Integration of photoperiod and cold temperature signals into flowering genetic pathways in Arabidopsis

Appropriate timing of flowering is critical for propagation and reproductive success in plants. Therefore, flowering time is coordinately regulated by endogenous developmental programs and external signals, such as changes in photoperiod and temperature. Flowering is delayed by a transient shift to cold temperatures that frequently occurs during early spring in the temperate zones. It is known that the delayed flowering by short-term cold stress is mediated primarily by the floral repressor FLOWERING LOCUS C (FLC). However, how the FLC-mediated cold signals are integrated into flowering genetic pathways is not fully understood. We have recently reported that the INDUCER OF CBF EXPRESSION 1 (ICE1), which is a master regulator of cold responses, FLC, and the floral integrator SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) constitute an elaborated feedforward-feedback loop that integrates photoperiod and cold temperature signals to regulate seasonal flowering in Arabidopsis. Cold temperatures promote the binding of ICE1 to FLC promoter to induce its expression, resulting in delayed flowering. However, under floral inductive conditions, SOC1 induces flowering by blocking the ICE1 activity. We propose that the ICE1-FLC-SOC1 signaling network fine-tunes the timing of photoperiodic flowering during changing seasons.     

Photoperiod and gibberellic acid – control of the cell cycle in the meristem of Silene armeria and its effects on flowering

Plants of Silene armeria L., strain S_2.1, a quantitative long-day (LD) species which is known to react to GA₃ by flowering after attaining, the'intermediate stage', were induced by two LD or by two GA₃ applications. Changes in the mitotic index and DNA content (microdensitometric estimation) of cells in the axial zone, lateral zone and rib meristem of the shoot apex were observed during the first 48 h of each treatment. Similar mitotic activation occurred in response to LD or GA₃ after a 6-8 h lag period. This was preceded by a decrease in the proportion of nuclei with a 2C DNA content, indicating that in this species the control point for the shortening of the cell cycle was essentially in G₁. A second mitotic peak was observed 16 h later in photoinduced meristems, resulting in more pronounced cellular synchronization. These further events were not seen in GA₃-treated plants where only the meristematic activity was slightly, but reproducibly higher than in the control. Thus, two successive synchronizations of cell division are a typical feature of LD induction. The data are discussed with regard to the competence of shoot apical cells to be reactivated. The essential changes for the transition to flowering depend on these differential patterns of cell reactivation.     

A MADS domain gene involved in the transition to flowering in Arabidopsis

Flowering time in many plants is triggered by environmental factors that lead to uniform flowering in plant populations, ensuring higher reproductive success. So far, several genes have been identified that are involved in flowering time control. AGL20 (AGAMOUS LIKE 20) is a MADS domain gene from Arabidopsis that is activated in shoot apical meristems during the transition to flowering. By transposon tagging we have identified late flowering agl20 mutants, showing that AGL20 is involved in flowering time control. In previously described late flowering mutants of the long-day and constitutive pathways of floral induction the expression of AGL20 is down-regulated, demonstrating that AGL20 acts downstream to the mutated genes. Moreover, we can show that AGL20 is also regulated by the gibberellin (GA) pathway, indicating that AGL20 integrates signals of different pathways of floral induction and might be a central component for the induction of flowering. In addition, the constitutive expression of AGL20 in Arabidopsis is sufficient for photoperiod independent flowering and the over-expression of the orthologous gene from mustard, MADSA, in the classical short-day tobacco Maryland Mammoth bypasses the strict photoperiodic control of flowering.     

INDUCER OF CBF EXPRESSION 1 integrates cold signals into FLOWERING LOCUS C‐mediated flowering pathways in Arabidopsis

Plants constantly monitor changes in photoperiod and temperature throughout the year to synchronize flowering with optimal environmental conditions. In the temperate zones, both photoperiod and temperature fluctuate in a somewhat predictable manner through the seasons, although a transient shift to low temperature is also encountered during changing seasons, such as early spring. Although low temperatures are known to delay flowering by inducing the floral repressor FLOWERING LOCUS C (FLC), it is not fully understood how temperature signals are coordinated with photoperiodic signals in the timing of seasonal flowering. Here, we show that the cold signaling activator INDUCER OF CBF EXPRESSION 1 (ICE1), FLC and the floral promoter SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) constitute an elaborate signaling network that integrates cold signals into flowering pathways. The cold‐activated ICE1 directly induces the gene encoding FLC, which represses SOC1 expression, resulting in delayed flowering. In contrast, under floral promotive conditions, SOC1 inhibits the binding of ICE1 to the promoters of the FLC gene, inducing flowering with a reduction of freezing tolerance. These observations indicate that the ICE1‐FLC‐SOC1 signaling network contributes to the fine‐tuning of flowering during changing seasons.     

Natural variation in GmGBP1 promoter affects photoperiod control of flowering time and maturity in soybean

The present study screened for polymorphisms in coding and non‐coding regions of the GmGBP1 gene in 278 soybean accessions with variable maturity and growth habit characteristics under natural field conditions in three different latitudes in China. The results showed that the promoter region was highly diversified compared with the coding sequence of GmGBP1. Five polymorphisms and four haplotypes were closely related to soybean flowering time and maturity through association and linkage disequilibrium analyses. Varieties with the polymorphisms SNP_‐796G, SNP_‐770G, SNP_‐307T, InDel_‐242normal, SNP_353A, or haplotypes Hap‐3 and Hap‐4 showed earlier flowering time and maturity in different environments. The shorter growth period might be largely due to higher GmGBP1 expression levels in soybean that were caused by the TCT‐motif with SNP_‐796G in the promoter. In contrast, the lower expression level of GmGBP1 in soybean caused by RNAi interference of GmGBP1 resulted in a longer growth period under different day lengths. Furthermore, the gene interference of GmGBP1 also caused a reduction in photoperiod response sensitivity (PRS) before flowering in soybean. RNA‐seq analysis on GmGBP1 underexpression in soybean showed that 94 and 30 predicted genes were significantly upregulated and downregulated, respectively. Of these, the diurnal photoperiod‐specific expression pattern of three significant flowering time genes GmFT2a, GmFT5a, and GmFULc also showed constantly lower mRNA levels in GmGBP1‐i soybean than in wild type, especially under short day conditions. Together, the results showed that GmGBP1 functioned as a positive regulator upstream of GmFT2a and GmFT5a to activate the expression of GmFULc to promote flowering on short days.     

Mutations in the Arabidopsis SWC6 gene, encoding a component of the SWR1 chromatin remodelling complex, accelerate flowering time and alter leaf and flower development

Mutations affecting the Arabidopsis SWC6 gene encoding a putativeorthologue of a component of the SWR1 chromatin remodellingcomplex in plants have been characterized. swc6 mutations causeearly flowering, shortened inflorescence internodes, and alteredleaf and flower development. These phenotypic defects resemblethose of the photoperiod independent early flowering 1 (pie1)and early in short days 1 (esd1) mutants, also affected in homologuesof the SWR1 complex subunits. SWC6 is a ubiquitously expressednuclear HIT-Zn finger-containing protein, with the highest levelsfound in pollen. Double mutant analyses suggest that swc6 abolishesthe FLC-mediated late-flowering phenotype of plants carryingactive alleles of FRI and of mutants of the autonomous pathway.It was found that SWC6 is required for the expression of theFLC repressor to levels that inhibit flowering. However, theeffect of swc6 in an flc null background and the down-regulationof other FLC-like/MAF genes in swc6 mutants suggest that floweringinhibition mediated by SWC6 occurs through both FLC- and FLC-likegene-dependent pathways. Both genetic and physical interactionsbetween SWC6 and ESD1 have been demonstrated, suggesting thatboth proteins act in the same complex. Using chromatin immunoprecipitation,it has been determined that SWC6, as previously shown for ESD1,is required for both histone H3 acetylation and H3K4 trimethylationof the FLC chromatin. Altogether, these results suggest thatSWC6 and ESD1 are part of an Arabidopsis SWR1 chromatin remodellingcomplex involved in the regulation of diverse aspects of plantdevelopment, including floral repression through the activationof FLC and FLC-like genes. Key words: Arabidopsis, chromatin remodelling, floral repression, HIT-Zn finger, phase transition, SWR1 complex     

Salicylic acid regulates flowering time and links defence responses and reproductive development

Flowering relies on signaling networks that integrate endogenous and external cues. Normally, plants flower at a particular season, reflecting day length and/or temperature cues. However, plants can surpass this seasonal regulation and show precocious flowering under stress environmental conditions. Here, we show that UV-C light stress activates the transition to flowering in Arabidopsis thaliana through salicylic acid (SA). Moreover, SA also regulates flowering time in non-stressed plants, as SA-deficient plants are late flowering. The regulation of flowering time by SA seems to involve the photoperiod and autonomous pathways, but it does not require the function of the flowering time genes CONSTANS (CO), FCA, or FLOWERING LOCUS C (FLC).     

Dwarf pea response to gibberellic acid applied to soil through a drip irrigation system,and gibberellic acid biodegradation in soil

Gibberellic acid (29 or 290 M) injected into drip irrigation lines significantly stimulated internode elongation of dwarf peas, and the 290-M soil treatment produced significantly taller plants than did the 29-M treatment. GA₃ uptake may limit GA-induced internode elongation when GA₃ is applied to soil, in contrast to results obtained for hydroponically grown plants, where uptake initially appeared to exceed the rate of hormone metabolism (andersonet al.). It is likely that biodegradation or chemical inactivation limited the plant-availability of GA₃ in the soil. Degradation of moderate GA₃ concentrations in a moist, aerobic loamy fine sand was nearly complete within five days, indicating that the inefficiency of soil applications may outweight the benefits provided by reducing labor costs associated with foliar-spray applications.     

Reciprocal control of flowering time by OsSOC1 in transgenic Arabidopsis and by FLC in transgenic rice

In a screen for MADS box genes which activate and/or repress flowering in rice, we identified a gene encoding a MADS domain protein (OsSOC1) related to the Arabidopsis gene AtSOC1. AtSOC1 and OsSOC1 show a 97% amino acid similarity in their MADS domain. The rice gene contains a large first intron of 27.6 kb compared to the 1 kb intron in Arabidopsis. OsSOC1 is located on top of the short arm of chromosome 3, tightly linked to the heading date locus, Hd9. OsSOC1 is expressed in vegetative tissues, and expression is elevated at the time of floral initiation, 40-50 days after sowing, and remains uniformly high thereafter, similar to the expression pattern of AtSOC1. The constitutive expression of OsSOC1 in Arabidopsis results in early flowering, suggesting that the rice gene is a functional equivalent of AtSOC1. We were not able to identify FLC-like sequences in the rice genome; however, we show that ectopic expression of the Arabidopsis FLC delays flowering in rice, and the up-regulation of OsSOC1 at the onset of flowering initiation is delayed in the AtFLC transgenic lines. The reciprocal recognition and flowering time effects of genes introduced into either Arabidopsis or rice suggest that some components of the flowering pathways may be shared. This points to a potential application in the manipulation of flowering time in cereals using well characterized Arabidopsis genes.