Regulation of Flowering in the Long-Day Grass Lolium temulentum by Gibberellins and the FLOWERING LOCUS T Gene

Seasonal control of flowering often involves leaf sensing of daylength coupled to time measurement and generation and transport of florigenic signals to the shoot apex. We show that transmitted signals in the grass Lolium temulentum may include gibberellins (GAs) and the FLOWERING LOCUS T (FT) gene. Within 2 h of starting a florally inductive long day (LD), expression of a 20-oxidase GA biosynthetic gene increases in the leaf; its product, GA 20 , then increases 5.7-fold versus short day; its substrate, GA19, decreases equivalently; and a bioactive product, GA5, increases 4-fold. A link between flowering, LD, GAs, and GA biosynthesis is shown in three ways: (1) applied GA19 became florigenic on exposure to LD; (2) expression of LtGA20ox1, an important GA biosynthetic gene, increased in a florally effective LD involving incandescent lamps, but not with noninductive fluorescent lamps; and (3) paclobutrazol, an inhibitor of an early step of GA biosynthesis, blocked flowering, but only if applied before the LD. Expression studies of a 2-oxidase catabolic gene showed no changes favoring a GA increase. Thus, the early LD increase in leaf GA5 biosynthesis, coupled with subsequent doubling in GA5 content at the shoot apex, provides a substantial trail of evidence for GA5 as a LD florigen. LD signaling may also involve transport of FT mRNA or protein because expression of LtFTand LtCONSTANS increased rapidly, substantially (.80-fold for FT), and independently of GA. However, because a LD from fluorescent lamps induced LtFTexpression but not flowering, the nature of the light response of FT requires clarification. It has been more than 90 years since Tournois, in 1914, and, independently, Garner and Allard, in 1920, proposed that seasonal control of flowering was a response to daylength; long-day (LD) species flower with an increase in the daily hours of light, and shortday (SD) species flower in shorter daylengths. The leaf was the predominant organ of daylength perception, and, based on his grafting studies, Chailakhyan, in 1937, coined the term florigen for the florally inductive signals transported from the leaf to the shoot apex (for review, see Lang, 1965). Based on recent studies with