The amplitude of phototropic curvature to blue light is enhanced by a prior exposure of seedlings to red light. This enhancement i s mediated by phytochrome. Fluence-response relationships have been constructed for red-light-induced enhancement in the phytochrome A (phyA) null mutant, the phytochrome B-(phyB) deficient mutant, and in two transgenic lines of Arabidopsis thaliana that overexpress either phyA or phyB. These fluence-response relationships demonstrate the existence of two responses in enhancement, a response in the very-low-to-low-fluence range, and a response in the high-fluence range. Only the response in the high-fluence range is present in the phyA null mutant. In contrast, the phyB-deficient mutant is indistinguishable from the wild-type parent in red-light responsiveness. These data indicate that phyA is necessary for the very-low-to-low but not the high-fluence response, and that phyB i s not necessary for either response range. Based on these results, the high-fluence response, if controlled by a single phytochrome, must be controlled by a phytochrome other than phyA or phyB. Overexpression of phyA has a negative effect and overexpression of phyB has an enhancing effect in the high-fluence range. These results suggest that overexpression of either phytochrome perturbs the function of the endogenous photoreceptor system in an unpredictable fashion.

1997. Both phytochrome A and phytochrome B are required for the normal expression of phototropism m Arabidopsis thaliana seedlings. -Physiol. Plant. 101: 278-282.The role of phytochrome A (phyA) and phytochrome B (phyB) in phototropism was investigated by using the phytochrome-deficient mutants phyA-101, phyB-I and a phyA/phyB double mutant. The red-light-induced enhancement of phototropism, which is normally observed in wild-type seedlings, could not be detected in the phyA/phyB mutant at fluences of red light between 0.1 and 19 000 |amol m~'. The loss of phyB has been shown to have no apparent effect on enhancement, while the loss of phyA resulted in a loss of enhancement only in the low fluence range (Janoudi et al. 1997). The conclusions of the aforementioned study can now be modified based on the current results which indicate that phototropic enhancement in the high fluence range is mediated by either phyA or phyB, and that other phytochromes have no role in enhancement. First positive phototropism was unaffected in phyA-IOl and phyB-I. However, the magnitude of first positive phototropism in the phyA/phyB mutant was significantly lower than that of the wild-type Landsberg parent, thus, the presence of either phyA or phyB is required for normal expression of first positive phototropism. The time threshold for second positive phototropism is unaltered in the phyA-lOI and phyB-1 mutants. However, the time threshold in the phyA/phyB mutant is about 2 h, approximately six times that of the wild type. Finally, the magnitude of second positive phototropism in both phyA-101 and phyB-I is diminished in comparison with the wild-type response. Thus, phyA and phyB, acting independently or in combination, regulate the magnitude of phototropic curvature and the time threshold for second positive phototropism. We conclude that the presence of phyA and phyB is required, but not sufficient, for the expression of normal phototropism.

With urea as sole nitrogen source, the addition of 5×10(-5) M nickel sulfate to axenic cultures of Lemna paucicostata 6746 approximately doubles the rate of vegetative growth. Under a standard light-dark schedule, Ni(2+) changes the daily pattern of respiratory CO2 output on urea from one having a single daily peak to one with two daily peaks which resembles that on ammonium or nitrate as sole nitrogen source. It also increases CO2 output by as much as 3-fold on a fresh-weight basis. These data represent the first confirmation in an intact higher plant of proposals, based on enzymology and tissue culture responses, for a role of Ni(2+) in urea metabolism. Further, they indicate the possible existence of two distinct pathways of urea utilization.

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Eight constitutive isoperoxidases were separated by the disc method of polyacrylamide gel electrophoresis from a lyophilized extract of 8-day-old oat (Avena sativa L., cv. Victory) coleoptiles. Both anodic and cathodic isoperoxidases were studied and differences in electrophoretic mobilities and hydrogen donor substrate specificities were revealed. In addition, by enzyme assay, cathodic and anodic isoenzymes were shown to possess differences in peroxidase and IAA (indole-3-acetic acid) oxidase activities.Treatment of coleoptiles with 0.07 mM IAA for 24 h resulted in the repression of two slow-migrating anodic isoperoxidases; however, the same treatment also resulted in the induction of two slow-migrating cathodic isoenzymes which were shown to exhibit peroxidase and IAA oxidase activities.