Sequence analysis of five gene families that were isolated from tobacco thin cell layer explants initiating floral development [Meeks-Wagner et al. (1989). Plant Cell 1, [25][26][27][28][29][30][31][32][33][34][35] showed that two encode the pathogenesis-related proteins basic chitinase and basic ,B-l,3-glucanase, while a third encodes the cell wall protein extensin, which also accumulates during pathogen attack. Another sequence family encodes the water stress-induced protein osmotin [Singh et al. (1989). Plant Physiol. 90, 1096-11011. We found that osmotin was also induced by vira1 infection and wounding and, hence, could be considered a pathogenesis-related protein. These genes, which were highly expressed in explants during de novo flower formation but not in explants forming vegetative shoots [MeeksWagner et al. (1989). Plant Cell 1, 25-35], were also regulated developmentally in day-neutra1 and photoresponsive tobacco plants with high expression levels in the roots and moderate-to low-leve1 expression in other plant organs including flowers. An unidentified gene family, FB7-4, had its highest leve1 of expression in the basal internodes. Our findings indicate that these genes, some of which are conventionally considered to encode pathogen-related proteins, also have a complex association with normal developmental processes, including the floral response, in healthy plants.
Sequence analysis of five gene families that were isolated from tobacco thin cell layer explants initiating floral development [Meeks-Wagner et al. (1989). Plant Cell 1, 25-35] showed that two encode the pathogenesis-related proteins basic chitinase and basic ,B-l,3-glucanase, while a third encodes the cell wall protein extensin, which also accumulates during pathogen attack. Another sequence family encodes the water stress-induced protein osmotin [Singh et al. (1989). Plant Physiol. 90, 1096-11011. We found that osmotin was also induced by vira1 infection and wounding and, hence, could be considered a pathogenesis-related protein. These genes, which were highly expressed in explants during de novo flower formation but not in explants forming vegetative shoots [Meeks-Wagner et al. (1989). Plant Cell 1, 25-35], were also regulated developmentally in day-neutra1 and photoresponsive tobacco plants with high expression levels in the roots and moderate-to low-leve1 expression in other plant organs including flowers. An unidentified gene family, FB7-4, had its highest leve1 of expression in the basal internodes. Our findings indicate that these genes, some of which are conventionally considered to encode pathogen-related proteins, also have a complex association with normal developmental processes, including the floral response, in healthy plants.
The fine structure of young root hairs of radish was studied, with special attention to cytoplasm-wall relationships. Hairs up to 130/~ in length were examined after fixation of root tips in glutaraldehyde followed by osmium tetroxide. Microtubules occur axially aligned in the cytoplasm just beneath the plasmalemma, and extend from the base of the hair to within 2 to 3 ~ of the tip. Poststaining with uranyl acetate and lead citrate clearly reveals in thin sections the presence of the two layers of cellulose microfibrils known from studies on shadowed wall preparations: an outer layer of randomly arranged microfibrils arising at the tip, and a layer of axially oriented microfibrils deposited on the inside of this layer along the sides. The youngest microfibrils of the inner, oriented layer first appear at a distance of about 25 /z from the tip. Although the microfibrils of the inner layer and the adjacent microtubules are similarly oriented, the oriented microtubules also extend through the 20-to 25-/z zone near the tip where the wall structure consists of random microfibrils. This suggests that the role of microtubules in wall deposition or orientation may be indirect.
Summary The cyanobacterium, Nostoc, and the angiosperm, Gunnera, form a symbiotic association in which glandular cells host an intracellular population of heterocystous Nostoc cells. Symbiotic and free‐living isolates of Nostoc and Anabaena were assayed for their ability to infect, fix nitrogen and supply nitrogen to Gunnera. The range of cyanobacteria acceptable to Gunnera was found to be circumscribed by the genus, Nostoc, but included six different isolates within that genus. The effectiveness of the various Nostoc isolates was compared in terms of their activities at both the inter and intracellular levels of symbiotic association.
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