.

Knockdown of Arabidopsis ROOT UVB SENSITIVE4 Disrupts Anther Dehiscence by Suppressing Secondary Thickening in the Endothecium

ROOT UV-B SENSITIVE4 (RUS4) encodes a protein with no known function that contains a conserved Domain of Unknown Function 647 (DUF647). The DUF647-containing proteins RUS1 and RUS2 have previously been associated with root UV-B-sensing pathway that plays a major role in Arabidopsis early seedling morphogenesis and development. Here, we show that RUS4 knockdown Arabidopsis plants, referred to as amiR-RUS4, were severely reduced in male fertility with indehiscent anthers. Light microscopy of anther sections revealed a significantly reduced secondary wall thickening in the endothecium of amiR-RUS4 anthers. We further show that the transcript abundance of the NAC domain genes NAC SECONDARY WALL THICKENING PROMOTING FACTOR1 (NST1) and NST2, which have been shown to regulate the secondary cell wall thickenings in the anther endothecium, were dramatically reduced in the amiR-RUS4 floral buds. Expression of the secondary cell wall-associated MYB transcription factor genes MYB103 and MYB85 were also strongly reduced in floral buds of the amiR-RUS4 plants. Overexpression of RUS4 led to increased secondary thickening in the endothecium. However, the rus4-2 mutant exhibited no obvious phenotype. Promoter-GUS analysis revealed that the RUS4 promoter was highly active in the anthers, supporting its role in anther development. Taken together, these results suggest that RUS4, probably functions redundantly with other genes, may play an important role in the secondary thickening formation in the anther endothecium by indirectly affecting the expression of secondary cell wall biosynthetic genes.

Olivooides-like tube aperture in early Cambrian carinachitids (Medusozoa, Cnidaria)

The early Cambrian Carinachitidae, a family in the subclass Conulata, are intriguing and important small shelly fossils. Their gently tapering, tube-shaped skeletons consist of convex faces separated from each other by broad, deep corner sulci, and they exhibit triradial, pentaradial, or predominantly tetraradial symmetry. However, the morphology of the aperture and the modes of growth of carinachitid skeletons as well as the anatomy of their soft parts are unknown. Examination of a single new, exceptionally well-preserved specimen of tetramerousCarinachites spinatusQian, 1977, collected from the lower Cambrian Kuanchuanpu Formation in South China, reveals: (1) that its aperture is connected to a small mass of relic soft tissue and (2) that the apertural end of each of the four faces is developed into a subtriangular lappet or oral lobe that is smoothly folded toward the long axis of the tube, partially closing the tube aperture. Similarities between thorn-like spines on the faces and the oral lobes indicate that the transverse ribs were periodically displaced from the perradial portion of the aperture during formation of new ribs. In addition, the tube walls may have undergone secondary thickening during growth. The growth pattern of the tube and the spatial relationships between the tube aperture and soft parts are analogous to those of co-occurring olivooids. These findings further strengthen the previously proposed hypothesis that coeval carinachitids, olivooids, hexangulaconulariids, and Paleozoic conulariids are closely related taxa within the subphylum Medusozoa. Finally, carinachitids most likely represent an evolutionary intermediate between olivooids and hexangulaconulariids.

General and peculiar features of vascular organization and development in shoots of Bougainvillaea glabra Choisy (Nyctaginaceae)

In the present study developmental relations between peripheral meristematic cylinder and vascular bundles of the young parts of the shoots and the relations between these bundles from the young elongating parts and those formed during secondary thickening has been elucidated. The suggestion is advanced that the limited lateral extension of bundles is an important developmental feature of the vascular system of <i>Bougainvillaea glabra</i> Choisy. The organization of the vascular system in thick fast growing sprouts and in the thin slow growing lateral shoots are described. It is shown that the quantitative differences in vascular organization, between both types of shoots are the manifestation of a close morphological harmonistation of the entire shoot, and particularly of its entire vascular system.

Is the secondary thickening in palms always diffuse?

Unlike other arboreal monocotyledons, the secondary growth of palms has for the past 100 years been described as diffuse. Solely cell enlargement and random parenchyma divisions, without the activity of a meristem, characterize such growth. Some previous works of the early 20th century have, however, mentioned the presence of a secondary meristem in the stems of palms, but this information was forgotten since then. Addressing to this question, we analysed palm stems of four species, with the aim to understand the possible presence of such secondary growth. We found that a meristematic band occurs between the cortex and the central cylinder and gives rise to new vascular bundles and parenchyma internally, producing parenchyma and fibres externally. It appears secondarily, i.e., it undergoes meristematic activity in the median and basal stem regions, far away from the apical region. In fact, a meristematic band is present and may be more common than currently believed, but uneasy to detect in certain palms for being restricted to specific regions of their stems. In conclusion, the diffuse secondary thickening is here shown not to be the only mechanism of secondary growth in palms. The presence of a meristem band in the stems of palms merits careful reconsideration.

Primary and secondary thickening in the stem of Cordyline fruticosa (Agavaceae)

The growth in thickness of monocotyledon stems can be either primary, or primary and secondary. Most of the authors consider this thickening as a result of the PTM (Primary Thickening Meristem) and the STM (Secondary Thickening Meristem) activity. There are differences in the interpretation of which meristem would be responsible for primary thickening. In Cordyline fruticosa the procambium forms two types of vascular bundles: collateral leaf traces (with proto and metaxylem and proto and metaphloem), and concentric cauline bundles (with metaxylem and metaphloem). The procambium also forms the pericycle, the outermost layer of the vascular cylinder consisting of smaller and less intensely colored cells that are divided irregularly to form new vascular bundles. The pericycle continues the procambial activity, but only produces concentric cauline bundles. It was possible to conclude that the pericycle is responsible for the primary thickening of this species. Further away from the apex, the pericyclic cells undergo periclinal divisions and produce a meristematic layer: the secondary thickening meristem. The analysis of serial sections shows that the pericycle and STM are continuous in this species, and it is clear that the STM originates in the pericycle.The endodermis is acknowledged only as the innermost layer of the cortex.