GIGANTEA enables drought escape response via abscisic acid-dependent activation of the florigens and SUPPRESSOR OF OVEREXPRESSION OF CONSTANS

Toward a Molecular Understanding of Abscisic Acid Actions in Floral Transition

The transition from the vegetative growth phase to flowering is a crucial checkpoint for plant reproduction and survival, especially under environmental stress conditions. Numerous factors regulate flowering time, including exogenous environmental cues such as day length and temperature, as well as salt and drought stresses, and endogenous phytohormone signaling cascades. Gibberellins and ABA are one classic combination of phytohormones which antagonistically regulate several biological processes, including seed dormancy and germination, primary root growth and seedling development. As regards control of flowering time, gibberellin exhibits a positive role, and represents an important pathway in the regulation of floral transition. However, over the past decades, numerous investigations have demonstrated that the contribution of the stress hormone ABA to floral transition is still controversial, as both positive and negative effects have been documented. It is important to determine why and how ABA shows this contradictory effect on flowering time. In this up to date review, primarily based on recent publications and emerging data, we summarize the distinct and contrasting roles of ABA on floral transition, while the detailed molecular mechanisms underlying these roles are discussed. Finally, the remaining challenges and open questions in this topic are presented.

No Time to Waste: Transcriptome Study Reveals that Drought Tolerance in Barley May Be Attributed to Stressed-Like Expression Patterns that Exist before the Occurrence of Stress

Plant survival in adverse environmental conditions requires a substantial change in the metabolism, which is reflected by the extensive transcriptome rebuilding upon the occurrence of the stress. Therefore, transcriptomic studies offer an insight into the mechanisms of plant stress responses. Here, we present the results of global gene expression profiling of roots and leaves of two barley genotypes with contrasting ability to cope with drought stress. Our analysis suggests that drought tolerance results from a certain level of transcription of stress-influenced genes that is present even before the onset of drought. Genes that predispose the plant to better drought survival play a role in the regulatory network of gene expression, including several transcription factors, translation regulators and structural components of ribosomes. An important group of genes is involved in signaling mechanisms, with significant contribution of hormone signaling pathways and an interplay between ABA, auxin, ethylene and brassinosteroid homeostasis. Signal transduction in a drought tolerant genotype may be more efficient through the expression of genes required for environmental sensing that are active already during normal water availability and are related to actin filaments and LIM domain proteins, which may function as osmotic biosensors. Better survival of drought may also be attributed to more effective processes of energy generation and more efficient chloroplasts biogenesis. Interestingly, our data suggest that several genes involved in a photosynthesis process are required for the establishment of effective drought response not only in leaves, but also in roots of barley. Thus, we propose a hypothesis that root plastids may turn into the anti-oxidative centers protecting root macromolecules from oxidative damage during drought stress. Specific genes and their potential role in building up a drought-tolerant barley phenotype is extensively discussed with special emphasis on processes that take place in barley roots. When possible, the interconnections between particular factors are emphasized to draw a broader picture of the molecular mechanisms of drought tolerance in barley.

Transcription Factor WRKY75 Interacts with DELLA Proteins to Affect Flowering

Flowering time is tightly controlled by both endogenous and exogenous signals. Although several lines of evidence have suggested the involvement of WRKY transcription factors in floral initiation, the underlying mechanisms and signaling pathways involved remain elusive. Here, we newly identified Arabidopsis (Arabidopsis thaliana) WRKY DNA binding protein75 (WRKY75) as a positive regulator of flowering initiation. Mutation of WRKY75 resulted in a delay in flowering, whereas overexpression of WRKY75 significantly accelerated flowering in Arabidopsis. Gene expression analysis showed that the transcript abundance of the flowering time integrator gene FLOWERING LOCUS T (FT) was lower in wrky75 mutants than in the wild type, but greater in WRKY75-overexpressing plants. Chromatin immunoprecipitation assays revealed that WRKY75 directly binds to the promoter of FT Both in vivo and in vitro biochemical analyses demonstrated that WRKY75 interacts with DELLA proteins. We found that both REPRESSOR OF ga1-3 (RGA) RGA-LIKE1 (RGL1) and GA INSENSITIVE (GAI) can repress the activation ability of WRKY75, thereby attenuating expression of its regulon. Genetic analyses indicated that WRKY75 positively regulates flowering in a FT-dependent manner and overexpression of RGL1 or gain-of-function of GAI could partially rescue the early flowering phenotype of WRKY75-overexpressing plants. Taken together, our results demonstrate that WRKY75 may function as a new component of the GA-mediated signaling pathway to positively regulate flowering in Arabidopsis.

Abscisic Acid and Gibberellins Antagonistically Mediate Plant Development and Abiotic Stress Responses

Phytohormones regulate numerous important biological processes in plant development and biotic/abiotic stress response cascades. More than 50 and 100 years have passed since the initial discoveries of the phytohormones abscisic acid (ABA) and gibberellins (GA), respectively. Over the past several decades, numerous elegant studies have demonstrated that ABA and GA antagonistically regulate many plant developmental processes, including seed maturation, seed dormancy and germination, root initiation, hypocotyl and stem elongation, and floral transition. Furthermore, as a well-established stress hormone, ABA plays a key role in plant responses to abiotic stresses, such as drought, flooding, salinity and low temperature. Interestingly, recent evidence revealed that GA are also involved in plant response to adverse environmental conditions. Consequently, the complex crosstalk networks between ABA and GA, mediated by diverse key regulators, have been extensively investigated and documented. In this updated mini-review, we summarize the most recent advances in our understanding of the antagonistically regulatory roles of ABA and GA in different stages of plant development and in various plant-environment interactions, focusing on the crosstalk between ABA and GA at the levels of phytohormone metabolism and signal transduction.

Antisense transcription represses Arabidopsis seed dormancy QTL DOG1 to regulate drought tolerance

Plants have developed multiple strategies to sense the external environment and to adapt growth accordingly. Delay of germination 1 (DOG1) is a major quantitative trait locus (QTL) for seed dormancy strength in Arabidopsis thaliana that is reported to be expressed exclusively in seeds. DOG1 is extensively regulated, with an antisense transcript (asDOG1) suppressing its expression in seeds. Here, we show that asDOG1 shows high levels in mature plants where it suppresses DOG1 expression under standard growth conditions. Suppression is released by shutting down antisense transcription, which is induced by the plant hormone abscisic acid (ABA) and drought. Loss of asDOG1 results in constitutive high-level DOG1 expression, conferring increased drought tolerance, while inactivation of DOG1 causes enhanced drought sensitivity. The unexpected role of DOG1 in environmental adaptation of mature plants is separate from its function in seed dormancy regulation. The requirement of asDOG1 to respond to ABA and drought demonstrates that antisense transcription is important for sensing and responding to environmental changes in plants.

Fine-tuning timing: natural variation informs the mechanistic basis of the switch to flowering in Arabidopsis thaliana

The evolution of diverse life history strategies has allowed Arabidopsis thaliana to adapt to worldwide locations, spanning a range of latitudinal and environmental conditions. Arabidopsis thaliana accessions are either vernalization-requiring winter annuals or rapid cyclers, with extensive natural variation in vernalization requirement and response. Genetic and molecular analysis of this variation has enhanced our understanding of the mechanisms involved in life history determination, with translation to both natural and crop systems in the Brassicaceae and beyond.

Early Flowering as a Drought Escape Mechanism in Plants: How Can It Aid Wheat Production?

Drought escape (DE) is a classical adaptive mechanism which involves rapid plant development to enable the completion of the full life-cycle prior to a coming drought event. This strategy is widely used in populations of native plants, and is also applicable to cereal crops such as wheat. Early flowering time and a shorter vegetative phase can be very important for wheat production in conditions of terminal drought since this can minimize exposure to dehydration during the sensitive flowering and post-anthesis grain filling periods. A gradual shift toward early flowering has been observed over the last century of wheat breeding in countries with a Mediterranean-type climate and frequent terminal drought. This trend is predicted to continue for wheat production in the coming years in response to global climate warming. The advantage of early flowering wheat is apparent under conditions of impending terminal drought, and modern varieties are significantly more productive due to minimization of the risk associated with drought stress. Under favorable conditions, a short vegetative phase can result in reduced plant biomass due to the reduction in time available for photosynthetic production and seed nutrient accumulation. However, high yield potential has been reported for the development of both shallow and deep roots, representing plasticity in response to drought in combination with the early flowering trait. Wheat productivity can be high both in well-watered and drought-affected field trials, where an efficient strategy of DE was associated with quick growth, yield potential and water use efficiency. Therefore, early flowering provides a promising strategy for the production of advanced drought-adapted wheat cultivars.

Long photoperiod affects the maize transition from vegetative to reproductive stages: a proteomic comparison between photoperiod-sensitive inbred line and its recurrent parent

Maize (Zea mays L.) is a typical short-day plant that is produced as an important food product and industrial material. The photoperiod is one of the most important evolutionary mechanisms enabling the adaptation of plant developmental phases to changes in climate conditions. There are differences in the photoperiod sensitivity of maize inbred lines from tropical to temperate regions. In this study, to identify the maize proteins responsive to a long photoperiod (LP), the photoperiod-insensitive inbred line HZ4 and its near-isogenic line H496, which is sensitive to LP conditions, were analyzed under long-day conditions using isobaric tags for relative and absolute quantitation. We identified 5259 proteins in maize leaves exposed to the LP condition between the vegetative and reproductive stages. These proteins included 579 and 576 differentially accumulated proteins in H496 and HZ4 leaves, respectively. The differentially accumulated proteins (e.g., membrane, defense, and energy- and ribosome-related proteins) exhibited the opposite trends in HZ4 and H496 plants during the transition from the vegetative stage to the reproductive stage. These results suggest that the photoperiod-associated fragment in H496 plants considerably influences various proteins to respond to the photoperiod sensitivity. Overall, our data provide new insights into the effects of long-day treatments on the maize proteome, and may be useful for the development of new germplasm.

Cloning and characterization of a novel GIGANTEA gene in sweet potato

The transition from vegetative to reproductive growth, a key event in the lifecycle of a plant, is affected by environmental stresses. The flowering-time regulator GIGANTEA (GI) may be contributing to susceptibility of the regulation of photoperiodic flowering, circadian rhythm control, and abiotic stress resistance in Arabidopsis. However, the role of GI in sweet potato remains unknown. Here, we isolated and characterized a GI gene (IbGI) from sweet potato (Ipomoea batatas [L.] Lam). The IbGI cDNA sequence was isolated based on information from a sweet potato transcriptome database. IbGI mRNA transcript levels showed robust circadian rhythm control during the light-dark transition, and the expression of IbGI was stronger in leaves and roots than in stems. IbGI protein is predominantly localized to the nucleus. IbGI expression was upregulated by high temperature, drought, and salt stress but downregulated by cold stress. Overexpressing IbGI in the Arabidopsis gi-2 mutant background rescued its late flowering phenotype and reduced its salt tolerance. Taken together, these results indicate that IbGI shares functions in regulating flowering, the circadian rhythm, and tolerance to some stresses with other GI orthologs.

TCP4-dependent induction of CONSTANS transcription requires GIGANTEA in photoperiodic flowering in Arabidopsis

Photoperiod is one of the most reliable environmental cues for plants to regulate flowering timing. In Arabidopsis thaliana, CONSTANS (CO) transcription factor plays a central role in regulating photoperiodic flowering. In contrast to posttranslational regulation of CO protein, still little was known about CO transcriptional regulation. Here we show that the CINCINNATA (CIN) clade of class II TEOSINTE BRANCHED 1/ CYCLOIDEA/ PROLIFERATING CELL NUCLEAR ANTIGEN FACTOR (TCP) proteins act as CO activators. Our yeast one-hybrid analysis revealed that class II CIN-TCPs, including TCP4, bind to the CO promoter. TCP4 induces CO expression around dusk by directly associating with the CO promoter in vivo. In addition, TCP4 binds to another flowering regulator, GIGANTEA (GI), in the nucleus, and induces CO expression in a GI-dependent manner. The physical association of TCP4 with the CO promoter was reduced in the gi mutant, suggesting that GI may enhance the DNA-binding ability of TCP4. Our tandem affinity purification coupled with mass spectrometry (TAP-MS) analysis identified all class II CIN-TCPs as the components of the in vivo TCP4 complex, and the gi mutant did not alter the composition of the TCP4 complex. Taken together, our results demonstrate a novel function of CIN-TCPs as photoperiodic flowering regulators, which may contribute to coordinating plant development with flowering regulation.

For plant adaptation to seasonal environments, a crucial developmental event is flowering, as proper timing of flowering affects reproductive success. Although plants monitor various environmental parameters to optimize this timing, photoperiod information is important for plants to regulate seasonal flowering time, because changes in photoperiod occur in a predictable manner throughout the year. The model plant Arabidopsis thaliana responds to photoperiodic changes and flowers under long-day conditions. Based on genetic analyses using mutants defective in the photoperiodic flowering response, we learned that the transcription factor referred to as CONSTANS (CO) plays a central role in regulating the timing of flowering by directly controlling the expression of florigen (flowering-inducing substrate) gene. Long-day afternoon expression of CO is critical for this regulation; however, we had limited knowledge of CO transcriptional regulation. Here we identified that a group of plant-specific transcription factors belonging to the TCP gene family function as novel CO transcriptional activators. We demonstrated that TCP transcription factors regulate CO transcription together with known regulators of CO. Our results imply that plants utilize multiple transcription factors to precisely coordinate the expression of the key regulator gene, CO, which will directly affect flowering time.

The control of flowering time by environmental factors

The timing of flowering is determined by endogenous genetic components as well as various environmental factors, such as day length, temperature, and stress. The genetic elements and molecular mechanisms that rule this process have been examined in the long-day-flowering plant Arabidopsis thaliana and short-day-flowering rice (Oryza sativa). However, reviews of research on the role of those factors are limited. Here, we focused on how flowering time is influenced by nutrients, ambient temperature, drought, salinity, exogenously applied hormones and chemicals, and pathogenic microbes. In response to such stresses or stimuli, plants either begin flowering to produce seeds for the next generation or else delay flowering by slowing their metabolism. These responses vary depending upon the dose of the stimulus, the plant developmental stage, or even the cultivar that is used. Our review provides insight into how crops might be managed to increase productivity under various environmental challenges.

Retrograde Signals: Integrators of Interorganellar Communication and Orchestrators of Plant Development

Interorganellar cooperation maintained via exquisitely controlled retrograde-signaling pathways is an evolutionary necessity for maintenance of cellular homeostasis. This signaling feature has therefore attracted much research attention aimed at improving understanding of the nature of these communication signals, how the signals are sensed, and ultimately the mechanism by which they integrate targeted processes that collectively culminate in organellar cooperativity. The answers to these questions will provide insight into how retrograde-signal-mediated regulatory mechanisms are recruited and which biological processes are targeted, and will advance our understanding of how organisms balance metabolic investments in growth against adaptation to environmental stress. This review summarizes the present understanding of the nature and the functional complexity of retrograde signals as integrators of interorganellar communication and orchestrators of plant development, and offers a perspective on the future of this critical and dynamic area of research.

Ethylene Role in Plant Growth, Development and Senescence: Interaction with Other Phytohormones

The complex juvenile/maturity transition during a plant's life cycle includes growth, reproduction, and senescence of its fundamental organs: leaves, flowers, and fruits. Growth and senescence of leaves, flowers, and fruits involve several genetic networks where the phytohormone ethylene plays a key role, together with other hormones, integrating different signals and allowing the onset of conditions favorable for stage progression, reproductive success and organ longevity. Changes in ethylene level, its perception, and the hormonal crosstalk directly or indirectly regulate the lifespan of plants. The present review focused on ethylene's role in the development and senescence processes in leaves, flowers and fruits, paying special attention to the complex networks of ethylene crosstalk with other hormones. Moreover, aspects with limited information have been highlighted for future research, extending our understanding on the importance of ethylene during growth and senescence and boosting future research with the aim to improve the qualitative and quantitative traits of crops.

Hormonal control of the floral transition: Can one catch them all?

The transition to flowering marks a key adaptive developmental switch in plants which impacts on their survival and fitness. Different signaling pathways control the floral transition, conveying both endogenous and environmental cues. These cues are often relayed and/or modulated by different hormones, which might confer additional developmental flexibility to the floral process in the face of varying conditions. Among the different hormonal pathways, the phytohormone gibberellic acid (GA) plays a dominant role. GA is connected with the other floral pathways through the GA-regulated DELLA proteins, acting as versatile interacting modules for different signaling proteins. In this review, I will highlight the role of DELLAs as spatial and temporal modulators of different consolidated floral pathways. Next, building on recent data, I will provide an update on some emerging themes connecting other hormone signaling cascades to flowering time control. I will finally provide examples for some established as well as potential cross-regulatory mechanisms between hormonal pathways mediated by the DELLA proteins.

Floral transitions in wheat and barley: interactions between photoperiod, abiotic stresses, and nutrient status

The timing of plant reproduction has a large impact on yield in crop plants. Reproductive development in temperate cereals comprises two major developmental transitions. During spikelet initiation, the identity of the shoot meristem switches from the vegetative to the reproductive stage and spikelet primordia are formed on the apex. Subsequently, floral morphogenesis is initiated, a process strongly affected by environmental variation. Recent studies in cereal grasses have suggested that this later phase of inflorescence development controls floret survival and abortion, and is therefore crucial for yield. Here, we provide a synthesis of the early morphological and the more recent genetic studies on shoot development in wheat and barley. The review explores how photoperiod, abiotic stress, and nutrient signalling interact with shoot development, and pinpoints genetic factors that mediate development in response to these environmental cues. We anticipate that research in these areas will be important in understanding adaptation of cereal grasses to changing climate conditions.

Geographic Variation of Plant Circadian Clock Function in Natural and Agricultural Settings

The increasing demand for improved agricultural production will require more efficient breeding for traits that maintain yield under heterogeneous environments. The internal circadian oscillator is essential for perceiving and coordinating environmental cues such as day length, temperature, and abiotic stress responses within physiological processes. To investigate the contribution of the circadian clock to local adaptability, we have analyzed circadian period by leaf movement in natural populations of Mimulus guttatus and domesticated cultivars of Glycine max. We detected consistent variation in circadian period along a latitudinal gradient in annual populations of the wild plant and the selectively bred crop, and this provides novel evidence of natural and artificial selection for circadian performance. These findings provide new support that the circadian clock acts as a central regulator of plant adaptability and further highlight the potential of applying circadian clock gene variation to marker-assisted breeding programs in crops.

Identification of Genes Associated with Lemon Floral Transition and Flower Development during Floral Inductive Water Deficits: A Hypothetical Model

Water deficit is a key factor to induce flowering in many woody plants, but reports on the molecular mechanisms of floral induction and flowering by water deficit are scarce. Here, we analyzed the morphology, cytology, and different hormone levels of lemon buds during floral inductive water deficits. Higher levels of ABA were observed, and the initiation of floral bud differentiation was examined by paraffin sections analysis. A total of 1638 differentially expressed genes (DEGs) were identified by RNA sequencing. DEGs were related to flowering, hormone biosynthesis, or metabolism. The expression of some DEGs was associated with floral induction by real-time PCR analysis. However, some DEGs may not have anything to do with flowering induction/flower development; they may be involved in general stress/drought response. Four genes from the phosphatidylethanolamine-binding protein family were further investigated. Ectopic expression of these genes in Arabidopsis changed the flowering time of transgenic plants. Furthermore, the 5' flanking region of these genes was also isolated and sequence analysis revealed the presence of several putative cis-regulatory elements, including basic elements and hormone regulation elements. The spatial and temporal expression patterns of these promoters were investigated under water deficit treatment. Based on these findings, we propose a model for citrus flowering under water deficit conditions, which will enable us to further understand the molecular mechanism of water deficit-regulated flowering in citrus.

HIGHLIGHT

Based on gene activity during floral inductive water deficits identified by RNA sequencing and genes associated with lemon floral transition, a model for citrus flowering under water deficit conditions is proposed.

Changing Responses to Changing Seasons: Natural Variation in the Plasticity of Flowering Time

The mechanisms by which the environment regulates flowering time have evolved as crops and wild populations have adapted to diverse climates, and the specific variants involved are increasingly known.

ABA-dependent control of GIGANTEA signalling enables drought escape via up-regulation of FLOWERING LOCUS T in Arabidopsis thaliana

One strategy deployed by plants to endure water scarcity is to accelerate the transition to flowering adaptively via the drought escape (DE) response. In Arabidopsis thaliana, activation of the DE response requires the photoperiodic response gene GIGANTEA (GI) and the florigen genes FLOWERING LOCUS T (FT) and TWIN SISTER OF FT (TSF). The phytohormone abscisic acid (ABA) is also required for the DE response, by promoting the transcriptional up-regulation of the florigen genes. The mode of interaction between ABA and the photoperiodic genes remains obscure. In this work we use a genetic approach to demonstrate that ABA modulates GI signalling and consequently its ability to activate the florigen genes. We also reveal that the ABA-dependent activation of FT, but not TSF, requires CONSTANS (CO) and that impairing ABA signalling dramatically reduces the expression of florigen genes with little effect on the CO transcript profile. ABA signalling thus has an impact on the core genes of photoperiodic signalling GI and CO by modulating their downstream function and/or activities rather than their transcript accumulation. In addition, we show that as well as promoting flowering, ABA simultaneously represses flowering, independent of the florigen genes. Genetic analysis indicates that the target of the repressive function of ABA is the flowering-promoting gene SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1), a transcription factor integrating floral cues in the shoot meristem. Our study suggests that variations in ABA signalling provide different developmental information that allows plants to co-ordinate the onset of the reproductive phase according to the available water resources.

Reduction of GIGANTEA expression in transgenic Brassica rapa enhances salt tolerance

Here we report the enhancement of tolerance to salt stress in Brassica rapa (Chinese cabbage) through the RNAi-mediated reduction of GIGANTEA ( GI ) expression. Circadian clocks integrate environmental signals with internal cues to coordinate diverse physiological outputs. The GIGANTEA (GI) gene was first discovered due to its important contribution to photoperiodic flowering and has since been shown to be a critical component of the plant circadian clock and to contribute to multiple environmental stress responses. We show that the GI gene in Brassica rapa (BrGI) is similar to Arabidopsis GI in terms of both expression pattern and function. BrGI functionally rescued the late-flowering phenotype of the Arabidopsis gi-201 loss-of-function mutant. RNAi-mediated suppression of GI expression in Arabidopsis Col-0 and in the Chinese cabbage, B. rapa DH03, increased tolerance to salt stress. Our results demonstrate that the molecular functions of GI described in Arabidopsis are conserved in B. rapa and suggest that manipulation of gene expression through RNAi and transgenic overexpression could enhance tolerance to abiotic stresses and thus improve agricultural crop production.

Hd3a, RFT1 and Ehd1 integrate photoperiodic and drought stress signals to delay the floral transition in rice

Plants show a high degree of developmental plasticity in response to external cues, including day length and environmental stress. Water scarcity in particular can interfere with photoperiodic flowering, resulting in the acceleration of the switch to reproductive growth in several species, a process called drought escape. However, other strategies are possible and drought stress can also delay flowering, albeit the underlying mechanisms have never been addressed at the molecular level. We investigated these interactions in rice, a short day species in which drought stress delays flowering. A protocol that allows the synchronization of drought with the floral transition was set up to profile the transcriptome of leaves subjected to stress under distinct photoperiods. We identified clusters of genes that responded to drought differently depending on day length. Exposure to drought stress under floral-inductive photoperiods strongly reduced transcription of EARLY HEADING DATE 1 (Ehd1), HEADING DATE 3a (Hd3a) and RICE FLOWERING LOCUS T 1 (RFT1), primary integrators of day length signals, providing a molecular connection between stress and the photoperiodic pathway. However, phenotypic and transcriptional analyses suggested that OsGIGANTEA (OsGI) does not integrate drought and photoperiodic signals as in Arabidopsis, highlighting molecular differences between long and short day model species.

Stress-induced flowering: the third category of flowering response

The switch from vegetative growth to reproductive growth, i.e. flowering, is the critical event in a plant's life. Flowering is regulated either autonomously or by environmental factors; photoperiodic flowering, which is regulated by the duration of the day and night periods, and vernalization, which is regulated by low temperature, have been well studied. Additionally, it has become clear that stress also regulates flowering. Diverse stress factors can induce or accelerate flowering, or inhibit or delay it, in a wide range of plant species. This article focuses on the positive regulation of flowering via stress, i.e. the induction or acceleration of flowering in response to stress that is known as stress-induced flowering - a new category of flowering response. This review aims to clarify the concept of stress-induced flowering and to summarize the full range of characteristics of stress-induced flowering from a predominately physiological perspective.

Integrative effect of drought and low temperature on litchi (Litchi chinensis Sonn.) floral initiation revealed by dynamic genome-wide transcriptome analysis

Floral induction in litchi is influenced by multiple environment cues including temperature and soil water condition. In the present study, we determined that a combined treatment consisting of 14-day drought imposed prior to exposure to 35-day low temperature (T3) significantly promoted litchi flowering relative to the low temperature alone (T2), suggesting an integrative effect of drought and low temperature on litchi floral initiation. Analysis of transcriptomic changes in leaves from different treatments showed that 2,198 and 4,407 unigenes were differentially expressed in response to drought and low temperature, respectively. 1,227 of these unigenes were expressed in response to both treatments, implying an interaction of drought and low temperature on expression of genes involved in litchi floral initiation. Additionally, 932 unigenes were consistently differentially expressed during floral induction between T2 and T3 plants, which potentially accounts for the difference of flowering time. Thirty-eight transcription factors out of these 932 unigenes were identified as hub genes with central roles in regulation of litchi floral induction. The expression of litchi homologs of well-known flowering genes was also investigated, and one Flowering Locus T (FT) homolog may play a crucial role in litchi flowering in responses to drought and low temperature.

A New Insight of Salt Stress Signaling in Plant

Many studies have been conducted to understand plant stress responses to salinity because irrigation-dependent salt accumulation compromises crop productivity and also to understand the mechanism through which some plants thrive under saline conditions. As mechanistic understanding has increased during the last decades, discovery-oriented approaches have begun to identify genetic determinants of salt tolerance. In addition to osmolytes, osmoprotectants, radical detoxification, ion transport systems, and changes in hormone levels and hormone-guided communications, the Salt Overly Sensitive (SOS) pathway has emerged to be a major defense mechanism. However, the mechanism by which the components of the SOS pathway are integrated to ultimately orchestrate plant-wide tolerance to salinity stress remains unclear. A higher-level control mechanism has recently emerged as a result of recognizing the involvement of GIGANTEA (GI), a protein involved in maintaining the plant circadian clock and control switch in flowering. The loss of GI function confers high tolerance to salt stress via its interaction with the components of the SOS pathway. The mechanism underlying this observation indicates the association between GI and the SOS pathway and thus, given the key influence of the circadian clock and the pathway on photoperiodic flowering, the association between GI and SOS can regulate growth and stress tolerance. In this review, we will analyze the components of the SOS pathways, with emphasis on the integration of components recognized as hallmarks of a halophytic lifestyle.

A Drought-Inducible Transcription Factor Delays Reproductive Timing in Rice

The molecular mechanisms underlying photoperiod or temperature control of flowering time have been recently elucidated, but how plants regulate flowering time in response to other external factors, such as water availability, remains poorly understood. Using a large-scale Hybrid Transcription Factor approach, we identified a bZIP transcriptional factor, O. sativa ABA responsive element binding factor 1 (OsABF1), which acts as a suppressor of floral transition in a photoperiod-independent manner. Simultaneous knockdown of both OsABF1 and its closest homologous gene, OsbZIP40, in rice (Oryza sativa) by RNA interference results in a significantly earlier flowering phenotype. Molecular and genetic analyses demonstrate that a drought regime enhances expression of the OsABF1 gene, which indirectly suppresses expression of the Early heading date 1 (Ehd1) gene that encodes a key activator of rice flowering. Furthermore, we identified a drought-inducible gene named OsWRKY104 that is under the direct regulation of OsABF1 Overexpression of OsWRKY104 can suppress Ehd1 expression and confers a later flowering phenotype in rice. Together, these findings reveal a novel pathway by which rice modulates heading date in response to the change of ambient water availability.

Mutation of OsGIGANTEA Leads to Enhanced Tolerance to Polyethylene Glycol-Generated Osmotic Stress in Rice

Water deficit is one of the most important environmental stresses limiting plant growth and crop yield. While the identification of many key factors involved in the plant water deficit response has greatly increased our knowledge about the regulation system, the mechanisms underlying dehydration tolerance in plants are still not well understood. In our current study, we investigated the roles of the key flowering time regulator, OsGIGANTEA (OsGI), in the osmotic stress tolerance in rice. Results showed that mutation of OsGI conferred tolerance to osmotic stress generated by polyethylene glycol (PEG), increased proline and sucrose contents, and accelerated stomata movement. In addition, qRT-PCR and microarray analysis revealed that the transcript abundance of some osmotic stress response genes, such as OsDREB1E, OsAP37, OsAP59, OsLIP9, OsLEA3, OsRAB16A, and OsSalT, was significantly higher in osgi than in WT plants, suggesting that OsGI might be a negative regulator in the osmotic stress response in rice.

Drought responsive gene expression regulatory divergence between upland and lowland ecotypes of a perennial C4 grass

Climatic adaptation is an example of a genotype-by-environment interaction (G×E) of fitness. Selection upon gene expression regulatory variation can contribute to adaptive phenotypic diversity; however, surprisingly few studies have examined how genome-wide patterns of gene expression G×E are manifested in response to environmental stress and other selective agents that cause climatic adaptation. Here, we characterize drought-responsive expression divergence between upland (drought-adapted) and lowland (mesic) ecotypes of the perennial C4 grass,Panicum hallii, in natural field conditions. Overall, we find that cis-regulatory elements contributed to gene expression divergence across 47% of genes, 7.2% of which exhibit drought-responsive G×E. While less well-represented, we observe 1294 genes (7.8%) with transeffects.Trans-by-environment interactions are weaker and much less common than cis G×E, occurring in only 0.7% oft rans-regulated genes. Finally, gene expression heterosis is highly enriched in expression phenotypes with significant G×E. As such, modes of inheritance that drive heterosis, such as dominance or overdominance, may be common among G×E genes. Interestingly, motifs specific to drought-responsive transcription factors are highly enriched in the promoters of genes exhibiting G×E and transregulation, indicating that expression G×E and heterosis may result from the evolution of transcription factors or their binding sites.P. hallii serves as the genomic model for its close relative and emerging biofuel crop, switchgrass (Panicum virgatum). Accordingly, the results here not only aid in the discovery of the genetic mechanisms that underlie local adaptation but also provide a foundation to improve switchgrass yield under water-limited conditions.

Transcriptome analysis reveals the role of the root hairs as environmental sensors to maintain plant functions under water-deficiency conditions

An important part of the root system is the root hairs, which play a role in mineral and water uptake. Here, we present an analysis of the transcriptomic response to water deficiency of the wild-type (WT) barley cultivar 'Karat' and its root-hairless mutant rhl1.a. A comparison of the transcriptional changes induced by water stress resulted in the identification of genes whose expression was specifically affected in each genotype. At the onset of water stress, more genes were modulated by water shortage in the roots of the WT plants than in the roots of rhl1.a. The roots of the WT plants, but not of rhl1.a, specifically responded with the induction of genes that are related to the abscisic acid biosynthesis, stomatal closure, and cell wall biogenesis, thus indicating the specific activation of processes that are related to water-stress signalling and protection. On the other hand, the processes involved in the further response to abiotic stimuli, including hydrogen peroxide, heat, and high light intensity, were specifically up-regulated in the leaves of rhl1.a. An extended period of severe stress caused more drastic transcriptome changes in the roots and leaves of the rhl1.a mutant than in those of the WT. These results are in agreement with the much stronger damage to photosystem II in the rhl1.a mutant than in its parent cultivar after 10 d of water stress. Taking into account the putative stress sensing and signalling features of the root hair transcriptome, we discuss the role of root hairs as sensors of environmental conditions.

Overexpression of soybean miR172c confers tolerance to water deficit and salt stress, but increases ABA sensitivity in transgenic Arabidopsis thaliana

MiRNAs play crucial roles in many aspects of plant development and the response to the environment. The miR172 family has been shown to participate in the control of flowering time and the response to abiotic stress. This family regulates the expression of APETALA2 (AP2)-like transcription factors in Arabidopsis. In the present study, soybean (Glycine max L. Merr.) miR172c, a member of the miR172 family, and its target gene were investigated for abiotic stress responses in transgenic Arabidopsis. gma-miR172c was induced by abscisic acid (ABA) treatments and abiotic stresses, including salt and water deficit. 5'-RACE (5'-rapid amplification of cDNA ends) assays indicated that miR172c directed Glyma01g39520 mRNA cleavage in soybeans. Overexpression of gma-miR172c in Arabidopsis resulted in reduced leaf water loss and increased survival rate under stress conditions. Meanwhile, the root length, germination rate, and cotyledon greening of transgenic plants were improved during both high salt and water deficit conditions. In addition, transgenic plants exhibited hypersensitivity to ABA during both the seed germination and post-germination seedling growth stages. Stress-related physiological indicators and the expression of stress/ABA-responsive genes were affected by abiotic treatments. The overexpression of gma-miR172c in Arabidopsis promoted earlier flowering compared with the wild type through modulation of the expression of flowering genes, such as FT and LFY during long days, especially under drought conditions. Glyma01g39520 weakened ABA sensitivity and reduced the tolerance to drought stress in the snz mutant of Arabidopsis by reducing the expression of ABI3 and ABI5. Overall, the present results demonstrate that gma-miR172c confers water deficit and salt tolerance but increased ABA sensitivity by regulating Glyma01g39520, which also accelerates flowering under abiotic stresses.

WRKY71 accelerates flowering via the direct activation of FLOWERING LOCUS T and LEAFY in Arabidopsis thaliana

Flowering is crucial for achieving reproductive success. A large number of well-delineated factors affecting flowering are involved in complex genetic networks in Arabidopsis thaliana. However, the underlying part played by the WRKY transcription factors in this process is not yet clear. Here, we report that WRKY71 is able to accelerate flowering in Arabidopsis. An activation-tagged mutant WRKY71-1D and a constitutive over-expresser of WRKY71 both flowered earlier than the wild type (WT). In contrast, both the RNA interference-based multiple WRKY knock-out mutant (w71w8 + 28RNAi) and the dominant repression line (W71-SRDX) flowered later. Gene expression analysis showed that the transcript abundance of the flowering time integrator gene FLOWERING LOCUS T (FT) and the floral meristem identity genes LEAFY (LFY), APETALA1 (AP1) and FRUITFULL (FUL) were greater in WRKY71-1D than in the WT, but lower in w71w8 + 28RNAi and W71-SRDX. Further, WRKY71 was shown to bind to the W-boxes in the FT and LFY promoters in vitro and in vivo. The suggestion is that WRKY71 activity hastens flowering via the direct activation of FT and LFY.

The link between flowering time and stress tolerance

Evolutionary success in plants is largely dependent on the successful transition from vegetative to reproductive growth. In the lifetime of a plant, flowering is not only an essential part of the reproductive process but also a critical developmental stage that can be vulnerable to environmental stresses. Exposure to stress during this period can cause substantial yield losses in seed-producing plants. However, it is becoming increasingly evident that altering flowering time is an evolutionary strategy adopted by plants to maximize the chances of reproduction under diverse stress conditions, ranging from pathogen infection to heat, salinity, and drought. Here, recent studies that have revealed new insights into how biotic and abiotic stress signals can be integrated into floral pathways are reviewed. A better understanding of how complex environmental variables affect plant phenology is important for future genetic manipulation of crops to increase productivity under the changing climate.

ABSCISIC ACID-INSENSITIVE 4 negatively regulates flowering through directly promoting Arabidopsis FLOWERING LOCUS C transcription

During the life cycle of a plant, one of the major biological processes is the transition from the vegetative to the reproductive stage. In Arabidopsis, flowering time is precisely controlled by extensive environmental and internal cues. Gibberellins (GAs) promote flowering, while abscisic acid (ABA) is considered as a flowering suppressor. However, the detailed mechanism through which ABA inhibits the floral transition is poorly understood. Here, we report that ABSCISIC ACID-INSENSITIVE 4 (ABI4), a key component in the ABA signalling pathway, negatively regulates floral transition by directly promoting FLOWERING LOCUS C (FLC) transcription. The abi4 mutant showed the early flowering phenotype whereas ABI4-overexpressing (OE-ABI4) plants had delayed floral transition. Consistently, quantitative reverse transcription-PCR (qRT-PCR) assay revealed that the FLC transcription level was down-regulated in abi4, but up-regulated in OE-ABI4. The change in FT level was consistent with the pattern of FLC expression. Chromatin immunoprecipitation-qPCR (ChIP-qPCR), electrophoretic mobility shift assay (EMSA), and tobacco transient expression analysis showed that ABI4 promotes FLC expression by directly binding to its promoter. Genetic analysis demonstrated that OE-ABI4::flc-3 could not alter the flc-3 phenotype. OE-FLC::abi4 showed a markedly delayed flowering phenotype, which mimicked OE-FLC::WT, and suggested that ABI4 acts upstream of FLC in the same genetic pathway. Taken together, these findings suggest that ABA inhibits the floral transition by activating FLC transcription through ABI4.

AtRAV1 and AtRAV2 overexpression in cotton increases fiber length differentially under drought stress and delays flowering

There is a longstanding problem of an inverse relationship between cotton fiber qualities versus high yields. To better understand drought stress signaling and adaptation in cotton (Gossypium hirsutum) fiber development, we expressed the Arabidopsis transcription factors RELATED_TO_ABA-INSENSITIVE3/VIVIPAROUS1/(RAV1) and AtRAV2, which encode APETALA2-Basic3 domain proteins shown to repress transcription of FLOWERING_LOCUS_T (FT) and to promote stomatal opening cell-autonomously. In three years of field trials, we show that AtRAV1 and AtRAV2-overexpressing cotton had ∼5% significantly longer fibers with only marginal decreases in yields under well-watered or drought stress conditions that resulted in 40-60% yield penalties and 3-7% fiber length penalties in control plants. The longer transgenic fibers from drought-stressed transgenics could be spun into yarn which was measurably stronger and more uniform than that from well-watered control fibers. The transgenic AtRAV1 and AtRAV2 lines flowered later and retained bolls at higher nodes, which correlated with repression of endogenous GhFT-Like (FTL) transcript accumulation. Elevated expression early in development of ovules was observed for GhRAV2L, GhMYB25-Like (MYB25L) involved in fiber initiation, and GhMYB2 and GhMYB25 involved in fiber elongation. Altered expression of RAVs controlling critical nodes in developmental and environmental signaling hierarchies has the potential for phenotypic modification of crops.

Gibberellic acid signaling is required for ambient temperature-mediated induction of flowering in Arabidopsis thaliana

Distinct molecular mechanisms integrate changes in ambient temperature into the genetic pathways that govern flowering time in Arabidopsis thaliana. Temperature-dependent eviction of the histone variant H2A.Z from nucleosomes has been suggested to facilitate the expression of FT by PIF4 at elevated ambient temperatures. Here we show that, in addition to PIF4, PIF3 and PIF5, but not PIF1 and PIF6, can promote flowering when expressed specifically in phloem companion cells (PCC), where they can induce FT and its close paralog, TSF. However, despite their strong potential to promote flowering, genetic analyses suggest that the PIF genes seem to have only a minor role in adjusting flowering in response to photoperiod or high ambient temperature. In addition, loss of PIF function only partially suppressed the early flowering phenotype and FT expression of the arp6 mutant, which is defective in H2A.Z deposition. In contrast, the chemical inhibition of gibberellic acid (GA) biosynthesis resulted in a strong attenuation of early flowering and FT expression in arp6. Furthermore, GA was able to induce flowering at low temperature (15°C) independently of FT, TSF, and the PIF genes, probably directly at the shoot apical meristem. Together, our results suggest that the timing of the floral transition in response to ambient temperature is more complex than previously thought and that GA signaling might play a crucial role in this process.

Expression profiling of potato cultivars with contrasting tuberization at elevated temperature using microarray analysis

Temperature is one of the most significant factors affecting potato yield. Night temperature beyond 18-22 °C drastically reduces tuber formation, constraining potato cultivation in tropics and subtropics. Identification of genes and pathways affected by high temperature is crucial for developing thermo tolerant cultivars for these regions. In the present study, two cultivars with contrasting tuberization behavior at night temperatures (24 °C) were selected for gene expression analysis using a customized microarray chip representing 39,031 potato genes. A total of 2500 genes were differentially expressed on 21 d and 4096 genes on 14 d after stress. Gene ontology and pathway analysis provided insights into the probable biological processes and pathways governing tuberization at elevated temperature. Pathway maps were constructed to graphically represent the gene expression patterns. Genes associated with photosynthesis, hormonal activity, sugar transporters and transcription factors were differentially expressed. The results are presented and discussed in terms of tuberization at high temperature. The effect of high temperature on expression of genes controlling tuberization was also analyzed. This study provided useful information on potato tuberization at elevated temperature and make available a framework for further investigations into heat stress in potato.

Arabidopsis INCURVATA2 Regulates Salicylic Acid and Abscisic Acid Signaling, and Oxidative Stress Responses

Epigenetic regulatory states can persist through mitosis and meiosis, but the connection between chromatin structure and DNA replication remains unclear. Arabidopsis INCURVATA2 (ICU2) encodes the catalytic subunit of DNA polymerase α, and null alleles of ICU2 have an embryo-lethal phenotype. Analysis of icu2-1, a hypomorphic allele of ICU2, demonstrated that ICU2 functions in chromatin-mediated cellular memory; icu2-1 strongly impairs ICU2 function in the maintenance of repressive epigenetic marks but does not seem to affect ICU2 polymerase activity. To better understand the global function of ICU2 in epigenetic regulation, here we performed a microarray analysis of icu2-1 mutant plants. We found that the genes up-regulated in the icu2-1 mutant included genes encoding transcription factors and targets of the Polycomb Repressive Complexes. The down-regulated genes included many known players in salicylic acid (SA) biosynthesis and accumulation, ABA signaling and ABA-mediated responses. In addition, we found that icu2-1 plants had reduced SA levels in normal conditions; infection by Fusarium oxysporum induced SA accumulation in the En-2 wild type but not in the icu2-1 mutant. The icu2-1 plants were also hypersensitive to salt stress and exogenous ABA in seedling establishment, post-germination growth and stomatal closure, and accumulated more ABA than the wild type in response to salt stress. The icu2-1 mutant also showed high tolerance to the oxidative stress produced by 3-amino-1,2,4-triazole (3-AT). Our results uncover a role for ICU2 in the regulation of genes involved in ABA signaling as well as in SA biosynthesis and accumulation.

Circadian Clock Genes Universally Control Key Agricultural Traits

Circadian clocks are endogenous timers that enable plants to synchronize biological processes with daily and seasonal environmental conditions in order to allocate resources during the most beneficial times of day and year. The circadian clock regulates a number of central plant activities, including growth, development, and reproduction, primarily through controlling a substantial proportion of transcriptional activity and protein function. This review examines the roles that alleles of circadian clock genes have played in domestication and improvement of crop plants. The focus here is on three groups of circadian clock genes essential to clock function in Arabidopsis thaliana: PSEUDO-RESPONSE REGULATORs, GIGANTEA, and the evening complex genes early flowering 3, early flowering 4, and lux arrhythmo. homologous genes from each group underlie quantitative trait loci that have beneficial influences on key agricultural traits, especially flowering time but also yield, biomass, and biennial growth habit. Emerging insights into circadian clock regulation of other fundamental plant processes, including responses to abiotic and biotic stresses, are discussed to highlight promising avenues for further crop improvement.

Investigating the Association between Flowering Time and Defense in the Arabidopsis thaliana-Fusarium oxysporum Interaction

Plants respond to pathogens either by investing more resources into immunity which is costly to development, or by accelerating reproductive processes such as flowering time to ensure reproduction occurs before the plant succumbs to disease. In this study we explored the link between flowering time and pathogen defense using the interaction between Arabidopsis thaliana and the root infecting fungal pathogen Fusarium oxysporum. We report that F. oxysporum infection accelerates flowering time and regulates transcription of a number of floral integrator genes, including FLOWERING LOCUS C (FLC), FLOWERING LOCUS T (FT) and GIGANTEA (GI). Furthermore, we observed a positive correlation between late flowering and resistance to F. oxysporum in A. thaliana natural ecotypes. Late-flowering gi and autonomous pathway mutants also exhibited enhanced resistance to F. oxysporum, supporting the association between flowering time and defense. However, epistasis analysis showed that accelerating flowering time by deletion of FLC in fve-3 or fpa-7 mutants did not alter disease resistance, suggesting that the effect of autonomous pathway on disease resistance occurs independently from flowering time. Indeed, RNA-seq analyses suggest that fve-3 mediated resistance to F. oxysporum is most likely a result of altered defense-associated gene transcription. Together, our results indicate that the association between flowering time and pathogen defense is complex and can involve both pleiotropic and direct effects.

The evolution of drought escape and avoidance in natural herbaceous populations

While the functional genetics and physiological mechanisms controlling drought resistance in crop plants have been intensely studied, less research has examined the genetic basis of adaptation to drought stress in natural populations. Drought resistance adaptations in nature reflect natural rather than human-mediated selection and may identify novel mechanisms for stress tolerance. Adaptations conferring drought resistance have historically been divided into alternative strategies including drought escape (rapid development to complete a life cycle before drought) and drought avoidance (reducing water loss to prevent dehydration). Recent studies in genetic model systems such as Arabidopsis, Mimulus, and Panicum have begun to elucidate the genes, expression profiles, and physiological changes responsible for ecologically important variation in drought resistance. Similar to most crop plants, variation in drought escape and avoidance is complex, underlain by many QTL of small effect, and pervasive gene by environment interactions. Recently identified major-effect alleles point to a significant role for genetic constraints in limiting the concurrent evolution of both drought escape and avoidance strategies, although these constraints are not universally found. This progress suggests that understanding the mechanistic basic and fitness consequences of gene by environment interactions will be critical for crop improvement and forecasting population persistence in unpredictable environments.

Allelic polymorphism of GIGANTEA is responsible for naturally occurring variation in circadian period in Brassica rapa

GIGANTEA (GI) was originally identified by a late-flowering mutant in Arabidopsis, but subsequently has been shown to act in circadian period determination, light inhibition of hypocotyl elongation, and responses to multiple abiotic stresses, including tolerance to high salt and cold (freezing) temperature. Genetic mapping and analysis of families of heterogeneous inbred lines showed that natural variation in GI is responsible for a major quantitative trait locus in circadian period in Brassica rapa. We confirmed this conclusion by transgenic rescue of an Arabidopsis gi-201 loss of function mutant. The two B. rapa GI alleles each fully rescued the delayed flowering of Arabidopsis gi-201 but showed differential rescue of perturbations in red light inhibition of hypocotyl elongation and altered cold and salt tolerance. The B. rapa R500 GI allele, which failed to rescue the hypocotyl and abiotic stress phenotypes, disrupted circadian period determination in Arabidopsis. Analysis of chimeric B. rapa GI alleles identified the causal nucleotide polymorphism, which results in an amino acid substitution (S264A) between the two GI proteins. This polymorphism underlies variation in circadian period, cold and salt tolerance, and red light inhibition of hypocotyl elongation. Loss-of-function mutations of B. rapa GI confer delayed flowering, perturbed circadian rhythms in leaf movement, and increased freezing and increased salt tolerance, consistent with effects of similar mutations in Arabidopsis. Collectively, these data suggest that allelic variation of GI-and possibly of clock genes in general-offers an attractive target for molecular breeding for enhanced stress tolerance and potentially for improved crop yield.

The GI-CDF module of Arabidopsis affects freezing tolerance and growth as well as flowering

Plants monitor and integrate temperature, photoperiod and light quality signals to respond to continuous changes in their environment. The GIGANTEA (GI) protein is central in diverse signaling pathways, including photoperiodic, sugar and light signaling pathways, stress responses and circadian clock regulation. Previously, GI was shown to activate expression of the key floral regulators CONSTANS (CO) and FLOWERING LOCUS T (FT) by facilitating degradation of a family of CYCLING DOF FACTOR (CDF) transcriptional repressors. However, whether CDFs are implicated in other processes affected by GI remains unclear. We investigated the contribution of the GI-CDF module to traits that depend on GI. Transcriptome profiling indicated that mutations in GI and the CDF genes have antagonistic effects on expression of a wider set of genes than CO and FT, whilst other genes are regulated by GI independently of the CDFs. Detailed expression studies followed by phenotypic assays showed that the CDFs function downstream of GI, influencing responses to freezing temperatures and growth, but are not necessary for proper clock function. Thus GI-mediated regulation of CDFs contributes to several processes in addition to flowering, but is not implicated in all of the traits influenced by GI.

A possible role of an anthocyanin filter in low-intensity light stress-induced flowering in Perilla frutescens var. crispa

The red-leaved form of Perilla frutescens var. crispa was induced to flower by low-intensity light stress. The leaves of this form are normally red, but turned green under low-intensity light due to anthocyanin depletion in the epidermis. Flowering did not occur when plants were grown under light passed through a red-colored cellophane paper, which has an absorption spectrum similar to that of anthocyanins. High-concentration anthocyanins may play the role of a red-colored optical filter under normal light conditions, and this filter effect may be lost under low-intensity light, causing a change in the wavelength characteristics of the light with which the mesophyll cells are irradiated. This change may induce a photobiological effect leading to flowering. The gene expression and enzyme activity of phenylalanine ammonia-lyase (PAL), the key enzyme for anthocyanin biosynthesis, decreased under low-intensity light. L-2-aminooxy-3-phenylpropionic acid (AOPP), which is widely used as a PAL inhibitor, inhibited low-intensity light stress-induced flowering and increased PAL activity and anthocyanin content. The inhibition of flowering by AOPP in P. frutescens may be through different mechanisms than PAL inhibition.

GIGANTEA - an emerging story

GIGANTEA (GI) is a plant specific nuclear protein and functions in diverse physiological processes such as flowering time regulation, light signaling, hypocotyl elongation, control of circadian rhythm, sucrose signaling, starch accumulation, chlorophyll accumulation, transpiration, herbicide tolerance, cold tolerance, drought tolerance, and miRNA processing. It has been five decades since its discovery but the biochemical function of GI and its different domains are still unclear. Although it is known that both GI transcript and GI protein are clock controlled, the regulation of its abundance and functions at the molecular level are still some of the unexplored areas of intensive research. Since GI has many important pleotropic functions as described above scattered through literature, it is worthwhile and about time to encapsulate the available information in a concise review. Therefore, in this review, we are making an attempt to summarize (i) the various interconnected roles that GI possibly plays in the fine-tuning of plant development, and (ii) the known mutations of GI that have been instrumental in understanding its role in distinct physiological processes.

Photoperiodic Regulation of Florigen Function in Arabidopsis thaliana

One mechanism through which flowering in response to seasonal change is brought about is by sensing the fluctuation in day-length; the photoperiod. Flowering induction occurs through the production of the florigenic protein FLOWERING LOCUS T (FT) and its movement from the phloem companion cells in the leaf vasculature into the shoot apex, where meristematic reprogramming occurs. FT activation in response to photoperiod condition is accomplished largely through the activity of the transcription factor CONSTANS (CO). Regulation of CO expression and protein stability, as well as the timing of other components via the circadian clock, is a critical mechanism by which plants are able to respond to photoperiod to initiate the floral transition. Modulation of FT expression in response to external and internal stimuli via components of the flowering network is crucial to mediate a fluid flowering response to a variety of environmental parameters. In addition, the regulated movement of FT protein from the phloem to the shoot apex, and interactions that determine floral meristem cell fate, constitute novel mechanisms through which photoperiodic information is translated into flowering time.

Arabidopsis flower specific defense gene expression patterns affect resistance to pathogens

We investigated whether the Arabidopsis flower evolved protective measures to increase reproductive success. Firstly, analyses of available transcriptome data show that the most highly expressed transcripts in the closed sepal (stage 12) are enriched in genes with roles in responses to chemical stimuli and cellular metabolic processes. At stage 15, there is enrichment in transcripts with a role in responses to biotic stimuli. Comparative analyses between the sepal and petal in the open flower mark an over-representation of transcripts with a role in responses to stress and catalytic activity. Secondly, the content of the biotic defense-associated phytohormone salicylic acid (SA) in sepals and petals is significantly higher than in leaves. To understand whether the high levels of stress responsive transcripts and the higher SA content affect defense, wild-type plants (Col-0) and transgenic plants defective in SA accumulation (nahG) were challenged with the biotrophic fungus Golovinomyces cichoracearum, the causal agent of powdery mildew, and the necrotrophic fungus Botrytis cinerea. NahG leaves were more sensitive than those of Col-0, suggesting that in leaves SA has a role in the defense against biotrophs. In contrast, sepals and petals of both genotypes were resistant to G. cichoracearum, indicating that in the flower, resistance to the biotrophic pathogen is not critically dependent on SA, but likely dependent on the up-regulation of stress-responsive genes. Since sepals and petals of both genotypes are equally susceptible to B. cinerea, we conclude that neither stress-response genes nor increased SA accumulation offers protection against the necrotrophic pathogen. These results are interpreted in the light of the distinctive role of the flower and we propose that in the early stages, the sepal may act as a chemical defense barrier of the developing reproductive structures against biotrophic pathogens.

The relationship between flowering time and growth responses to drought in the Arabidopsis Landsberg erecta x Antwerp-1 population

Limited water availability is one of the most prominent abiotic constraints to plant survival and reproduction. Thus, plants have evolved different strategies to cope with water deficit, including modification of their growth and timing of developmental events such as flowering. In this work, we explore the link between flowering time and growth responses to moderate drought stress in Arabidopsis thaliana using natural variation for these traits found in the Landsberg erecta x Antwerp-1 recombinant inbred line population. We developed and phenotyped near isogenic lines containing different allelic combinations at three interacting quantitative trait loci (QTL) affecting both flowering time and growth in response to water deficit. We used these lines to confirm additive and epistatic effects of the three QTL and observed a strong association between late flowering and reduced sensitivity to drought. Analyses of growth responses to drought over time revealed that late flowering plants were able to recover their growth in the second half of their vegetative development. In contrast, early flowering, a common drought escape strategy that ensures plant survival under severe water deficit, was associated with strongly impaired plant fitness. The results presented here indicate that late flowering may be advantageous under continuous mild water deficit as it allows stress acclimatization over time.

Dlf1, a WRKY transcription factor, is involved in the control of flowering time and plant height in rice

Flowering time and plant height are important agronomic traits for crop production. In this study, we characterized a semi-dwarf and late flowering (dlf1) mutation of rice that has pleiotropic effects on these traits. The dlf1 mutation was caused by a T-DNA insertion and the cloned Dlf1 gene was found to encode a WRKY transcription factor (OsWRKY11). The dlf1 mutant contains a T-DNA insertion at the promoter region, leading to enhanced accumulation of Dlf1 transcripts, resulting in a semidominant mutation. The dlf1 mutation suppressed the transcription of Ehd2/RID1/OsId1 and its downstream flowering-time genes including Hd1, Ehd1 and Hd3a under both long-day (LD) and short-day (SD) conditions. Knock-down of Dlf1 expression exhibited early flowering at LD condition related to the wild-type plants. Accumulation of Dlf1 mRNA was observed in most tissues, and two splicing forms of Dlf1 cDNAs were obtained (OsWRKY11.1 and OsWRKY11.2). These two proteins showed transactivation activity in yeast cells. Dlf1 protein was found to be localized in the nucleus. Enhanced expression of OsWRKY11.2 or its 5′ truncated gene showed similar phenotypes to the dlf1 mutant, suggesting that it might function as a negative regulator. We conclude that Dlf1 acts as a transactivator to downregulate Ehd2/RID1/OsId1 in the signal transduction pathway of flowering and plays an important role in the regulation of plant height in rice.

Stress enhances the gene expression and enzyme activity of phenylalanine ammonia-lyase and the endogenous content of salicylic acid to induce flowering in pharbitis

The involvement of salicylic acid (SA) in the regulation of stress-induced flowering in the short-day plant pharbitis (also called Japanese morning glory) Ipomoea nil (formerly Pharbitis nil) was studied. Pharbitis cv. Violet was induced to flower when grown in 1/100-strength mineral nutrient solution under non-inductive long-day conditions. All fully expanded true leaves were removed from seedlings, leaving only the cotyledons, and flowering was induced under poor-nutrition stress conditions. This indicates that cotyledons can play a role in the regulation of poor-nutrition stress-induced flowering. The expression of the pharbitis homolog of PHENYLALANINE AMMONIA-LYASE, the enzyme activity of phenylalanine ammonia-lyase (PAL; E.C. 4.3.1.5) and the content of SA in the cotyledons were all up-regulated by the stress treatment. The Violet was also induced to flower by low-temperature stress, DNA demethylation and short-day treatment. Low-temperature stress enhanced PAL activity, whereas non-stress factors such as DNA demethylation and short-day treatment decreased the activity. The PAL enzyme activity was also examined in another cultivar, Tendan, obtaining similar results to Violet. The exogenously applied SA did not induce flowering under non-stress conditions but did promote flowering under weak stress conditions in both cultivars. These results suggest that stress-induced flowering in pharbitis is induced, at least partly, by SA, and the synthesis of SA is promoted by PAL.