Gibberellic acid and the inhibition of aerial tuberisation in Solanum tuberosum L.

The sensitivity of aerial and subterranean tuberisation to photoperiod was studied in potato (Solanum tuberosum cv. Aracy). Although photoperiodic sensitivity varied with the position along the stem, all buds could be induced to develop tubers under SD. Gibberellic acid (GA₃) applied to induced (30 short days) cuttings inhibited the photoperiodic effect. No tubers were formed and orthotropic shoots developed instead. The GA₃ caused a reduction in starch content in induced buds, lowering it to the same level as found in long-day treated plants. However, -amylase activity of buds of induced plants was not affected by GA₃, suggesting that GA₃ does not inhibit tuberisation by promotion of starch hydrolysis.     

Over-expression of <Emphasis Type="Italic">CONSTANS-LIKE 5</Emphasis> can induce flowering in short-day grown <Emphasis Type="Italic">Arabidopsis</Emphasis>

To ensure that the initiation of flowering occurs at the correct time of year, plants need to integrate a diverse range of external and internal signals. In Arabidopsis, the photoperiodic flowering pathway is controlled by a set of regulators that include CONSTANS (CO). In addition, Arabidopsis plants also have a family of genes with homologies to CO known as CO-LIKE (COL) about which relatively little is known. In this paper, we describe the regulation and interactions of a novel member of the family, COL5. The expression of COL5 is under circadian and diurnal regulation, but COL5 itself does not appear to affect circadian rhythms. COL5, like CO, is regulated by GIGANTEA. Furthermore, COL5 is expressed in the vascular tissue. Using COL5 over-expressing lines we show that, under short days, constitutive expression of COL5 affects flowering time and the expression of the floral integrator genes, FLOWERING LOCUS T and SUPPRESSOR OF OVEREXPRESSION OF CO 1. Constitutive expression of COL5 partially suppresses the late flowering phenotype of the co-mutant plants. However, plants with loss of COL5 function do not show altered flowering. Taken together, our results suggest that COL5 has COL activity, but may either not have a role in regulating flowering in wild-type plants or may act redundantly with other flowering regulators. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

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.     

Gibberellins and the photoperiodic control of stem elongation in the long-day plant Agrostemma githago L.

Agrostemma githago is a long-day rosette plant in which transfer from short days (SD) to long days (LD) results in rapid stem elongation, following a lag phase of 7–8 d. Application of gibberellin A₂₀ (GA₂₀) stimulated stem elongation in plants under SD, while 2-isopropyl-4-dimethylamino-5-methylphenyl-1-piperidine-carboxylate methyl chloride (AMO-1618, an inhibitor of GA biosynthesis) inhibited stem elongation in plants exposed to LD. This inhibition of stem elongation by AMO-1618 was overcome by simultaneous application of GA₂₀, indicating that GAs play a role in the photoperiodic control of stem elongation in this species. Endogenous GA-like substances were analyzed using reverse-phase high-performance liquid chromatography and the d-5 corn (Zea mays L.) assay. Three zones with GA-like activity were detected and designated, in order of decreasing polarity, as A, B, and C. A transient, 10-fold increase in the activity of zone B occurred after 8–10 LD, coincident with the transition from lag phase to the phase of rapid stem elongation. After 16 LD the activity in this zone had returned to a level similar to that under SD, even though the plants were elongating rapidly by this time. However, when AMO-1618 was applied to plants after 11 LD, there was a rapid reduction in the rate of stem elongation, indicating that continued GA biosynthesis was necessary following the transient increase in activity of zone B, if stem elongation was to continue under LD. It was concluded that control of stem elongation in A. githago involves more than a simple qualitative or quantitative change in the levels of endogenous GAs, and that photoperiodic induction alters both the sensitivity to GAs and the rate of turnover of endogenous GAs.Abbreviations AMO-1618 2-isopropyl-4-dimethylamino-5-methylphenyl-1-piperidine-carboxylate methyl chloride - GA(s) gibberellin(s) - LD long day(s) - LDP long-day plant(s) - SD short day(s)     

Are cuttings suitable for assessing maturity type in potato (Solanum tuberosum)?

Two experiments were carried out to evaluate the potential of single‐node cuttings of potato (Solanum tuberosum) as a tool to assess genotypic differences in maturity type. Plants were exposed to different photoperiodic treatments (different photoperiods, different numbers of photoperiodic cycles), and cuttings were taken at different plant ages. Cuttings from early (and to a lesser extent also late) maturing varieties exposed to short photoperiods showed strong induction to tuberise, irrespective of plant age; the induction increased with an increase in the number of short photoperiodic cycles. The response of cuttings taken from early‐maturing varieties exposed to long photoperiods depended on plant age: cuttings showed stronger induction when mother plants were older; cuttings from late‐maturing varieties hardly tuberised after exposure to long photoperiods. The tuberisation of the cuttings did not depend on the length of the long photoperiod (18 or 24 h) or on the number of cycles of a photoperiod of 18 h. Tuberisation on cuttings did not properly reflect the tuber formation on the mother plants, although within varieties, significant correlations between tuberisation on cuttings and tuber yield per plant 9 weeks after planting were found with different numbers of photoperiodic cycles of 12 h. Our experiments show that the cutting technique cannot be used on older plants to assess the maturity type of potato varieties, as there are interactions between photoperiod, genotype, plant age and number of photoperiodic cycles, in the reflection of the degree of induction to tuberise on single‐node cuttings.     

A genetic and physiological analysis of late flowering mutants in Arabidopsis thaliana

Summary Monogenic mutants of the early ecotype Landsberg erecta were selected on the basis of late flowering under long day (LD) conditions after treatment with ethyl methanesulphonate or irradiation. In addition to later flowering the number of rosette and cauline leaves is proportionally higher in all mutants, although the correlation coefficient between the two parameters is not the same for all genotypes. Forty-two independently induced mutants were found to represent mutations at 11 loci. The mutations were either recessive, intermediate (co locus) or almost completely dominant (fwa locus). The loci are located at distinct positions on four of the five Arabidopsis chromosomes. Recombinants carrying mutations at different loci flower later than or as late as the later parental mutant. This distinction led to the assignment of eight of the loci to three epistatic groups. In wild type, vernalization promotes flowering to a small extent. For mutants at the loci fca, fve, fy and fpa, vernalization has a large effect both under LD and short day (SD) conditions, whereas co, gi, fd and fwa mutants are almost completely insensitive to this treatment. SD induces later flowering except for mutants at the co and gi loci, which flower with the same number of leaves under LD and SD conditions. This differential response of the mutants to environmental factors and their subdivision into epistatic groups is discussed in relation to a causal model for floral initiation in Arabidopsis thaliana.     

Mapping loci controlling vernalization requirement in Brassica rapa

Brassica cultivars are classified as biennial or annual based on their requirement for a period of cold treatment (vernalization) to induce flowering. Genes controlling the vernalization requirement were identified in a Brassica rapa F₂ population derived from a cross between an annual and a biennial oilseed cultivar by using an RFLP linkage map and quantitative trait locus (QTL) analysis of flowering time in F₃ lines. Two genomic regions were strongly associated with variation for flowering time of unvernalized plants and alleles from the biennial parent in these regions delayed flowering. These QTLs had no significant effect on flowering time after plants were vernalized for 6 weeks, suggesting that they control flowering time through the requirement for vernalization. The two B. rapa linkage groups containing these QTLs had RFLP loci in common with two B. napus linkage groups that were shown previously to contain QTLs for flowering time. An RFLP locus detected by the cold-induced gene COR6.6 cloned from Arabidopsis thaliana mapped very near to one of the B. rapa QTLs for flowering time.     

Stimulation of in vitro root and shoot growth of potato by increasing sucrose concentration in the presence of fluridone,an inhibitor of abscisic acid synthesis

Fluridone, an inhibitor of abscisic acid (ABA) biosynthesis, strongly stimulated rooting of nodal stem segments of potato (Solanum tuberosum L.) cultivar Arran Banner cultured in darkness on tuberisation medium. Inclusion of 10^-6 M ABA in the culture medium prevented this rooting response, indicating that root proliferation in the presence of fluridone could be due to inhibition of ABA synthesis. The rooting response to fluridone (increased total root number and root fresh weight) was obtained only at high sucrose concentrations (0.175 and 0.234 M) and was demonstrated with two potato cultivars and two culture media; one which favoured tuberisation and one which did not. Shoot numbers were also increased, but to a lesser extent than root numbers, and total fresh weight of plant material per culture was greatly increased by inclusion of both fluridone (10^-6 or 10^-5 M) and 0.234 M sucrose in the culture medium. The role of sucrose was not simply osmotic because when the osmolarity of fluridone medium was increased using mixtures of mannitol and sucrose, no root proliferation occurred unless sucrose predominated in the mixture.     

Genes for dwarfing and photoperiod flowering response inPharbitis nil choisy

Dwarfing and sensitivity to the duration of a single inductive dark period for flowering ofPharbitis nil in F₂ progeny of a cross between the tall strain Tendan, and the dwarf, Kidachi appear to be controlled by the alleles at two independent loci. Progeny of a similar cross between the tall strain Violet and the dwarf Kidachi at F₂ and F₃ also showed single locus segregation for tall: dwarf plants. In this cross, differences in photoperiodic response could be identified in F₃ families but they were not simply inherited. There was some evidence of difficulties with classification of the F₂ plants, but also, the flowering of the F₁ between the two less sensitive strains Tendan and Violet indicated complex inheritance of their photoperiodic response. Complementary dominant alleles at three independent loci may be necessary for flowering in even shorter dark periods with the sensitive strain Kidachi. The dwarf strain Kidachi has a reduced gibberellin (GA) content (Barendse and Lang 1972), it flowers in a short dark period without terminal flowering, and it responds positively to GA application both for flowering and growth. However, since control of dwarfing and photoperiodic sensitivity can be separated genetically, there is no strick link between the gibberellin responsiveness of Kidachi for its growth and flowering. Despite the complexity of flowering genetics in Violet×Kidachi, a short-dark-period-sensitive, terminal flowering and tall F₇ line was obtained in a pedigree previously held heterozygous for the dwarf: tall character but not selected for flowering time. Thus, flowering in a short dark period can also be obtained in the presence of the non-dwarfing allele from strain Violet, again demonstrating genetic independence.     

LOV KELCH PROTEIN2 and ZEITLUPE repress Arabidopsis photoperiodic flowering under non-inductive conditions, dependent on FLAVIN-BINDING KELCH REPEAT F-BOX1

LOV KELCH PROTEIN2 (LKP2), ZEITLUPE (ZTL)/LOV KELCH PROTEIN1 (LKP1) and FLAVIN‐BINDING KELCH REPEAT F‐BOX1 (FKF1) constitute a family of Arabidopsis F‐box proteins that regulate the circadian clock. Over‐expression of LKP2 or ZTL causes arrhythmicity of multiple clock outputs under constant light and in constant darkness. Here, we show the significance of LKP2 and ZTL in the photoperiodic control of flowering time in Arabidopsis. In plants over‐expressing LKP2, CO and FT expression was down‐regulated under long‐day conditions. LKP2 and ZTL physically interacted with FKF1, which was recruited from the nucleus into cytosolic speckles. LKP2 and ZTL inhibited the interaction of FKF1 with CYCLING DOF FACTOR 1, a ubiquitination substrate for FKF1 that is localized in the nucleus. The Kelch repeat regions of LKP2 and ZTL were sufficient for their physical interaction with FKF1 and translocation of FKF1 to the cytoplasm. Over‐expression of LKP2 Kelch repeats induced late flowering under long‐day conditions. lkp2 ztl double mutant plants flowered earlier than wild‐type plants under short‐day (non‐inductive) conditions, and both CO and FT expression levels were up‐regulated in the double mutant plants. The early flowering of lkp2 ztl was dependent on FKF1. LKP2, ZTL or both affected the accumulation of FKF1 protein during the early light period. These results indicate that an important role of LKP2 and ZTL in the photoperiodic pathway is repression of flowering under non‐inductive conditions, and this is dependent on FKF1.     

Changes in mRNA level rhythmicity in the leaves ofSinapis alba during a lengthening of the photoperiod which induces flowering

A previous study has shown that mRNAs exhibit complex patterns of diurnal rhythms in their quantity in the leaves ofSinapis alba during an 8 h light/16 h dark short day (SD). In order to determine whether this situation is rapidly modified in plants subjected to an extended light treatment, we have usedin vitro translation and two-dimensional polyacrylamide gel electrophoresis, together with a strict gel comparison procedure giving aP=0.03 certitude level, to analyse the mRNA complement at different times during a 22 h light/2 h dark long day (LD).During this LD, complex changes affected about 10% of the mRNAs. Thirty-four different patterns were observed. Some diurnal rhythms present in SD are not modified by the lengthening of the light period, but most are affected. Moreover, we have shown that some mRNAs presenting a constant quantity under a SD regime show an increase or a decrease during the first hours of the photoperiod lengthening.InSinapis, this LD also induces flowering. All the changes in mRNA quantity detected thus parallel the photoperiodic induction of flowering in the leaves and are quantitative; no mRNA was shown to appear or to disappear.     

Photoperiodic flowering occurs under internal and external coincidence

Determining the proper time to flower is important to ensure the reproductive success of plants. The model plant Arabidopsis is able to measure day-length and promotes flowering in long day (LD) conditions. One of the most prominent mechanisms in photoperiodic flowering is the clock-regulated gene expression of CONSTANS (CO) and the stabilization and activation of CO protein by light (regarded as external coincidence). We recently demonstrated that timing of the blue-light dependent formation of FLAVIN-BINDING, KELCH REPEAT, F-BOX 1 (FKF1) and GIGANTEA (GI) protein complex is crucial for regulating the timing of CO gene expression. The expression of FKF1 and GI is clock regulated, and their expression patterns have the same phase in LD (regarded as internal coincidence) but not in short day (SD) conditions, where floral induction is greatly delayed. Hence, timing of the FKF1-GI complex formation is regulated by the coincidence of both external and internal cues. Here, we propose a molecular mechanism for CO regulation by FKF1-GI complex formation.Key words: Arabidopsis, circadian clock, photoperiodic flowering, CONSTANS, GIGANTEA, FKF1, CDF1