NO flowering

The complex control of flowering time ensures that plants flower in conditions favourable for reproductive success. A recent study adds another dimension to this established complexity by revealing that nitric oxide (NO) represses flowering in Arabidopsis. The analysis of recently identified mutants that either overproduce or are compromised in endogenous NO production has identified NO-sensitive features of the circuitry of flowering time control: NO acts to repress the amplification of gene expression dependent on the circadian clock and promotes the accumulation of mRNA encoding a key repressor of flowering, FLC.     

Stress-induced flowering

Many plant species can be induced to flower by responding to stress factors. The short-day plants Pharbitis nil and Perilla frutescens var. crispa flower under long days in response to the stress of poor nutrition or low-intensity light. Grafting experiments using two varieties of P. nil revealed that a transmissible flowering stimulus is involved in stress-induced flowering. The P. nil and P. frutescens plants that were induced to flower by stress reached anthesis, fruited and produced seeds. These seeds germinated, and the progeny of the stressed plants developed normally. Phenylalanine ammonialyase inhibitors inhibited this stress-induced flowering, and the inhibition was overcome by salicylic acid (SA), suggesting that there is an involvement of SA in stress-induced flowering. PnFT2, a P. nil ortholog of the flowering gene FLOWERING LOCUS T (FT) of Arabidopsis thaliana, was expressed when the P. nil plants were induced to flower under poor-nutrition stress conditions, but expression of PnFT1, another ortholog of FT, was not induced, suggesting that PnFT2 is involved in stress-induced flowering.Key words: flowering, stress, phenylalanine ammonia-lyase, salicylic acid, FLOWERING LOCUS T, Pharbitis nil, Perilla frutescensFlowering in many plant species is regulated by environmental factors, such as night-length in photoperiodic flowering and temperature in vernalization. On the other hand, a short-day (SD) plant such as Pharbitis nil (synonym Ipomoea nil) can be induced to flower under long days (LD) when grown under poor-nutrition, low-temperature or high-intensity light conditions.^1–9 The flowering induced by these conditions is accompanied by an increase in phenylalanine ammonia-lyase (PAL) activity.¹⁰ Taken together, these facts suggest that the flowering induced by these conditions might be regulated by a common mechanism. Poor nutrition, low temperature and high-intensity light can be regarded as stress factors, and PAL activity increases under these stress conditions.¹¹ Accordingly, we assumed that such LD flowering in P. nil might be induced by stress. Non-photoperiodic flowering has also been sporadically reported in several plant species other than P. nil, and a review of these studies suggested that most of the factors responsible for flowering could be regarded as stress. Some examples of these factors are summarized in 12^–14

Table 1

Some cases of stress-induced flowering

Reversion of flowering

Reversion from floral to vegetative growth is under environmental control in many plant species. However the factors regulating floral reversion, and the events at the shoot apex that take place when it occurs, have received less attention than those associated with the transition to flowering. Reversions may be categorized as flower reversion, in which the flower meristem resumes leaf production, or inflorescence reversions, in which the meristem ceases to initiate bracts with flowers in their axils and begins instead to make leaves with vegetative branches in their axils. Related to these two types of reversion, but distinct from them, are examples of partial flowering, when non-floral meristems grow out so that the plant begins to grow vegetatively again. Anomalous or proliferous flowers may form as a result of unfavourable growth conditions or viral infection, but these do not necessarily involve flower reversions.     

A stochastic flowering model describing an asynchronically flowering set of trees

A general stochastic model is presented that simulates the time course of flowering of individual trees and populations, integrating the synchronization of flowering both between and within trees. Making some hypotheses, a simplified expression of the model, called the 'shoot' model, is proposed, in which the synchronization of flowering both between and within trees is characterized by specific parameters. Two derived models, the 'tree' model and the 'population' model, are presented. They neglect the asynchrony of flowering, respectively, within trees, and between and within trees. Models were fitted and tested using data on flowering of Psidium cattleianum observed at study sites at elevations of 200, 520 and 890 m in Reunion Island. The 'shoot' model fitted the data best and reproduced the strong irregularities in flowering shown by empirical data. The asynchrony of flowering in P. cattleianum was more pronounced within than between trees. Simulations showed that various flowering patterns can be reproduced by the 'shoot' model. The use of different levels of organization of the general model is discussed.     

Climate, size and flowering history determine flowering pattern of an orchid

The flowering pattern of plant species, including orchid species, may fluctuate irregularly. Several explanations are given in the literature to explain that pattern, including: costs associated with reproduction, herbivory effects, intrinsically triggered unpredictable variation of the system, and external conditions (i.e. weather). The influence of age is discussed, but is difficult to determine because relevant long-term field observations are generally absent in the literature. The influence of age, size, reproductive effort and climatic conditions on flowering variability of Himantoglossum hircinum are examined using data collected in a long-term project (1976–2001) in Germany. PCA and multiple regression analysis were used to analyse variability in flowering pattern over the years as a function of size and weather variability. We studied future size after flowering to quantify costs of reproduction. Flowering probability was strongly determined by plant size, while there was no significant influence of age class on flowering probability of the population. Costs associated with reproduction resulted in a decrease in plant size, causing reduced flowering probability of the plants in the following year. The weather explained about 50% of the yearly variation in the proportion of large plants and thus had an indirect, strong influence on the flowering percentage. We conclude that variability in flowering is caused mainly by the variability of weather conditions in the previous and current year, whereby reproductive effort causes further variability in flowering at the individual and, consequently, the population levels.  © 2006 The Linnean Society of London, Botanical Journal of the Linnean Society , 2006, 151 , 511–526.     

Self-incompatibility in flowering plants

Self-pollination in some groups of plants is prevented by a sophisticated biochemical signalling system. The molecule active in the female emerges as a highly charged glycoprotein, but the identity of the male determinant remains unknown. Studies of both the molecular biology and the physiology of the interaction suggest that the female polypeptide belongs to a family of glycoproteins which may play an additional, and more general, role in pollination. Pollen compatibility is controlled by one of two genetic systems and new information indicates a mechanism by which they may have arisen, together with the different stigma types with which they are correlated.     

Chromatin regulation of flowering

The transition to flowering is a major developmental switch in the life cycle of plants. In Arabidopsis (Arabidopsis thaliana), chromatin mechanisms play critical roles in flowering-time regulation through the expression control of key flowering-regulatory genes. Various conserved chromatin modifiers, plant-specific factors, and long noncoding RNAs are involved in chromatin regulation of FLOWERING LOCUS C (FLC, a potent floral repressor). The well-studied FLC regulation has provided a paradigm for chromatin-based control of other developmental genes. In addition, chromatin modification plays an important role in the regulation of FLOWERING LOCUS T (FT, encoding florigen), which is widely conserved in angiosperm species. The chromatin mechanisms underlying FT regulation in Arabidopsis are likely involved in the regulation of FT relatives and, therefore, flowering-time control in other plants.     

Vernalization and flowering time

Vernalization is the process by which flowering is promoted by prolonged exposure to the cold of a typical winter. In certain plant species, the role of vernalization is to suppress the expression of genes that encode repressors of flowering. In Arabidopsis, this suppression is an epigenetic phenomenon in the sense that it is mitotically stable in the spring after the inducing signal, cold, is no longer perceived. This epigenetic silencing results from the modification of the chromatin of flowering repressors.     

Control of flowering time

The multiple promotive and repressive pathways controlling flowering have been further defined by analysis of genetic interactions and the activation of floral meristem identity genes. Cloning of additional genes in these pathways has uncovered some of the molecular processes that control the timing of the transition to reproductive development.     

The flowering of interferon

A microsomal vesicle fraction was prepared from rat liver homogenate by centrifugation in gradients of Percoll. The microsomes were subjected to gel filtration on Sephacryl S-1000 Superfine, which resolved the microsomes from Percoll. The elution pattern of the microsomal marker enzyme NADPH-cytochrome c reductase showed that the main part of the enzyme was present in a peak at Kav about 0.1, while Percoll eluted in a broad peak at Kav about 0.7. The total yield of eluted enzyme activity was 85%. The gel filtration had to be carried out in the presence of 10 mM tris or NaCl. At lower ionic strength or in 0.25 M sucrose alone, anomalous behaviour of the Percoll particles and microsomes on the gel was observed. Electron microscopy of samples from the void volume fraction of the Sephacryl S-1000 Superfine column showed an almost complete removal of Percoll from the microsomes. Furthermore, the vesicle preparation was essentially free of membrane fragments.     

Environmental regulation of flowering

The timing of flower initiation is a highly plastic developmental process. To achieve reproductive success, plants must select the most favourable season to initiate reproductive development; this in turn requires continuous monitoring of environmental factors and a properly response. Environmental factors which change in a predictable fashion along the year, such as light and temperature, are the most relevant in terms of selection of the flowering season. In Arabidopsis and more recently in a few other species, molecular genetic analyses are providing a way to identify the genes involved in the regulation of flowering time. From gene sequences it is possible to develop hypotheses regarding molecular function and to infer some of the molecular mechanisms involved in the environmental regulation of flowering time. In this paper, we summarize recent discoveries concerning the mechanisms which plants use to perceive and respond to major environmental factors (light and temperature) and their different components. We focus mainly on annual plants and especially on Arabidopsis because most of the available molecular and functional data come from this species. However, additional information arising from other plant systems is also considered.     

Epigenetic regulation of flowering

The acceleration of flowering by prolonged low temperature treatment (vernalization) has unique properties including the floral transition occurring at a time separate from the vernalization treatment. This implies the vernalization condition is inherited through mitotic divisions, but this vernalized state is not inherited from one generation to the next. FLC, the key gene mediating this response in the Arabidopsis is repressed by histone modifications involving the VRN2 protein complex. Other protein complexes participate in activating the gene. While many plant species depend on vernalization for optimising flowering time, the genes involved differ between dicot and monocot plants in both Arabidopsis and cereals, vernalization regulates photoperiod control of flowering by preventing the induction of the floral promoter FT by long days in autumn but allowing induction of FT in spring and hence flowering occurs at an optimal time in the annual life cycle.