Selective deactivation of gibberellins below the shoot apex is critical to flowering but not to stem elongation of Lolium

Gibberellins (GAs) cause dramatic increases in plant height and a genetic block in the synthesis of GA₁ explains the dwarfing of Mendel's pea. For flowering, it is GA₅ which is important in the long-day (LD) responsive grass, Lolium. As we show here, GA₁ and GA₄ are restricted in their effectiveness for flowering because they are deactivated by C-2 hydroxylation below the shoot apex. In contrast, GA₅ is effective because of its structural protection at C-2. Excised vegetative shoot tips rapidly degrade [¹⁴C]GA₁, [¹⁴C]GA ₄, and [¹⁴C]GA₂₀ (>80% in 6 h), but not [¹⁴C]GA₅. Coincidentally, genes encoding two 2β-oxidases and a putative 16-17-epoxidase were most expressed just below the shoot apex (<3 mm). Further down the immature stem (>4 mm), expression of these GA deactivation genes is reduced, so allowing GA₁ and GA₄ to promote sub-apical stem elongation. Subsequently, GA degradation declines in florally induced shoot tips and these GAs can become active for floral development. Structural changes which stabilize GA₄ confirm the link between florigenicity and restricted GA 2β-hydroxylation (e.g. 2α-hydroxylation and C-2 di-methylation). Additionally, a 2-oxidase inhibitor (Trinexapac Ethyl) enhanced the activity of applied GA₄, as did limiting C-16,17 epoxidation in 16,17-dihydro GAs or after C-13 hydroxylation. Overall, deactivation of GA₁ and GA₄ just below the shoot apex effectively restricts their florigenicity in Lolium and, conversely, with GA₅, C-2 and C-13 protection against deactivation allows its high florigenicity. Speculatively, such differences in GA access to the shoot apex of grasses may be important for separating floral induction from inflorescence emergence and thus could influence their survival under conditions of herbivore predation.