The nature of floral signals in Arabidopsis. II. Roles for FLOWERING LOCUS T (FT) and gibberellin

Signals produced in leaves are transported to the shoot apex where they cause flowering. Protein of the gene FLOWERING LOCUS T (FT) is probably a long day (LD) signal in Arabidopsis. In the companion paper, rapid LD increases in FT expression associated with flowering driven photosynthetically in red light were documented. In a far red (FR)-rich LD, along with FT there was a potential role for gibberellin (GA). Here, with the GA biosynthesis dwarf mutant ga1-3, GA(4)-treated plants flowered after 26 d in short days (SD) but untreated plants were still vegetative after 6 months. Not only was FT expression low in SD but applied GA bypassed some of the block to flowering in ft-1. On transfer to LD, ga1-3 only flowered when treated simultaneously with GA, and FT expression increased rapidly (<19.5 h) and dramatically (15-fold). In contrast, in the wild type in LD there was little requirement for GA for FT increase and flowering so its endogenous GA content was near to saturating. Despite this permissive role for endogenous GA in Columbia, RNA interference (RNAi) silencing of the GA biosynthesis gene, GA 20-OXIDASE2, revealed an additional, direct role for GA in LD. Flowering took twice as long after silencing the LD-regulated gene, GA 20-OXIDASE2. Such independent LD input by FT and GA reflects their non-sympatric expression (FT in the leaf blade and GA 20-OXIDASE2 in the petiole). Overall, FT acts as the main LD floral signal in Columbia and GA acts on flowering both via and independently of FT.     

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, GA(19), decreases equivalently; and a bioactive product, GA(5), increases 4-fold. A link between flowering, LD, GAs, and GA biosynthesis is shown in three ways: (1) applied GA(19) 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 GA(5) biosynthesis, coupled with subsequent doubling in GA(5) content at the shoot apex, provides a substantial trail of evidence for GA(5) as a LD florigen. LD signaling may also involve transport of FT mRNA or protein because expression of LtFT and LtCONSTANS increased rapidly, substantially (>80-fold for FT), and independently of GA. However, because a LD from fluorescent lamps induced LtFT expression but not flowering, the nature of the light response of FT requires clarification.     

Mobile signals in day length-regulated flowering: Gibberellins, flowering locus T, and sucrose

Despite low activity for stem growth, the gibberellins GA₅ and GA₆ act as long-day (LD) florigens in Lolium temulentum L. This claim is based on extensive evidence covering GA synthesis in LD in the induced leaf and their transport to the shoot apex where they act in a dose-dependent manner. GAs also act as a LD florigen in association with cold vernalization of L. perenne. In contrast, highly bioactive GA₄ and, possibly, GA₁ are important florigens in Arabidopsis thaliana (L.) Heynh. This species contrast reflects differences in GA deactivation, which is unimportant for Arabidopsis but dominant in L. temulentum. It is unclear if GAs participate in flowering responses of short-day (SD) species since it is LD, which up-regulate enzymes for GA biosynthesis. Sugars (sucrose) may also act directly as a florigen and, specifically, with increase in photosynthesis as in LD or when light intensity is increased in SD. In addition, in LD sucrose can indirectly cause flowering by up-regulating FT expression, the FT protein acting as a further leaf-to-apex transported florigen. Thus, there are not only multiple florigens but there can be complex interactions between the signaling pathways controlling production of these various florigens.     

Circadian rhythms and the induction of flowering in the long-day grass Lolium temulentum L.

Plants of Lolium temulentum L. strain Ceres were grown in 8-h short day (SD) for 45 d before being exposed either to a single long day (LD) or to a single 8-h SD given during an extended dark period. For LD induction, the critical photoperiod was between 12 and 14 h, and more than 16 h were needed for a maximal flowering response. During exposure to a single 24-h LD, the translocation of the floral stimulus began between the fourteenth and the sixteenth hours after the start of the light period, and was completed by the twenty-fourth hour. Full flowering was also induced by one 8-h SD beginning 4 or 28 h after the start of a 40-h dark period, i.e. by shifting 12 h forward or beyond the usual SD. The effectiveness of a so-called ‘displaced short day’ (DSD) was not affected by light quality and light intensity. With a mixture of incandescent and fluorescent lights at a total photosynthetic photon flux density of 400 μmol m⁻² s⁻¹, a 4-h light exposure beginning 4 h after the start of a 40-h dark period was sufficient to induce 100% flowering. The flower-inducing effect of a single 8-h DSD was also assessed during a 64-h dark period. Results revealed two maxima at a 20-h interval. This fluctuation in light sensitivity suggests that a circadian rhythm is involved in the control of flowering of L. temulentum.     

CONSTANS delays Arabidopsis flowering under short days

Long days (LD) promote flowering of Arabidopsis thaliana compared with short days (SD) by activating the photoperiodic pathway. Here we show that growth under very‐SD (3 h) or darkness (on sucrose) also accelerates flowering on a biological scale, indicating that SD actively repress flowering compared with very‐SD. CONSTANS (CO) repressed flowering under SD, and the early flowering of co under SD required FLOWERING LOCUS T (FT). FT was expressed at a basal level in the leaves under SD, but these levels were not enhanced in co. This indicates that the action of CO in A. thaliana is not the mirror image of the action of its homologue in rice. In the apex, CO enhanced the expression of TERMINAL FLOWER 1 (TFL1) around the time when FT expression is important to promote flowering. Under SD, the tfl1 mutation was epistatic to co and in turn ft was epistatic to tfl1. These observations are consistent with the long‐standing but not demonstrated model where CO can inhibit FT induction of flowering by affecting TFL1 expression.     

Terminal flower2, an Arabidopsis homolog of heterochromatin protein1, counteracts the activation of flowering locus T by constans in the vascular tissues of leaves to regulate flowering time

The flowering time of plants is tightly regulated by both promotive and repressive factors. Molecular genetic studies using Arabidopsis have identified several epigenetic repressors that regulate flowering time. Terminal flower2, (TFL2), which encodes a homolog of heterochromatin protein1 represses flowering locus T (FT) expression, which is induced by the activator constans (CO) in response to the long-day signal. Here, we show that TFL2, CO, and FT are expressed together in leaf vascular tissues and that TFL2 represses FT expression continuously throughout development. Mutations in TFL2 derepress FT expression within the vascular tissues of leaves, resulting in daylength-independent early flowering. TFL2 can reduce FT expression even when CO is overexpressed. However, FT expression reaches a level sufficient for floral induction even in the presence of TFL2, suggesting that TFL2 does not maintain FT in a silent state or inhibit it completely; rather, it counteracts the effect of CO on FT activation.     

Postflowering photoperiod regulates vegetative growth and reproductive development of soybean

Soybean development is controlled by environmental factors, primarily photoperiod and temperature. To date, photoperiod effects on flowering have been well studied but the performances and mechanism of postflowering photoperiod responses have not been fully understood, especially for the photoperiod effects on vegetative growth after flowering. In the present study, the responses of vegetative growth and reproductive development in soybean to different postflowering photoperiod regimes were investigated in four separate experiments. Three varieties of different maturity groups (MG) including the early (Dongnong 36, MG 000), medium (Dandou 5, MG IV), and late (Zigongdongdou, MG IX) were exposed to two photoperiods, short (10, 12 h) and long (15, 16 or 18 h). The results showed that postflowering photoperiod not only regulated reproductive development but also affected vegetative growth. Even when flowers and pods were removed, short-day (SD) treatment promoted leaf senescence. The onset of leaf senescence among varieties tested appeared to be dependent on photoperiod sensitivity. Leaf senescence of the late-maturing variety of Zigongdongdou (sensitive to photoperiod) was delayed more significantly than that of the medium and early-maturing varieties (less sensitive to photoperiod). Long-day (LD) treatments delayed leaf senescence and seed maturation in the late-maturing variety of Zigongdongdou plants with only the SD-induced leaves produced before flowering. LD treatments imposed from the beginning bloom, beginning pod setting or beginning seed filling delayed leaf senescence and seed maturation of late-maturing soybean variety (Zigongdongdou). Results of night-break with red (R) and far-red (FR) light demonstrated that postflowering photoperiod responses of soybean were R/FR reversible reactions and the phytochromes seemed to be functional as receptors of photoperiod signals even after flowering. It was proposed that the regulation of photoperiod on development of soybean was effective from emergence through maturation, and the postflowering photoperiod signals were also mediated by phytochromes similar to those before flowering. The flowering reversion in late-MG soybean varieties under LD was a direct result of LD and was not due to secondary effect of abscission of pods and flowers. Soybean leaves not only received SD signals but also LD signals; furthermore, the LD effects reversed the SD effects and vice versa.     

The alleles at the E1 locus impact the expression pattern of two soybean FT-like genes shown to induce flowering in Arabidopsis

A small gene family of phosphatidyl ethanolamine-binding proteins (PEBP) has been shown to function as key regulators in flowering; in Arabidopsis thaliana the FT protein promotes flowering whilst the closely related TFL1 protein represses flowering. Control of flowering time in soybean [Glycine max (L.) Merrill] is important for geographic adaptation and maximizing yield. Soybean breeders have identified a series of loci, the E-genes, that control photoperiod-mediated flowering time, yet how these loci control flowering is poorly understood. The objectives of this study were to evaluate the expression of GmFT-like genes in the E1 near-isogenic line (NIL) background. Of the 20 closely related PEBP proteins in the soybean genome, ten are similar to the Arabidopsis FT protein. Expression analysis of these ten GmFT-like genes confirmed that only two are detectable in the conditions tested. Further analysis of these two genes in the E1 NILs grown under short-day (SD) and long-day (LD) conditions showed a diurnal expression and tissue specificity expression commensurate with soybean flowering time under SD and LD conditions, suggesting that these were good candidates for flowering induction in soybean. Arabidopsis ft mutant lines flowered early when transformed with the two soybean genes, suggesting that the soybean genes can complement the Arabidopsis FT function. Flowering time in E1 NILs is consistent with the differential expression of the two GmFT-like genes under SD and LD conditions, suggesting that the E1 locus, at least in part, impacts time to flowering through the regulation of soybean FT expression.     

Role of magnesium in carbon partitioning and alleviating photooxidative damage

Magnesium (Mg) deficiency exerts a major influence on the partitioning of dry matter and carbohydrates between shoots and roots. One of the very early reactions of plants to Mg deficiency stress is the marked increase in the shoot-to-root dry weight ratio, which is associated with a massive accumulation of carbohydrates in source leaves, especially of sucrose and starch. These higher concentrations of carbohydrates in Mg-deficient leaves together with the accompanying increase in shoot-to-root dry weight ratio are indicative of a severe impairment in phloem export of photoassimilates from source leaves. Studies with common bean and sugar beet plants have shown that Mg plays a fundamental role in phloem loading of sucrose. At a very early stage of Mg deficiency, phloem export of sucrose is severely impaired, an effect that occurs before any noticeable changes in shoot growth, Chl concentration or photosynthetic activity. These findings suggest that accumulation of carbohydrates in Mg-deficient leaves is caused directly by Mg deficiency stress and not as a consequence of reduced sink activity. The role of Mg in the phloem-loading process seems to be specific; resupplying Mg for 12 or 24 h to Mg-deficient plants resulted in a very rapid recovery of sucrose export. It appears that the massive accumulation of carbohydrates and related impairment in photosynthetic CO2 fixation in Mg-deficient leaves cause an over-reduction in the photosynthetic electron transport chain that potentiates the generation of highly reactive O2 species (ROS). Plants respond to Mg deficiency stress by marked increases in antioxidative capacity of leaves, especially under high light intensity, suggesting that ROS generation is stimulated by Mg deficiency in chloroplasts. Accordingly, it has been found that Mg-deficient plants are very susceptible to high light intensity. Exposure of Mg-deficient plants to high light intensity rapidly induced leaf chlorosis and necrosis, an outcome that was effectively delayed by partial shading of the leaf blade, although the Mg concentrations in different parts of the leaf blade were unaffected by shading. The results indicate that photooxidative damage contributes to development of leaf chlorosis under Mg deficiency, suggesting that plants under high-light conditions have a higher physiological requirement for Mg. Maintenance of a high Mg nutritional status of plants is, thus, essential in the avoidance of ROS generation, which occurs at the expense of inhibited phloem export of sugars and impairment of CO2 fixation, particularly under high-light conditions.     

A high proportion of blue light increases the photosynthesis capacity and leaf formation rate of Rosa × hybrida but does not affect time to flower opening

Alterations in light quality affect plant morphogenesis and photosynthetic responses but the effects vary significantly between species. Roses exhibit an irradiance‐dependent flowering control but knowledge on light quality responses is scarce. In this study we analyzed, the responses in morphology, photosynthesis and flowering of Rosa × hybrida to different blue (B) light proportions provided by light‐emitting diodes (LED, high B 20%) and high pressure sodium (HPS, low B 5%) lamps. There was a strong morphological and growth effect of the light sources but no significant difference in total dry matter production and flowering. HPS‐grown plants had significantly higher leaf area and plant height, yet a higher dry weight proportion was allocated to leaves than stems under LED. LED plants showed 20% higher photosynthetic capacity (A_max) and higher levels of soluble carbohydrates. The increase in A_max correlated with an increase in leaf mass per unit leaf area, higher stomata conductance and CO₂ exchange, total chlorophyll (Chl) content per area and Chl a/b ratio. LED‐grown leaves also displayed a more sun‐type leaf anatomy with more and longer palisade cells and a higher stomata frequency. Although floral initiation occurred at a higher leaf number in LED, the time to open flowers was the same under both light conditions. Thereby the study shows that a higher portion of B light is efficient in increasing photosynthesis performance per unit leaf area, enhancing growth and morphological changes in roses but does not affect the total Dry Matter (DM) production or time to open flower.