NT-3 Expression in Spared DRG and the Associated Spinal Laminae as well as Its Anterograde Transport in Sensory Neurons Following Removal of Adjacent DRG in Cats

Neurotrophin-3 plays an important role in survival and differentiation of sensory and sympathetic neurons, sprouting of neurites, synaptic reorganization, and axonal growth. The present study evaluated changes in expression of NT-3 in the spinal cord and L6 dorsal root ganglion (DRG), after ganglionectomy of adjacent dorsal roots in cats. NT-3 immunoreactivity increased at 3 days post-operation (dpo), but decreased at 10 dpo in spinal lamina II after ganglionectomy of L1–L5 and L7–S2 (leaving L6 DRG intact). Conversely, NT-3 immunoreactivity decreased on 3 dpo, but increased on 10 dpo in the nucleus dorsalis. Very little NT-3 mRNA signal was detected in the spinal cord, despite the changes in NT-3 expression. The above changes may be related to changes in NT-3 expression in the DRG. Ganglionectomy of L1–L5 and L7–S2 resulted in increase in NT-3 immunoreactivity and mRNA in small and medium-sized neurons, but decreased expression in large neurons of L6 DRG at 3 dpo. It is possible that increased NT-3 in spinal lamina II is derived from anterograde transport from small- and medium-sized neurons of L6 DRG, whereas decreased NT-3 immunoreactivity in the nucleus dorsalis is due to decreased transport of NT-3 from large neurons in the DRG at this time. This notion is supported by observations on NT-3 distribution in the dorsal root of L6 after ligation of the nerve root. The above results indicate that DRG may be a source of neurotrophic factors such as NT-3 to the spinal cord, and may contribute to plasticity in the spinal cord after injury.     

The influence of nematodes on below-ground processes in grassland ecosystems

This review summarises recent information on beneficial roles that soil nematodes play in the cycling of carbon and other plant nutrients in grassland ecosystems. In particular, we focus on the role of the two dominant functional groups of nematodes, namely the microbial- and root-feeders, and how their activities may enhance soil ecosystem-level processes of nutrient cycling and, ultimately, plant productivity in managed and unmanaged grassland ecosystems. We report recent experiments which show that low amounts of root herbivory by nematodes can increase the allocation of photoassimilate carbon to roots, leading to increased root exudation and microbial activity in the rhizosphere. The effects of these interactions on soil nutrient cycling and plant productivity are discussed. Evidence is presented to show that the feeding activities of microbial-feeding nematodes can enhance nutrient mineralization and plant nutrient uptake in grasslands, but that these responses are highly species-specific and appear to be strongly regulated by higher trophic groups of fauna (top-down regulation). We recommend that future studies of the roles of nematodes in grasslands ecosystems should consider these more complex trophic interactions and also the effects of species diversity of nematodes on soil ecosystem-level processes. This revised version was published online in June 2006 with corrections to the Cover Date.     

Role of auxins and cytokinins in the development of lateral roots in wheat plants with several roots removed

The removal of four of five roots of 7–8-day-old wheat plants resulted in the activation of lateral root growth and the initiation of lateral root primordia on the remained root as compared to the main root of intact plants. The extent of this growth response depended on placing cut surface above or beneath the surface of the nutrient solution. The measurement of the IAA and cytokinin contents showed accumulation of these hormones in the root of experimental plants as compared to the main root of intact plants. IAA accumulation was correlated with the number of lateral roots and their primordia. The analysis of hormonal balance and their transport from the shoot to the root permits discussing the involvement of these hormones and their interaction in the control of root growth at the stages of both primordium initiation and development and lateral root elongation.     

ASYMMETRY OF EYESPOT AND MATING STRUCTURE POSITIONS IN ULVA COMPRESSA (ULVALES,CHLOROPHYTA) REVEALED BY A NEW FIELD EMISSION SCANNING ELECTRON MICROSCOPY METHOD1

Gametes of the marine green alga Ulva compressa L. are biflagellate and pear shaped, with one eyespot at the posterior end of the cell. The species is at an early evolutionary stage between isogamy and anisogamy. In the past, zygote formation of green algae was categorized solely by the relative sizes of gametes produced by two mating types (+ and ?). Recently, however, locations of cell fusion sites and/or mating structures of gametes have been observed to differ between mating types in several green algae (asymmetry of cell fusion site and/or mating structure positions). To use this asymmetry for determining gamete mating type, we explored a new method, field emission scanning electron microscopy (FE‐SEM), for visualizing the mating structure of U. compressa. When gametes were subjected to drying stress in the process of a conventional critical‐point‐drying method, a round structure was observed on the cell surfaces. In the mating type MGEC‐1 (mt⁺), this structure was located on the same side of the cell as the eyespot, whereas it was on the side opposite the eyespot in the mating type MGEC‐2 (mt^?). The gametes fuse at the round structures. TEM showed an alignment of vesicles inside the cytoplasm directly below the round structures, which are indeed the mating structures. Serial sectioning and three‐dimensional construction of TEM micrographs confirmed the association of the mating structure with flagellar roots. The mating structure was associated with 1d root in the MGEC‐1 gamete but with 2d root in the MGEC‐2 gamete.     

Immunohistological localization and quantification of IAA in studies of root growth regulation

The content and distribution of auxins were studied in gravistimulated roots of maize (Zea mays L.) and primary roots of 7-day-old wheat (Triticum durum Desf.) seedlings, which branching was enhanced by excision of adventitious roots. IAA localization was observed immunohistochemically, using specific anti-IAA antibody in combination with second (anti-species) antibody labeled with colloidal gold. Differences in the IAA content (staining intensity) were found between upper and lower parts of gravistimulated maize roots. We also observed IAA accumulation in the primary wheat root after adventitious root excision; the cells of lateral root primordia were characterized by more intense IAA staining. The role of auxin redistribution in plants for lateral root initiation and development is discussed.     

Differential effect of ammonium on the induction of nitrate and nitrite reductase activities in roots of barley (Hordeum vulgare) seedlings

The effect of exogenous NH₄⁺ on the induction of nitrate reductase activity (NRA; EC 1.6.6.1) and nitrite reductase activity (NiRA; EC 1.7.7.1) in roots of 8-day-old intact barley (Hordeum vulgare L.) seedlings was studied. Enzyme activities were induced with 0.1, 1 or 10 mM NO₃⁺ in the presence of 0, 1 or 10 mM NH₄⁺, Exogenous NH₄⁺ partially inhibited the induction of NRA when roots were exposed to 0.1 mM, but not to 1 or 10 mM NO₃⁺, In contrast, the induction of NiRA was inhibited by NH₄⁺ at all NO₃⁺ levels. Maximum inhibition of the enzyme activities occurred at 1.0 mM NH₄⁺ Pre-treatment with NH₄⁺ had no effect on the subsequent induction of NRA in the absence of additional NH₄⁺ whereas the induction of NiRA in NH₄⁺-pretreated roots was inhibited in the absence of NH₄⁺ At 10 mM NO₃⁺ L-methionine sulfoximine stimulated the induction of NRA whether or not exogenous NH₄⁺ was present. In contrast, the induction of NiRA was inhibited by L-methionine sulfoximine irrespective of NH₄⁺ supply. During the postinduction phase, exogenous NH₄⁺ decreased NRA in roots supplied with 0.1 mM but not with 1mM NH₃⁺ whereas, NiRA was unaffected by NH₄⁺ at either substrate concentration. The results indicate that exogenous NH₄⁺ regulates the induction of NRA in roots by limiting the availability of NO₃⁺. Conversely, it has a direct effect, independent of the availability of NO₃⁺, on the induction of NiRA. The lack of an NH₄⁺ effect on NiRA during the postinduction phase is apparently due to a slower turnover rate of that enzyme.     

Growth responses of individual roots of Opuntia ficus-indica to salinity

Responses of individual roots of the widely cultivated cactus Opuntia ficus-indica to salinity stress were evaluated using a split-root system. Three roots from a plant with at least 20 roots were isolated from the remainder of the root system and exposed to 0, 30 or 100 mol m^-3 NaCl for up to 28 d. Cortical cells became shorter and lateral root development was substantially reduced as salinity increased. Compared with the control, the increase in dry weight for the isolated roots was reduced 40% by 30 mol m^-3 NaCl and 93% by 100mol m^-3 NaCl. The sodium content of roots increased only two-fold with increasing salinity. Respiration rates of roots exposed to 30 or 100 mol m^-3 NaCl were higher than those of the control. Carbon accumulation in roots measured 2 d after exposing shoots to ¹⁴CO₂ was not initially affected by 30 mol m^-3 NaCl but was substantially reduced at 100 mol m^-3 NaCl. Thus, roots exposed to short periods of moderate salinity stress maintained sufficient carbon sink strength for continued growth of the roots. Moreover, increased salinity led to decreased efficiency of carbon usage for the expansion of root surface area.     

Arbuscular mycorrhizal phosphate transport under monoxenic conditions using radio-labelled inorganic and organic phosphate

Compartmented monoxenic cultures of Ri T-DNA transformed carrot roots and a symbiotic arbuscular mycorrhizal fungus demonstrated for the first time that phosphate in an organic form (³²P-labelled AMP) may be hydrolysed by extra-radical mycorrhizal hyphae in the absence of other organisms, and subsequently utilized as a mineral nutrient source by the host plant after fungal transport.     

A universal molecular method for identifying underground plant parts to species

As part of a large project to determine rooting depth and resource uptake on the Edwards Plateau of central Texas, we developed a DNA-based technique that allows the below-ground parts of all plants to be identified to the level of genus and usually to species. Identification is achieved by comparing DNA sequences of the internal transcribed spacer (ITS) region of the 18S-26S nuclear ribosomal DNA repeat, derived from below-ground plant material, with a reference ITS region database for plants at a site. The method works throughout plants because the plant ITS region can be PCR amplified using a set of universal primers. Congeneric species can usually be identified because the ITS region evolves relatively rapidly. In our study, all roots were easily identified to the level of genus; most congeneric species were identified solely by ITS sequence differences but some required a combination of ITS sequence data and above-ground surveys of species at a site. In addition to showing the feasibility and efficacy of our technique, we compare it with another DNA-based technique used to identify below-ground plant parts. Finally, we also describe a DNA extraction and purification technique that reliably provides high-quality DNA of sufficient quantity from roots so that PCR can be readily accomplished. Our technique should allow the below-ground parts of plants in any system to be identified and thereby open new possibilities for the study of below-ground plant communities.