Bruce A. Bonner

The effect of abscisic acid on the uptake of potassium and chloride into Avena coleoptile sections

SummaryThe effect of abscisic acid (ABA) on uptake of potassium (86Bb+ or 42K+) by Avena sativa L. coleoptile sections was investigated. ABA lowered the potassium uptake rate within 30 min after its application and inhibition reached a maximum (ca. 75%) after 2 h. The inhibition of K+ uptake increased with ABA concentration over a range of 0.03 to 10 μg/ml ABA. At a higher K+ concentration (20 mM) the percentage inhibition decreased. The percentage inhibition of K+ uptake by ABA remained constant with external K+ varied from 0.04 to 1.0 mM. After a loading period in 20 mM K+ (86Rb+), apparent efflux of potassium was only slightly increased by ABA. Experiments in which growth was greatly reduced by mannitol or by omission of indole-3-acetic acid from the medium indicated there was no simple quantitative correspondence between ABA inhibition of coleoptile elongation and ABA inhibition of K+ uptake. Chloride uptake was also inhibited by ABA but to a smaller degree than was K+ uptake. No specificity for counterions was observed for K+ uptake. Uptake of 3,0-methylglucose and proline were inhibited by ABA to a much smaller extent (14 and 11%) than that of K+, a result which suggests that ABA acts on specific ion uptake mechanisms.

Labeling of porphobilinogen deaminase by radioactive 5-aminolevulinic acid in isolated developing pea chloroplasts

A protein had been previously described, which was labeled by radioactive 5-aminolevulinic acid in isolated developing chloroplasts. In the present study we have shown that this protein (Mr approximately equal to 43,000) probably exists as a monomer in the chloroplast stroma. The labeling is blocked if known inhibitors of 5-aminolevulinic acid dehydratase are added to the incubation mixture, and is markedly decreased in intensity if nonradioactive 5-aminolevulinate or porphobilinogen are added to the incubation mixture; other intermediates in the porphyrin biosynthetic pathway, uroporphyrinogen III, uroporphyrin III, and protoporphyrin IX, do not decrease the labeling of the 43-kDa protein appreciably. Nondenaturing gels of the proteins isolated from the incubation with radioactive 5-aminolevulinic acid were stained for porphobilinogen deaminase activity. A series of red fluorescent bands was obtained which coincided with the radioactive bands visualized by autoradiography. It is concluded that the soluble chloroplast protein that is labeled in organello by radioactive 5-aminolevulinic acid is porphobilinogen deaminase.