The nature of floral signals in Arabidopsis. I. Photosynthesis and a far-red photoresponse independently regulate flowering by increasing expression of FLOWERING LOCUS T (FT)

Arabidopsis flowers in long day (LD) in response to signals transported from the photoinduced leaf to the shoot apex. These LD signals may include protein of the gene FLOWERING LOCUS T (FT) while in short day (SD) with its slower flowering, signalling may involve sucrose and gibberellin. Here, it is shown that after 5 weeks growth in SD, a single LD up-regulated leaf blade expression of FT and CONSTANS (CO) within 4–8 h, and flowers were visible within 2–3 weeks. Plants kept in SDs were still vegetative 7 weeks later. This LD response was blocked in ft-1 and a co mutant. Exposure to different LD light intensities and spectral qualities showed that two LD photoresponses are important for up-regulation of FT and for flowering. Phytochrome is effective at a low intensity from far-red (FR)-rich incandescent lamps. Independently, photosynthesis is active in an LD at a high intensity from red (R)-rich fluorescent lamps. The photosynthetic role of a single high light LD is demonstrated here by the blocking of the flowering and FT increase on removal of atmospheric CO2 or by decreasing the LD light intensity by 10-fold. These conditions also reduced leaf blade sucrose content and photosynthetic gene expression. An SD light integral matching that in a single LD was not effective for flowering, although there was reasonable FT-independent flowering after 12 SD at high light. While a single photosynthetic LD strongly amplified FT expression, the ability to respond to the LD required an additional but unidentified photoresponse. The implications of these findings for studies with mutants and for flowering in natural conditions are discussed

CsFTL3, a chrysanthemum FLOWERING LOCUS T-like gene, is a key regulator of photoperiodic flowering in chrysanthemums

Chrysanthemum is a typical short-day (SD) plant that responds to shortening daylength during the transition from the vegetative to the reproductive phase. FLOWERING LOCUS T (FT)/Heading date 3a (Hd3a) plays a pivotal role in the induction of phase transition and is proposed to encode a florigen. Three FT-like genes were isolated from Chrysanthemum seticuspe (Maxim.) Hand.-Mazz. f. boreale (Makino) H. Ohashi & Yonek, a wild diploid chrysanthemum: CsFTL1, CsFTL2, and CsFTL3. The organ-specific expression patterns of the three genes were similar: they were all expressed mainly in the leaves. However, their response to daylength differed in that under SD (floral-inductive) conditions, the expression of CsFTL1 and CsFTL2 was down-regulated, whereas that of CsFTL3 was up-regulated. CsFTL3 had the potential to induce early flowering since its overexpression in chrysanthemum could induce flowering under non-inductive conditions. CsFTL3-dependent graft-transmissible signals partially substituted for SD stimuli in chrysanthemum. The CsFTL3 expression levels in the two C. seticuspe accessions that differed in their critical daylengths for flowering closely coincided with the flowering response. The CsFTL3 expression levels in the leaves were higher under floral-inductive photoperiods than under non-inductive conditions in both the accessions, with the induction of floral integrator and/or floral meristem identity genes occurring in the shoot apexes. Taken together, these results indicate that the gene product of CsFTL3 is a key regulator of photoperiodic flowering in chrysanthemums

Regulation of gibberellin biosynthesis and stem elongation by low temperature in Eustoma grandiflorum

Heat-induced rosetted Eustoma grandiflorum requires low temperature for induction of stem elongation and flowering. Although heat-induced rosetting is associated with a reduction of gibberellin A1 (GA1) content, how thermo-induction affects GA biosynthesis is unclear. Thus, we examined levels of GA, precursors including that of ent-kaurene which is the first committed step in GA biosynthesis. We used uniconazole, an ent-kaurene oxidase inhibitor to estimate the ent-kaurene biosynthesis activity.The accumulation level of ent-kaurene in stems of the cold-treated seedlings was approximately 1.8 times that of the non-cold-treated seedlings, whereas no difference was observed in the leaves. No change was observed in endogenous levels of GA1 and GA20 in stems of the heat-induced rosetted plants during the cold treatment, whereas their levels increased with stem elongation after transfer to warm conditions. In contrast to the levels of GA1 and GA20, endogenous levels of ent-kaurene, ent-kaurenoic acid, GA53, GA44 and GA19 in the stems markedly increased at the end of cold treatment. These results indicate that ent-kaurene biosynthesis and its metabolism early in the GA biosynthetic pathway are stimulated by low temperature and, later, the stimulation leads to an increment of endogenous levels of GA1 which is essential for stem elongation of the heat-induced rosetted E. grandiflorum

The relationship between endogenous gibberellins and rosetting in Eustoma grandiflorum

Treatments with gibberellin A1 (GA1), GA3 and GA20 promoted stem elongation in rosetted seedlings of Eustoma grandiflorum in the following order: GA1 = GA3 > GA20, whereas ent-kaurene (K), ent-kaurenoic acid (KA) and GA19 did not. GA1, GA3, GA19 and GA2

Identification of endogenous gibberellins in inflorescence of Ornithogalum thyrsoides

Endogenous gibberellins (GAs) were extracted from inflorescence of Ornithogalum thyrsoides and identified by using combined gas-chromatography / mass spectrometry (GC/MS). Three 13-hydroxylated GAs, GA19, GA20 and GA53, and thirteen 13-non-hydroxylated GAs, GA4, GA7, GA9, GA12, GA15, GA24, GA25, GA51, GA61, GA112, GA115, 1,2-didehydro GA9 (which is a novel GA, and has been assigned as GA120), and GA120 - isolactone were detected. The presence of these GAs suggests that both the early-13-hydroxylation GA biosynthesis pathway and the early-13-non-hydroxylated GA biosynthesis pathway were operating in the inflorescence of Ornithogalum. The presence of GA7, GA9 and GA120 suggests that GA120 could be considered as a metabolic intermediate in the conversion of GA9 into GA7 in O. thyrsoides

The role of gibberellin biosynthesis in the control of growth and flowering in Matthiola incana

Recently, it was found that stem elongation and flowering of stock Matthiola incana (L.) R. Br. are promoted by exogenous gibberellins (GAs), including GA4, and also by acylcyclohexanedione inhibitors of GA biosynthesis, such as prohexadione-calcium (PCa) and trinexapac-ethyl (TNE). Here, because it was unclear how GA biosynthetic inhibitors could promote stem elongation and flowering, their effect on GA biosynthesis has been examined by quantifying endogenous GA levels; also, the sensitivity of stem elongation and flowering to various GAs in combination with the inhibitors was examined. Stem elongation and flowering were most effectively promoted by GA4 when combined with PCa and, next in order, by 2,2-dimethyl-GA4, PCa, GA4 + TNE, TNE, GA9 + PCa and by GA4. There was little or no promotion by GA1, GA3, GA9, GA13, GA20 and 3-epi-2,2-dimethyl-GA4. Both the promotive effects of the acylcyclohexanediones on stem elongation and flowering, particularly when applied with GA4, and the fact that TNE caused a build-up of endogenous GA4 imply that one effect of TNE at the lower dose involved an inhibition of 2β-hydroxylation of GA4 rather than an inhibition of 20-oxidation and 3β-hydroxylation of GAs which were precursors of GA4. Overall, these results indicate that: (1) GAs with 3β-OH and without 13-OH groups (e.g. GA4) are the most important for stem elongation and flowering in M. incana; (2) growth promotion rather than inhibition can result if an acylcyclohexanedione acts predominantly to slow 2β-hydroxylation and so slows inactivation of active gibberellins, including GA4. It follows that a low dose of an acylcyclohexanedione can be a 'growth enhancer' for any applied GA that is liable to inactivation by 2β-hydroxylation