PHYTOCHROME INTERACTING FACTOR Research Articles

Temperature regulation of auxin-related gene expression and its implications for plant growth.

Twenty-five years ago, a seminal paper demonstrated that warm temperatures increase auxin levels to promote hypocotyl growth in Arabidopsis thaliana. Here we highlight recent advances in auxin-mediated thermomorphogenesis and identify unanswered questions. In the warmth, PHYTOCHROME INTERACTING FACTOR 4 (PIF4) and PIF7 bind the YUCCA8 gene promoter and, in concert with histone modifications, enhance its expression to increase auxin synthesis in the cotyledons. Once transported to the hypocotyl, auxin promotes cell elongation. The meta-analysis of expression of auxin-related genes in seedlings exposed to temperatures ranging from cold to hot shows complex patterns of response. Changes in auxin only partially account for these responses. The expression of many SMALL AUXIN UP RNA (SAUR) genes reaches a maximum in the warmth, decreasing towards both temperature extremes in correlation with the rate of hypocotyl growth. Warm temperatures enhance primary root growth, the response requires auxin, and the hormone levels increase in the root tip but the impacts on cell division and cell expansion are not clear. A deeper understanding of auxin-mediated temperature control of plant architecture is necessary to face the challenge of global warming.

PIF1, a phytochrome-interacting factor negatively regulates drought tolerance and carotenoids biosynthesis in tobacco

The phytochrome-interacting factors (PIFs) function crucially in multiple physiological processes, but the biological functions of some PIFs remain elusive in some species. Here, a PIF transcription factor NtPIF1 was cloned and characterized in tobacco (Nicotiana tabacum L.). The transcript of NtPIF1 was significantly induced by drought stress treatments, and it localized in the nuclear. Knockout of NtPIF1 by CRISPR/Cas9 system led to the improved drought tolerance of tobacco with increased osmotic adjustment, antioxidant activity, photosynthetic efficiency and decreased water loss rate. On the contrary, NtPIF1-overexpression plants displays drought-sensitive phenotypes. In addition, NtPIF1 reduced the biosynthesis of abscisic acid (ABA) and its upstream carotenoids by regulating the expression of genes involved in ABA and carotenoids biosynthetic pathway upon drought stress. Electrophoretic mobility shift and dual-luciferase assays illustrated that, NtPIF1 directly bind to the E-box elements within the promoters of NtNCED3, NtABI5, NtZDS and Ntβ-LCY to repress their transcription. Overall, these data suggested that NtPIF1 negatively regulate tobacco adaptive response to drought stress and carotenoids biosynthesis; moreover, NtPIF1 has the potential to develop drought-tolerant tobacco plants using CRISPR/Cas9 system.

Near-infrared light and PIF4 promote plant antiviral defense by enhancing RNA interference

The molecular mechanism underlying phototherapy and light treatment, which utilize various wavelength spectra of light, including near-infrared (NIR), to cure human and plant diseases, is obscure. Here we revealed that NIR light confers antiviral immunity by positively regulating PHYTOCHROME-INTERACTING FACTOR 4 (PIF4)-activated RNA interference (RNAi) in plants. PIF4, a central transcription factor involved in light signaling, accumulates to high levels under NIR light in plants. PIF4 directly induces the transcription of two essential components of RNAi, RNA-DEPENDENT RNA POLYMERASE 6 (RDR6) and ARGONAUTE 1 (AGO1), which play important roles in resistance to both DNA and RNA viruses. Moreover, the pathogenic determinant βC1 protein, which is evolutionarily conserved and encoded by betasatellites, interacts with PIF4 and inhibits its positive regulation of RNAi by disrupting PIF4 dimerization. These findings shed light on the molecular mechanism of PIF4-mediated plant defense and provide a new perspective for the exploration of NIR antiviral treatment.

Transcription factors KANADI 1, MYB DOMAIN PROTEIN 44, and PHYTOCHROME INTERACTING FACTOR 4 regulate long intergenic noncoding RNAs expressed in Arabidopsis roots.

Thousands of long intergenic noncoding RNAs (lincRNAs) have been identified in plant genomes. While some lincRNAs have been characterized as important regulators in different biological processes, little is known about the transcriptional regulation for most plant lincRNAs. Through the integration of 8 annotation resources, we defined 6,599 high-confidence lincRNA loci in Arabidopsis (Arabidopsis thaliana). For lincRNAs belonging to different evolutionary age categories, we identified major differences in sequence and chromatin features, as well as in the level of conservation and purifying selection acting during evolution. Spatiotemporal gene expression profiles combined with transcription factor (TF) chromatin immunoprecipitation (ChIP) data were used to construct a TF-lincRNA regulatory network containing 2,659 lincRNAs and 15,686 interactions. We found that properties characterizing lincRNA expression, conservation, and regulation differ between plants and animals. Experimental validation confirmed the role of 3 TFs, KANADI 1, MYB DOMAIN PROTEIN 44, and PHYTOCHROME INTERACTING FACTOR 4, as key regulators controlling root-specific lincRNA expression, demonstrating the predictive power of our network. Furthermore, we identified 58 lincRNAs, regulated by these TFs, showing strong root cell type-specific expression or chromatin accessibility, which are linked with genome-wide association studies genetic associations related to root system development and growth. The multilevel genome-wide characterization covering chromatin state information, promoter conservation, and chromatin immunoprecipitation-based TF binding, for all detectable lincRNAs across 769 expression samples, permits rapidly defining the biological context and relevance of Arabidopsis lincRNAs through regulatory networks.

MYB112 connects light and circadian clock signals to promote hypocotyl elongation in Arabidopsis.

Ambient light and the endogenous circadian clock play key roles in regulating Arabidopsis (Arabidopsis thaliana) seedling photomorphogenesis. PHYTOCHROME-INTERACTING FACTOR 4 (PIF4) acts downstream of both light and the circadian clock to promote hypocotyl elongation. Several members of the R2R3-MYB transcription factor (TF) family, the most common type of MYB TF family in Arabidopsis, have been shown to be involved in regulating photomorphogenesis. Nonetheless, whether R2R3-MYB TFs are involved in connecting the light and clock signaling pathways during seedling photomorphogenesis remains unknown. Here, we report that MYB112, a member of the R2R3-MYB family, acts as a negative regulator of seedling photomorphogenesis in Arabidopsis. The light signal promotes the transcription and protein accumulation of MYB112. myb112 mutants exhibit short hypocotyls in both constant light and diurnal cycles. MYB112 physically interacts with PIF4 to enhance the transcription of PIF4 target genes involved in the auxin pathway, including YUCCA8 (YUC8), INDOLE-3-ACETIC ACID INDUCIBLE 19 (IAA19), and IAA29. Furthermore, MYB112 directly binds to the promoter of LUX ARRHYTHMO (LUX), the central component of clock oscillators, to repress its expression mainly in the afternoon and relieve LUX-inhibited expression of PIF4. Genetic evidence confirms that LUX acts downstream of MYB112 in regulating hypocotyl elongation. Thus, the enhanced transcript accumulation and transcriptional activation activity of PIF4 by MYB112 additively promotes the expression of auxin-related genes, thereby increasing auxin synthesis and signaling and fine-tuning hypocotyl growth under diurnal cycles.

ELONGATED HYPOCOTYL5 (HY5) and HY5 HOMOLOGUE (HYH) maintain shade avoidance suppression in UV-B.

Reductions in red to far-red ratio (R:FR) provide plants with an unambiguous signal of vegetational shade and are monitored by phytochrome photoreceptors. Plants integrate this information with other environmental cues to determine the proximity and density of encroaching vegetation. Shade-sensitive species respond to reductions in R:FR by initiating a suite of developmental adaptations termed shade avoidance. These include the elongation of stems to facilitate light foraging. Hypocotyl elongation is driven by increased auxin biosynthesis promoted by PHYTOCHROME INTERACTING FACTORs (PIF) 4, 5 and 7. UV-B perceived by the UV RESISTANCE LOCUS 8 (UVR8) photoreceptor rapidly inhibits shade avoidance, in part by suppressing PIF4/5 transcript accumulation and destabilising PIF4/5 protein. Here, we show that longer-term inhibition of shade avoidance is sustained by ELONGATED HYPOCOTYL 5 (HY5) and HY5 HOMOLOGUE (HYH), which regulate transcriptional reprogramming of genes involved in hormone signalling and cell wall modification. HY5 and HYH are elevated in UV-B and suppress the expression of XYLOGLUCAN ENDOTRANSGLUCOSYLASE/HYDROLASE (XTH) genes involved in cell wall loosening. They additionally increase expression GA2-OXIDASE1 (GA2ox1) and GA2ox2, encoding gibberellin catabolism enzymes that act redundantly to stabilise the PIF-inhibiting DELLA proteins. UVR8 therefore regulates temporally distinct signalling pathways to first rapidly inhibit and subsequently maintain suppression of shade avoidance following UV-B exposure.

Arabidopsis thaliana rosette habit is controlled by combined light and energy signaling converging on transcriptional control of the TALE homeobox gene ATH1.

In the absence of light signals, Arabidopsis plants fail to develop the rosette habit typical for this species. Instead, plants display caulescent growth due to elongation of rosette internodes. This aspect of photomorphogenic development has been paid little attention and molecular events involved, downstream of photoreceptor signaling, remain to be identified. Using a combination of genetic and molecular approaches, we show that Arabidopsis rosette habit is a photomorphogenic trait controlled by induction of ARABIDOPSIS THALIANA HOMEOBOX GENE1 (ATH1) as downstream target of multiple photoreceptors. ATH1 induction prevents rosette internode elongation by maintaining the shoot apical meristem (SAM) rib zone area inactive and requires inactivation of photomorphogenesis inhibitors, including PHYTOCHROME INTERACTING FACTOR (PIF) proteins. ATH1 activity results in tissue-specific inhibition of PIF expression, establishing double-negative feedback-regulation at the SAM. Light-requirement for ATH1 expression can be overcome by high sugar availability to the SAM. Both sugar and light signals that induce ATH1 and, subsequently, rosette habit are mediated by TOR kinase. Collectively, our data reveal a SAM-specific, double-negative ATH1-PIF feedback loop at the basis of rosette habit. Upstream, TOR kinase functions as central hub integrating light and energy signals that control this for Arabidopsis quintessential trait.

Phytochrome-interacting factor PIF3 integrates phytochrome B and UVB signaling pathways to regulate gibberellin- and auxin-dependent growth in cucumber hypocotyls.

In Arabidopsis, the photoreceptors phytochrome B- (PhyB) and UVB resistance 8- (UVR8) mediated light responses play a major role in regulating photomorphogenic hypocotyl growth, but how they crosstalk to coordinate this process is not well understood. Here we report map-based cloning and functional characterization of a UVB-insensitive, long-hypocotyl mutant, lh1, and a wild-type-like lh2 mutant in cucumber, Cucumis sativus, which encodes a defective CsPhyB and a key gibberellic acid (GA) biosynthesis enzyme CsGA20ox-2, respectively. The lh2 mutation was epistatic to lh1 and partly suppressed long-hypocotyl phenotype in the lhl1lh2 double mutant. We identified phytochrome interacting factor (PIF) CsPIF3 that played a critical role in integrating the red/far-red and UVB light responses for hypocotyl growth. We show that two modules, CsPhyB-CsPIF3-CsGA20ox-2 (GA oxidase2)-DELLA and CsPIF3-CsARF18 (auxin response factor 18) mediate CsPhyB-regulated hypocotyl elongation through GA and auxin pathways, respectively, in which CsPIF3 binds to the G-/E-box motifs in the promoters of CsGA20ox-2 and CsARF18 to regulate their expression. We also identified a new physical interaction between CsPIF3 and CsUVR8 mediating CsPhyB-dependent, UVB-induced hypocotyl growth inhibition. Our work suggests that hypocotyl growth in cucumber involves a complex interplay of multiple photoreceptor- and phytohormone-mediated signaling pathways that show both conservation with and divergence from those in Arabidopsis.

Local phytochrome signalling limits root growth in light by repressing auxin biosynthesis.

In nature, plant shoots are exposed to light whereas the roots grow in relative darkness. Surprisingly, many root studies rely on in vitro systems that leave the roots exposed to light whilst ignoring the possible effects of this light on root development. Here, we investigated how direct root illumination affects root growth and development in Arabidopsis and tomato. Our results show that in light-grown Arabidopsis roots activation of local phytochrome A and B by far-red or red light inhibits respectively PHYTOCHROME INTERACTING FACTORs 1 or 4, resulting in decreased YUCCA4 and YUCCA6 expression. As a result, auxin levels in the root apex become suboptimal, ultimately resulting in reduced growth of light-grown roots. These findings highlight once more the importance of using in vitro systems where roots are grown in darkness, for studies that focus on root system architecture. Moreover, we show that the response and components of this mechanism are conserved in tomato roots, thus signifying its importance for horticulture as well. Our findings open up new research possibilities to investigate the importance of light-induced root growth inhibition for plant development, possibly by exploring putative correlations with responses to other abiotic signals, such as temperature, gravity, touch, or salt stress.

SALT OVERLY SENSITIVE2 stabilizes phytochrome-interacting factors PIF4 and PIF5 to promote Arabidopsis shade avoidance.

Sun-loving plants trigger the shade avoidance syndrome (SAS) to compete against their neighbors for sunlight. Phytochromes are plant red (R) and far-red (FR) light photoreceptors that play a major role in perceiving the shading signals and triggering SAS. Shade induces a reduction in the level of active phytochrome B (phyB), thus increasing the abundance of PHYTOCHROME-INTERACTING FACTORS (PIFs), a group of growth-promoting transcription factors. However, whether other factors are involved in modulating PIF activity in the shade remains largely obscure. Here, we show that SALT OVERLY SENSITIVE2 (SOS2), a protein kinase essential for salt tolerance, positively regulates SAS in Arabidopsis thaliana. SOS2 directly phosphorylates PIF4 and PIF5 at a serine residue close to their conserved motif for binding to active phyB. This phosphorylation thus decreases their interaction with phyB and posttranslationally promotes PIF4 and PIF5 protein accumulation. Notably, the role of SOS2 in regulating PIF4 and PIF5 protein abundance and SAS is more prominent under salt stress. Moreover, phyA and phyB physically interact with SOS2 and promote SOS2 kinase activity in the light. Collectively, our study uncovers an unexpected role of salt-activated SOS2 in promoting SAS by modulating the phyB-PIF module, providing insight into the coordinated response of plants to salt stress and shade.

Phytochromes enhance SOS2-mediated PIF1 and PIF3 phosphorylation and degradation to promote Arabidopsis salt tolerance.

Soil salinity is one of the most detrimental abiotic stresses affecting plant survival, and light is a core environmental signal regulating plant growth and responses to abiotic stress. However, how light modulates the plant's response to salt stress remains largely obscure. Here, we show that Arabidopsis (Arabidopsis thaliana) seedlings are more tolerant to salt stress in the light than in the dark, and that the photoreceptors phytochrome A (phyA) and phyB are involved in this tolerance mechanism. We further show that phyA and phyB physically interact with the salt tolerance regulator SALT OVERLY SENSITIVE2 (SOS2) in the cytosol and nucleus, and enhance salt-activated SOS2 kinase activity in the light. Moreover, SOS2 directly interacts with and phosphorylates PHYTOCHROME-INTERACTING FACTORS PIF1 and PIF3 in the nucleus. Accordingly, PIFs act as negative regulators of plant salt tolerance, and SOS2 phosphorylation of PIF1 and PIF3 decreases their stability and relieves their repressive effect on plant salt tolerance in both light and dark conditions. Together, our study demonstrates that photoactivated phyA and phyB promote plant salt tolerance by increasing SOS2-mediated phosphorylation and degradation of PIF1 and PIF3, thus broadening our understanding of how plants adapt to salt stress according to their dynamic light environment.

PIFs interact with SWI2/SNF2-related 1 complex subunit 6 to regulate H2A.Z deposition and photomorphogenesis in Arabidopsis

Light is an essential environmental signal perceived by a broad range of photoreceptors in plants. Among them, the red/far-red light receptor phytochromes function to promote photomorphogenesis, which is critical to the survival of seedlings after seeds germination. The basic-helix-loop-helix transcription factors phytochrome-interacting factors (PIFs) are the pivotal direct downstream components of phytochromes. H2A.Z is a highly conserved histone variant regulating gene transcription, and its incorporation into nucleosomes is catalyzed by SWI2/SNF2-related 1 complex, in which SWI2/SNF2-related 1 complex subunit 6 (SWC6) and actin-related protein 6 (ARP6) serve as core subunits. Here, we show that PIFs physically interact with SWC6 in vitro and in vivo, leading to the disassociation of HY5 from SWC6. SWC6 and ARP6 regulate hypocotyl elongation partly through PIFs in red light. PIFs and SWC6 coregulate the expression of auxin-responsive genes such as IAA6, IAA19, IAA20, and IAA29 and repress H2A.Z deposition at IAA6 and IAA19 in red light. Based on previous studies and our findings, we propose that PIFs inhibit photomorphogenesis, at least in part, through repression of H2A.Z deposition at auxin-responsive genes mediated by the interactions of PIFs with SWC6 and promotion of their expression in red light.

The FLOWERING LOCUS T gene expression is controlled by high-irradiance response and external coincidence mechanism in long days in Arabidopsis.

In natural long days, the florigen gene FLOWERING LOCUS T (FT) shows a bimodal expression pattern with morning and dusk peaks in Arabidopsis. This pattern differs from the one observed in the laboratory, and little is known about underlying mechanisms. A red : far-red (R : FR) ratio difference between sunlight and fluorescent light causes this FT pattern mismatch. We showed that bimodal FT expression patterns were induced in a day longer than 14 h with sunlight R : FR (= c. 1) conditions. By circadian gating experiments, we found that cumulative exposure of R : FR-adjusted light (R : FR ratio was adjusted to 1 with FR supplement) spanning from the afternoon to the next morning required full induction of FT in the morning. Conversely, only 2 h of R : FR adjustment in the late afternoon was sufficient for FT induction at dusk. We identified that phytochrome A (phyA) is required for the morning FT expression in response to the R : FR adjustment on the previous day. As a part of this mechanism, we showed that PHYTOCHROME-INTERACTING FACTOR 7 contributes to FT regulation. Our results suggest that phyA-mediated high-irradiance response and the external coincidence mechanism contribute to morning FT induction under natural long-day conditions.

Phosphorylation of PIF3 by MPK6 is required for coordinated regulation of miRNA biogenesis and hypocotyl elongation in Arabidopsis

Light is one of the most important environmental factors that affect overall growth and development in plants. PHYTOCHROME INTERACTING FACTORs (PIFs) are negative regulators of photomorphogenesis. PIFs mediate light responses by interacting with downstream molecular partners and are essential regulators for hypocotyl elongation in Arabidopsis. Light induce activation of phytochromes (phys), promotes rapid phosphorylation, ubiquitination and degradation of PIFs. However the kinase responsible for the phosphorylation of PIFs and the signaling mechanism governing the adaptive changes leading to hypocotyl elongation to differential light intensities is not well understood. Here, we report interaction and phosphorylation of PIF3 by a mitogen-activated protein kinase 6 (MPK6) both in vitro and in vivo. Phosphorylation was significantly abolished when all the PIF3 putative phosphorylation sites were mutated to alanine. Further, we found that PIF3 directly binds to the bHLH binding domain in the promoter of RL responsive miRNA, miR163. Interestingly, phosphorylation status of PIF3 substantially affects the binding of PIF3 and mutated proteins to miR163 promoter. Further, overexpression of PIF3 affects the expression of red light (RL) responsive miRNAs and downstream genes involved in hypocotyl elongation and seedling development. These results suggest that MPK6-PIF3 module functions upstream of RL-responsive miRNAs and PIF3-regulated genes involved in photomorphogenesis, thus interconnecting these pathways together.

Grape phytochrome-interacting factor VvPIF1 negatively regulates carotenoid biosynthesis by repressing VvPSY expression

Phytochrome-interacting factors (PIFs) play important roles in light-mediated secondary metabolism; however, the roles of PIFs in grape fruit carotenogenesis remain unclear. Here, by identifying the PIF family genes in grapes, we focused on the role of VvPIF1 in carotenoid metabolism. During grape berry development, VvPIF1 expression was negatively correlated with carotenoid accumulation and the transcription of phytoene synthase 1/2 (VvPSY1/2), which encodes the major flux-controlling enzymes for carotenoid biosynthesis. Light significantly repressed VvPIF1 expression, but induced the expression of carotenogenic genes including VvPSY1/2. VvPIF1 functioned as a nucleus-localized protein and interacted with the light photoreceptor VvphyB. Overexpression of VvPIF1 resulted in the downregulation of the endogenous PIF1 gene, which may unexpectedly induce carotenoid accumulation and PSY expression in tobacco leaves. The transgenic grape leaves and tomato fruits with high VvPIF1 expression produced a significant decrease in carotenoid concentrations, with suppressed transcription of PSY and other carotenogenic genes. Further biochemical assays demonstrated that VvPIF1 bound directly to the promoters of VvPSY1/2 to inhibit their transcription. Collectively, we conclude that VvPIF1 negatively regulates carotenoid biosynthesis by repressing VvPSY expression in grapes. These findings shed light on the role and mode of action of PIFs in the carotenoid regulatory network of grapes.

Identification and characterization of cotton PHYTOCHROME-INTERACTING FACTORS in temperature-dependent flowering.

PHYTOCHROME INTERACTING FACTORS (PIFs) integrate light and temperature signs to control plant growth and development. However, little is known about PIFs in crop plants such as cotton. Here, we identified 68 PIF proteins and their coding genes from an allotetraploid and three diploid ancestors. Cotton PIFs contain typical ACTIVEPHYA-BINDING (APA) and ACTIVE PHYB-BINDING (APB) motifs by which they bind to phytochrome phyA and phyB, respectively, and have a BASIC HELIX-LOOP-HELIX (bHLH) domain and a nuclear localization sequence necessary for bHLH-type transcription factors. Bioinformatics analysis showed that the promoter of each PIF gene contains multiple cis-acting elements and that the evolution of cotton genomes probably underwent loss, recombination, and tandem replication. Further observations indicated that the sensitivity of cotton PIF expression to high temperature was significantly different from that to low temperature. We found that allotetraploid Gossypium hirsutum PIF4a (GhPIF4a) was induced by high temperature. GhPIF4a promotes flowering in cotton and Arabidopsis and binds to the promoter of GhFT (G. hirsutum FLOWERING LOCUS T), and binding increased with increasing temperature. Our work identifies the evolutionary and structural characteristics and functions of PIF family members in cotton.

The Arabidopsis Phytoglobin 2 mediates phytochrome B (phyB) light signaling responses during somatic embryogenesis.

During the light induction of somatic embryogenesis, phyB-Pfr suppresses Phytoglobin 2, known to elevate nitric oxide (NO). NO depresses Phytochrome Interacting Factor 4 (PIF4) relieving its inhibition on embryogenesis through auxin. An obligatory step of many in vitro embryogenic systems is the somatic-embryogenic transition culminating with the formation of the embryogenic tissue. In Arabidopsis, this transition requires light and is facilitated by high levels of nitric oxide (NO) generated by either suppression of the NO scavenger Phytoglobin 2 (Pgb2), or its removal from the nucleus. Using a previously characterized induction system regulating the cellular localization of Pgb2, we demonstrated the interplay between phytochrome B (phyB) and Pgb2 during the formation of embryogenic tissue. The deactivation of phyB in the dark coincides with the induction of Pgb2 known to reduce the level of NO; consequently, embryogenesis is inhibited. Under light conditions, the active form of phyB depresses the levels of Pgb2 transcripts, thus expecting an increase in cellular NO. Induction of Pgb2 increases Phytochrome Interacting Factor 4 (PIF4) suggesting that high levels of NO repress PIF4. The PIF4 inhibition is sufficient to induce several auxin biosynthetic (CYP79B2, AMI1, and YUCCA 1, 2, and 6) and response (ARF5, 8, and 16) genes, conducive to the formation of the embryonic tissue and production of somatic embryos. Auxin responses mediated by ARF10 and 17 appear to be regulated by Pgb2, possibly through NO, in a PIF4-independent fashion. Overall, this work provides a new and preliminary model integrating Pgb2 (and NO) with phyB in the light regulation of in vitro embryogenesis.

MicroRNA156 conditions auxin sensitivity to enable growth plasticity in response to environmental changes in Arabidopsis

MicroRNAs (miRNAs) play diverse roles in plant development, but whether and how miRNAs participate in thermomorphogenesis remain ambiguous. Here we show that HYPONASTIC LEAVES 1 (HYL1)—a key component of miRNA biogenesis—acts downstream of the thermal regulator PHYTOCHROME INTERACTING FACTOR 4 in the temperature-dependent plasticity of hypocotyl growth in Arabidopsis. A hyl1-2 suppressor screen identified a dominant dicer-like1 allele that rescues hyl1-2’s defects in miRNA biogenesis and thermoresponsive hypocotyl elongation. Genome-wide miRNA and transcriptome analysis revealed microRNA156 (miR156) and its target SQUAMOSA PROMOTER-BINDING-PROTEIN-LIKE 9 (SPL9) to be critical regulators of thermomorphogenesis. Surprisingly, perturbation of the miR156/SPL9 module disengages seedling responsiveness to warm temperatures by impeding auxin sensitivity. Moreover, miR156-dependent auxin sensitivity also operates in the shade avoidance response at lower temperatures. Thus, these results unveil the miR156/SPL9 module as a previously uncharacterized genetic circuit that enables plant growth plasticity in response to environmental temperature and light changes.

Chrysanthemum morifolium β-carotene hydroxylase overexpression promotes Arabidopsis thaliana tolerance to high light stress

The β-carotene hydroxylase gene (BCH) regulates zeaxanthin production in response to high light levels ro protect Chrysanthemum morifolium plants against light-induced damage. In this study, the Chrysanthemum morifolium CmBCH1 and CmBCH2 genes were cloned and their functional importance was assessed by overexpressing them in Arabidopsis thaliana. These transgenic plants were evaluated for gene-related changes in phenotypic characteristics, photosynthetic activity, fluorescence properties, carotenoid biosynthesis, aboveground/belowground biomass, pigment content, and the expression of light-regulated genes under conditions of high light stress relative to wild-type (WT) plants. When exposed to high light stress, WT A. thaliana leaves turned yellow and the overall biomass was reduced compared to that of the transgenic plants. WT plants exposed to high light stress also exhibited significant reductions in the net photosynthetic rate, stomatal conductance, Fv/Fm, qP, and ETR, whereas these changes were not observed in the transgenic CmBCH1 and CmBCH2 plants. Lutein and zaxanthin levels were significantly increased in the transgenic CmBCH1 and CmBCH2 lines, with progressive induction with prolonged light exposure, whereas no significant changes were observed in light-exposed WT plants. The transgenic plants also expressed higher levels of most carotenoid biosynthesis pathway genes, including phytoene synthase (AtPSY), phytoene desaturase (AtPDS), lycopene-β-cyclase (AtLYCB), and ζ-carotene desaturase (AtZDS). The elongated hypocotyl 5 (HY5) and succinate dehydrogenase (SDH) genes were significantly induced following exposure to high light conditions for 12h, whereas phytochrome-interacting factor 7 (PIF7) was significantly downregulated in these plants.

Effect of redroot pigweed interference on antioxidant enzyme and light response of common bean (Phaseolus vulgaris L.) depends on cultivars and growth stages

Redroot Pigweed (Amaranthus retroflexus L.) is an important weed that is highly competitive with common bean. Photosynthetic pigments, the activity of antioxidant enzymes, the relative expression of a number of antioxidant enzyme and light response genes, were studied in three of common bean cultivars and in V4 and R7 stages under Redroot Pigweed free and infested. The presence of weeds reduced the content of chlorophyll, relative chlorophyll and anthocyanin of common bean leaves. With the increase of weed competition, the expression of antioxidant genes and enzymes increased, which indicates the increase of their activity in order to reduce the amount of reactive oxygen species. Among the studied antioxidant enzymes, the activity of catalase and ascorbate peroxidase produced in the leaves was higher than that of superoxide dismutase. With the increase of weed interference, the expression of phytochrome interacting factor 3 (PIF3) gene as a positive regulator of light signals is increased and the expression of phytochrome rapidly regulated1 (PAR1) gene as a negative regulator is decreased. Chlorophyll a/b-binding protein (CAB1) and auxin-responsive protein IAA8 (IAA8) genes also down-regulated with increasing competition. Along with the decrease of CAB expression in the conditions of competition with weeds, the chlorophyll a, b content also decreased. Correlation between gene expression and physiological traits related to them highlights the prominent role of CWCP in maintaining yield potential.