Long-Day Induction of Flowering in Lolium temulentumInvolves Sequential Increases in Specific Gibberellins at the Shoot Apex

Abstract

One challenge for plant biology has been to identify floral stimuli at the shoot apex. Using sensitive and specific gas chromatography-mass spectrometry techniques, we have followed changes in gibberellins (GAs) at the shoot apex during long day (LD)-regulated induction of flowering in the grass Lolium temulentum. Two separate roles of GAs in flowering are indicated. First, within 8 h of an inductive LD, i.e. at the time of floral evocation, the GA 5 content of the shoot apex doubled to about 120 ng g Ϫ1 dry weight. The concentration of applied GA 5 required for floral induction of excised apices (R.W. King, C. Blundell, L.T. Evans [1993] Aust J Plant Physiol 20: 337-348) was similar to that in the shoot apex. Leaf-applied [ 2 H 4 ] GA 5 was transported intact from the leaf to the shoot apex, flowering being proportional to the amount of GA 5 imported. Thus, GA 5 could be part of the LD stimulus for floral evocation of L. temulentum or, alternatively, its increase at the shoot apex could follow import of a primary floral stimulus. Later, during inflorescence differentiation and especially after exposure to additional LD, a second GA action was apparent. The content of GA 1 and GA 4 in the apex increased greatly, whereas GA 5 decreased by up to 75%. GA 4 applied during inflorescence differentiation strongly promoted flowering and stem elongation, whereas it was ineffective for earlier floral evocation although it caused stem growth at all times of application. Thus, we conclude that GA 1 and GA 4 are secondary, late-acting LD stimuli for inflorescence differentiation in L. temulentum.Plants of Lolium temulentum remain vegetative when grown in short days (SD), but flower after exposure of their leaves to a single long day (LD). The leaf gibberellin (GA) content increases in LD (Gocal et al., 1999) and applied GAs can cause flowering in noninductive SD (Evans, 1964;Pharis et al., 1987;Evans et al., 1990). Thus, GAs mimic LD responses and they could be a transmissible endogenous floral stimulus in this LD plant. A number of other LD plants but not all (for summary, see Metzger, 1995) flower in response to GA, and recent genetic and molecular studies with Arabidopsis support such a role for endogenous GAs in flowering in LD (Wilson et al., 1992;Weigel and Nilsson, 1995;Blásquez et al., 1997). Furthermore, where there are effects of LD exposure on stem elongation there are clear increases in the GA content of leaves, petioles, and shoot tips (Talon and Zeevaart, 1990;Talon et al., 1991;Zeevaart et al., 1993).Inflorescence initiation in L. temulentum after one LD precedes any acceleration of stem elongation. Therefore, if GAs were to play a role endogenously in floral evocation, they should have little effect on stem elongation. From application studies, we previously identified a number of GAs, including GA 5 , that meet this criterion of inducing flowering but with little or no effect on stem elongation (Evans et al., 1990(Evans et al., , 1994a(Evans et al., , 1994b. However, in a broad context, three lines of evi...

Effect of Gibberellic Acid on Growth, Yield, and Quality of Leaf Lettuce and Rocket Grown in a Floating System

“…It is well known that GA 3 treatments promote cell division and cell enlargements that may result in stem elongation [49][50][51][52] and that exogenous gibberellins can break the rosette or enhance and control the flowering of rosette plants by means of rapid enlargement of already differentiated tissues [53]. GAs promote cytogenesis and cell elongation in stems and can also stimulate flowering when they reach the shoot apex [54,55]. Below 10 −4 M GA 3 , the presence of GA 3 in the MNS was effective as a plant growth promoter and yield enhancer, especially at 10 −6 M GA 3 .…”

Dual Role of Gibberellin in Perennial Shoot Branching: Inhibition and Activation

“…However, plants can switch between pathways, depending on developmental phase or environmental conditions (Rieu et al, 2008b). For example, in a grass species GA 4 is produced during vegetative growth, while upon flowering it switched to GA 5 and GA 6 (King et al, 2001(King et al, , 2003. That GA2oxs play a role in this, is supported by studies in Jatropha, where overexpression of GA2ox6 induced a switch from the non-13hydroxylation pathway (GA 4 ) to the 13-hydroxylation pathway (GA 3 ), and led to dwarfing (Hu et al, 2017).…”

“…Only a small number of the more than 130 known GAs is biologically active, including GA 1 , GA 3 , GA 4 , GA 5 , GA 6 , and GA 7 (King et al, 2001(King et al, , 2003Yamaguchi, 2008;Hedden and Sponsel, 2015). GA biosynthesis starts with plastid-localized geranylgeranyl diphosphate (GGDP), which is converted to ent-kaurene ( Figure 1; Hedden and Phillips, 2000;Olszewski et al, 2002;Yamaguchi, 2008), and oxidized by cytochrome P450 mono-oxygenase in the endoplasmic reticulum to yield GA 12 (Helliwell et al, 2001).…”

The interplay of phloem-mobile signals in plant development and stress response

“…meristem (SAM) where flower development can begin. Depending on the plant species, phytohormones like GA, CKs, and ethylene participate in the regulation of flowering, either independently, as phloem-mobile signals themselves, or in FT/CONSTANS (CO)-dependent mechanisms [106][107][108][109][110]156]. In many plant species, the key florigenic signal is the protein FT.…”