Phytochrome B and Ethylene Rhythms in Sorghum: Biosynthetic Mechanism and Developmental Effects

Abstract

The sorghum (Sorghum bicolor) cultivar 58M exhibits reduced photoperiodic sensitivity resulting in early flowering and shows shade avoidance behaviour even under non-shaded conditions. This cultivar possesses a mutation in a PHYB gene, presumed to result in a non-functional phytochrome B protein. Both mutant and wild-type plants produce ethylene in a rhythmic cycle, with peaks near mid-day; however, peak ethylene production by the mutant (phyB) is about 10 times greater than the wild-type's. Under bright light, the ethylene rhythm is circadian, and correlates with rhythmic abundance of ACC oxidase [1-aminocyclopropane-1-carboxylate oxidoreductase](ACCO) mRNA and ACCO enzyme activity. Under simulated shade, in which the wild-type plant is a near phenocopy of the phyB mutant, the wild-type plant produces ethylene rhythms similar to those observed in the mutant. ACCO mRNA abundance shows a high amplitude rhythm in both cultivars, but does not translate into enzyme activity under these conditions. The high amplitude ethylene rhythms produced in both cultivars by simulated shading are diurnal but not circadian, and are caused by rhythmic accumulation of ACC. Fumigation of seedlings with 1 p.p.m. ethylene in a rhythm like that produced by the plants themselves results in a reduction of shoot elongation in both cultivars; however, the fresh and dry weights of the roots and shoots are reduced only in the wild-type. While most of the differential ethylene is produced by the shoot, roots of the phyB mutant produce two to three times as much ethylene as the wild-type. Hypoxic treatment of roots induces ethylene production and aerenchyma only in the wild-type. These results suggest that some aspects of seedling development may show reduced sensitivity to ethylene as a result of the phyB mutation