The Mechanism of Rhythmic Ethylene Production in Sorghum. The Role of Phytochrome B and Simulated Shading1

Mutant sorghum (Sorghum bicolor [L.] Moench) deficient in functional phytochrome B exhibits reduced photoperiodic sensitivity and constitutively expresses a shade-avoidance phenotype. Under relatively bright, high red:far-red light, ethylene production by seedlings of wild-type and phytochrome B-mutant cultivars progresses through cycles in a circadian rhythm; however, the phytochrome B mutant produces ethylene peaks with approximately 10 times the amplitude of the wild type. Time-course northern blots show that the mutant's abundance of the 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase mRNA SbACO2 is cyclic and is commensurate with ethylene production, and that ACC oxidase activity follows the same pattern. Both SbACO2 abundance and ACC oxidase activity in the wild-type plant are very low under this regimen. ACC levels in the two cultivars did not demonstrate fluctuations coincident with the ethylene produced. Simulated shading caused the wild-type plant to mimic the phenotype of the mutant and to produce high amplitude rhythms of ethylene evolution. The circadian feature of the ethylene cycle is conditionally present in the mutant and absent in the wild-type plant under simulated shading. SbACO2 abundance in both cultivars demonstrates a high-amplitude diurnal cycle under these conditions; however, ACC oxidase activity, although elevated, does not exhibit a clear rhythm correlated with ethylene production. ACC levels in both cultivars show fluctuations corresponding to the ethylene rhythm previously observed. It appears that at least two separate mechanisms may be involved in generating high-amplitude ethylene rhythms in sorghum, one in response to the loss of phytochrome B function and another in response to shading.

Phytochrome B and the Regulation of Circadian Ethylene Production in Sorghum1

The sorghum (Sorghum bicolor L. Moench) cultivar 58M, which contains the null mutant phytochrome B gene, shows reduced photoperiodic sensitivity and exhibits a shade-avoidance phenotype. Ethylene production by seedlings of wild-type and phytochrome B mutant cultivars was monitored every 3 h, and both cultivars were found to produce ethylene in a circadian rhythm, with peak production occurring during the day. The phytochrome B mutant produces rhythmic peaks of ethylene with approximately 10 times the amplitude of the wild-type counterpart with the same period and diurnal timing. The source of the mutant's additional ethylene is the shoot. The diurnal rhythm can be produced with either light or temperature cycles; however, both light and temperature cycles are required for circadian entrainment. The temperature signal overrides the light signal in the production of diurnal rhythms, because seedlings grown under thermoperiods reversed with the photoperiod produced ethylene peaks during the warm nights. To examine the effect of extreme shading on ethylene production, seedlings were grown under dim, far-red-enriched light. This treatment duplicated the phytochrome B mutant's shade-avoidance phenotype in the wild type and caused the wild type to produce ethylene peaks similar to those observed in the mutant. The results confirm that phytochrome B is not required for proper function of circadian timing, but it may be involved in modulating physiological rhythms driven by the biological clock oscillator.

UM Men's Cross Country Team. 1992

Back Row: Ron Warhurst, Jim Finlayson, Matt Joseph, Shawn Mackay, Carlos Paradelo, Jason Colvin, Ian Forsyth, Sean Sweat, Chris Childs, Theo Molla, volunteer coach Dan Heikkinen

Front Row: Mark Kwiatkowski, Jonathan Aubuchon, Brett Smith, Jay Schemanske, Matt Smith, Scott MacDonald, Kristopher Eggle

UM Men's Cross Country Team. 1993

Top Row: Nick Watson, Kevin Sullivan, Ian Forsyth, Theo Molla

Middle Row: David Barnett, Matt Schroeder, Ryan Burt, Kenneth Kozloff, Michael Mahler, Don MacDonald

Front Row: head coach Ron Warhurst, Kris Eggle, Jay Schemanske, Mark Kwiatkowski, Jon Aubuchon

Not pictured: Jim Finlayson, Scott MacDonald, Andy Hayes, Robert Lee, Shawn MacKay, Daniel Shea, Chad Tibbetts

The Dack family; a study in hereditary lack of emotional control,

Mode of access: Internet.

Minstrelsy of the Scottish border,

Mode of access: Internet.

Immunoglobulins : characteristics and uses of intravenous preparations /

Mode of access: Internet.

Scottish vernacular literature: a succinct history.

Bibliographical footnotes.

The casket letters and Mary queen of Scots;

Mode of access: Internet.

Sermons.

"A short account of the life and character of Dr. Hugh Blair," by James Finlayson: v. 5, p. [455]-475.

Sermons, /

Advertisements at end of v. 1-3, 5.

Sermons.

[Title-page of vol. V: To which is annexed, A short account of the life and character of the author, by James Finlayson.]

Sermons.

"A short account of the life and character of Dr. Hugh Blair," by James Finlayson; v. 5, p. [455]-475.

Scotland of to-day,

The evolution of the Scot.--The kirk and its story.--Education in school and college.--The law and the lawyers.--Architecture, ecclesiastical and other.--Painting and painters.--Literature.--"Edina, Scotia's darling seat."--The kingdom of Fife.--In Lothian fields.--The Lothian shore.--The city of St. Mungo.--The Clyde.--Burns and the Burns country.--The legend of the Covenant.--Yarrow and Traquair.--The Border and the Solway.--Stirling and Perth.--Dundee.--The Granite city.--A highland survey.--A note on Caithness.--Round the islands.--Sports and pastimes.--Music, old and new.--Scots food.--Scots drink.--Scots wit and humour.--The Scot abroad and the stranger in Scotland.