Takats, Stephen T. (Brookhaven Natl. Lab., Upton, N. Y.) An attempt to detect utilization of DNA breakdown products from the tapetum for DNA synthesis in the microspores of Lilium longiflorum. Amer. Jour. Bot. 49(7): 748–758. Illus. 1962.—The tapetum in anthers of Lilium longiflorum encloses the developing microspores and when it degenerates is a possible source of precursor material for DNA4 synthesis in the microspores. To check this, time‐course experiments were carried out tracing the fate of label introduced into the tapetal DNA. H3‐thymidine was given in vivo to anthers during late pachytene of meiosis, when the tapetum can be selectively labelled. Growth was then followed by sampling anthers until the tapetum degenerated and the microspores synthesized DNA and divided. Autoradiographs indicated that the label in tapetal nuclei was lost shortly before DNA synthesis in the microspores. The microspore walls were transiently labelled, but the microspore nuclei did not incorporate a detectable amount of label. The results are discussed in the light of related biochemical findings, and are explained on the basis of: (1) complete catabolism of tapetal DNA (or tapetal thymine); and (2) the existence of a non‐tapetal pool of precursors.
Symbiotic nodulation of the primary roots of soybeans (Glycine max L. Merrill cv. Pride 216) is regulated by the plant, and is suppressed in response to a high inoculum dose of Bradyrhizobium japonicum USDA strain I–110 (ARS)+ applied at one time to the root. If an optimal dose is followed 10 h later by a superoptimal dose, nodules from the first inoculum near the base of the primary root are suppressed in a dose‐dependent way similar to that observed after single inoculations. The nodules which appear are probably derived from infections initiated by the bacteria in both inocula.
Autoregulation of symbiotic root nodulation in soybean seedlings (Glycine max L. Merrill cv Pride 216) was studied following double inoculation of primary roots with Bradyrhizobium japonicum 110. When the second inoculation was given 10 or 17 hours after the first, the nodulation in the first-inoculated region of the root was suppressed. The effect was eliminated if B. japonicum 110 containing Tn5 insertions in the 'common' nod ABC genes was used for the second inoculation, indicating the requirement for changes in the root mediated by these bacterial genes. When the root cortex in the suppressed basal region was examined 3 days after inoculation, cell division centers were present in numbers not significantly different from the numbers in contrQl roots given a sham second inoculation; their size distribution, however, showed a failure of enlargement compared with controls.Feedback suppression of symbiotic root nodulation ('autoregulation') was discovered in 1952 by Nutman in red clover ( 16) and has since been demonstrated in several other legume plants (see 19 for review). Nutman interpreted it as an inhibition of new nodule production by already established nodules on the root, because surgical excision of established nodules or nodule meristems allowed additional nodulation (16). Split-root techniques have since been used (12) to demonstrate the systemic nature of the response in soybean plants (Glycine max L. Merrill). Grafting experiments with soybeans have shown that feedback suppression requires the normal shoot, since the response is lost when a mutant shoot which lacks the normal nodulation control ('supernodulating') is substituted for the normal one (8,17).Double inoculation experiments carried out by Pierce and Bauer (18) showed that feedback suppression was actually a plant response which could not be explained by limitation of the number of infective bacteria. Their experiments demonstrated that a second inoculation 15 h after the first produced virtually no nodules, provided that the bacterial concentration in the first inoculum was optimized for nodule yield.The present investigation was undertaken to determine the basis for the observation (22) that very early feedback regulation appears to control the nodulation not in the apical region but instead in the basal region of the root, which is initially most susceptible to infection by the bacteria (1, 2). This region is the first one inoculated in double inoculation protocols. The number of nodules which appeared in this region was greatly reduced when a large second inoculum was given 10 h after the first inoculum (22). This result was not 865 observed by Pierce and Bauer (18), whose protocol for double inoculation was different. Two questions were therefore asked: (a) Did the effect require the 'common' nod genes in B. japonicum which are responsible for nodulation? If so, it would indicate that the effect depended on specific gene expression and was not due to nonspecific phenomena incidental to the double inoculation protocol, e.g. di...
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