Regulation of rhythmic peaks in levels of endogenous gibberellins (GAs) by photoperiod was studied in the short-day monocot sorghum ( Sorghum bicolor [L.] Moench). Comparisons were made between three maturity (Ma) genotypes: 58M ( Ma 1 Ma 1 , Ma 2 Ma 2 , phyB-1phyB-1 , and Ma 4 Ma 4 [a phytochrome B null mutant]); 90M ( Ma 1 Ma 1 , Ma 2 Ma 2 , phyB-2phyB-2 , and Ma 4 Ma 4 ); and 100M ( Ma 1 Ma 1 , Ma 2 Ma 2 , PHYBPHYB , and Ma 4 Ma 4 ). Plants were grown for 14 d under 10-, 14-, 16-, 18-, and 20-h photoperiods, and GA levels were assayed by gas chromatography-mass spectrometry every 3 h for 24 h. Under inductive 10-h photoperiods, the peak of GA 20 and GA 1 levels in 90M and 100M was shifted from midday, observed earlier with 12-h photoperiods, to an early morning peak, and flowering was hastened. In addition, the early morning peaks in levels of GA 20 and GA 1 in 58M under conditions allowing early flowering (10-, 12-, and 14-h photoperiods) were shifted to midday by noninductive (18- and 20-h) photoperiods, and flowering was delayed. These results are consistent with the possibility that the diurnal rhythm of GA levels plays a role in floral initiation and may be one way by which the absence of phytochrome B causes early flowering in 58M under most photoperiods.

Photoperiod Control of Gibberellin Levels and Flowering in Sorghum

Comprehensive studies in grasses show that gibberellins (GAs) play a role as a florigen. For Lolium temulentum, which flowers in response to a single long day (LD), GAs are a transmitted signal, their content increasing in the leaf early in the LD and then, hours later, at the shoot apex. There is a continuous trail of evidence of hormonal action of these GAs for L. temulentum and support for a similar role in the flowering of other LD-responsive temperate grasses and cereals. A characteristic of the initial flowering responses of grasses and cereals is their limited stem elongation. Interestingly, it is GAs with low effectiveness for stem elongation, GA5 and GA6, that reach the shoot apex and, structurally, are probably not degraded by 2-oxidase enzymes. By contrast, GA1 and GA4 cause stem elongation, may be inactive for floral evocation, and do not reach the vegetative shoot apex apparently because of susceptibility to degradation. However, GA4 can be florally active if protected against 2-oxidases either structurally or by using a 2-oxidase inhibitor. Later in inflorescence development, GA1 and GA4 can be detected at the shoot apex and are florally active if applied. The 2-oxidase restricting accessibility to the apex has probably declined at this time so there is a second florigenic, LD-regulated GA action. A growing body of molecular evidence supporting these actions of GA may provide a future basis for manipulating flowering of grasses and cereals.

Gibberellins and Flowering of Grasses and Cereals: Prizing Open the Lid of the “Florigen” Black Box

Photoperiodism appears via one manifestation or another in many if not most eukaryotic organisms, including plants and animals. In plants, reproduction is often controlled or influenced by response to relative lengths of day and night, and form of a plant (e.g., internode lengths, stem heights, leaf shapes) is virtually always influenced by photoperiod. Other plant responses, including tuber formation and even relative concentrations of various growth regulators and metabolities, are also influenced by photoperiod. Clearly, most if not all plants and animals of virtually any ecosystem are as they are in response to the photoperiods to which they have been exposed. Hence, the complete understanding of ecosystems must include an understanding of the photoperiodic responses of their components organisms.

Responses to Photoperiod

Developmental responses associated with end-of-day far-red light (EOD-FR) signaling were investigated in maize (Zea mays subspecies mays) seedlings. A survey of genetically diverse inbreds of temperate and tropical/semitropical origins, together with teosinte (Zea mays subspecies parviglumis) and a modern hybrid, revealed distinct elongation responses. A mesocotyl elongation response to the EOD-FR treatment was largely absent in the tropical/semitropical lines, but both hybrid and temperate inbred responses were of the same magnitude as in teosinte, suggesting that EOD-FR-mediated mesocotyl responses were not lost during the domestication or breeding process. The genetic architecture underlying seedling responses to EOD-FR was investigated using the intermated B73 x Mo17 mapping population. Among the different quantitative trait loci identified, two were consistently detected for elongation and responsiveness under EOD-FR, but none were associated with known light signaling loci. The central role of phytochromes in mediating EOD-FR responses was shown using a phytochromeB1 phytochromeB2 (phyB1 phyB2) mutant series. Unlike the coleoptile and first leaf sheath, EOD-FR-mediated elongation of the mesocotyl appears predominantly controlled by gibberellin. EOD-FR also reduced abscisic acid (ABA) levels in the mesocotyl for both the wild type and phyB1 phyB2 double mutants, suggesting a FR-mediated but PHYB-independent control of ABA accumulation. EOD-FR elongation responses were attenuated in both the wild type and phyB1 phyB2 double mutants when a chilling stress was applied during the dark period, concomitant with an increase in ABA levels. We present a model for the EOD-FR response that integrates light and hormonal control of seedling elongation.

Physiological and genetic characterization of end-of-day far-red light response in maize seedlings

Barley (Hordeum vulgare L.) is a long-day plant whose flowering is enhanced when the photoperiod is supplemented with far-red light, and this promotion is mediated by phytochrome. A chemically mutagenized dwarf cultivar of barley was selected for early flowering time (barley maturity daylength response [BMDR]-1) and was made isogenic with the cultivar Shabet (BMDR-8) by backcrossing. BMDR-1 was found to contain higher levels of both phytochrome A and phytochrome B in the dark on immunoblots with monoclonal antibodies from oat (Avena sativa L.) that are specific to different members of the phytochrome gene family. Phytochrome A was light labile in both BMDR-1 and BMDR-8, decreasing to very low levels after 4 d of growth in the light. Phytochrome B was light stable in BMDR-8, being equal in both light and darkness. However, phytochrome B became light labile in BMDR-1 and this destabilization of phytochrome B appeared to make BMDR-1 insensitive to photoperiod. In addition, both the mutant and the wild type lacked any significant promotion of flowering in response to a pulse of far-red light given at the end of day, and the end-of-day, far-red inhibition of tillering is normal in both, suggesting that phytochrome B is not involved with these responses in barley.

A Photoperiod-Insensitive Barley Line Contains a Light-Labile Phytochrome B

Combinations of far-red light (FR) (4 min) and gibberellic acid (GA3), given at the beginning of a daily 12-h dark period in a growth room, were used to study floral induction in four maturity genotypes of the milo group of sorghum (Sorghum bicolor (L.) Moench). The 12-h dark period without GA3 application or FR induced flowering in only the early genotype; FR hastened initiation in the early genotype, while GA3 hastened floral initiation in the two intermidiate-flowering genotypes. GA3 and FR together had a strong synergistic effect, hastening floral initiation by 30 to more than 80 d in the early and intermediate genotypes. Red light (R) did not hasten flowering; FR preceded by R gave the same effect as FR alone. GA3 promoted stem elongation equally whether floral initiation occurred or not; thus, its effect on stem elongation was independent of floral initiation. The capacity of GA3 to induce flowering in sorghum, a short-day plant, seems to be enhanced by phytochrome being in the PR form at the beginning of the night when GA3 was applied.

Emily A. Williams

Papers

Floral initiation in sorghum hastened by gibberellic acid and far-red light