MYB30 Is a Key Negative Regulator of Arabidopsis Photomorphogenic Development That Promotes PIF4 and PIF5 Protein Accumulation in the Light

Green light mediates atypical photomorphogenesis by dual modulation of Arabidopsis phytochromes B and A

Although green light (GL) is located in the middle of the visible light spectrum and regulates a series of plant developmental processes, the mechanism by which it regulates seedling development is largely unknown. In this study, we demonstrated that GL promotes atypical photomorphogenesis in Arabidopsis thaliana via the dual regulations of phytochrome B (phyB) and phyA. Although the Pr-to-Pfr conversion rates of phyB and phyA under GL were lower than those under red light (RL) in a fluence rate-dependent and time-dependent manner, long-term treatment with GL induced high Pfr/Pr ratios of phyB and phyA. Moreover, GL induced the formation of numerous small phyB photobodies in the nucleus, resulting in atypical photomorphogenesis, with smaller cotyledon opening angles and longer hypocotyls in seedlings compared to RL. The abundance of phyA significantly decreased after short- and long-term GL treatments. We determined that four major PHYTOCHROME-INTERACTING FACTORs (PIFs: PIF1, PIF3, PIF4, and PIF5) act downstream of phyB in GL-mediated cotyledon opening. In addition, GL plays opposite roles in regulating different PIFs. For example, under continuous GL, the protein levels of all PIFs decreased, whereas the transcript levels of PIF4 and PIF5 strongly increased compared with dark treatment. Taken together, our work provides a detailed molecular framework for understanding the role of the antagonistic regulations of phyB and phyA in GL-mediated atypical photomorphogenesis.

BBX9 forms feedback loops with PIFs and BBX21 to promote photomorphogenic development

Light is one of the most essential environmental factors that tightly and precisely control various physiological and developmental processes in plants. B-box CONTAINING PROTEINs (BBXs) play central roles in the regulation of light-dependent development. In this study, we report that BBX9 is a positive regulator of light signaling. BBX9 interacts with the red light photoreceptor PHYTOCHROME B (phyB) and transcription factors PHYTOCHROME-INTERACTING FACTORs (PIFs). phyB promotes the stabilization of BBX9 in light, while BBX9 inhibits the transcriptional activation activity of PIFs. In turn, PIFs directly bind to the promoter of BBX9 to repress its transcription. On the other hand, BBX9 associates with the positive regulator of light signaling, BBX21, and enhances its biochemical activity. BBX21 associates with the promoter regions of BBX9 and transcriptionally up-regulates its expression. Collectively, this study unveiled that BBX9 forms a negative feedback loop with PIFs and a positive one with BBX21 to ensure that plants adapt to fluctuating light conditions.

Regulation of cryptochrome-mediated blue light signaling by the ABI4-PIF4 module

ABSCISIC ACID-INSENSITIVE 4 (ABI4) is a pivotal transcription factor which coordinates multiple aspects of plant growth and development as well as plant responses to environmental stresses. ABI4 has been shown to be involved in regulating seedling photomorphogenesis; however, the underlying mechanism remains elusive. Here, we show that the role of ABI4 in regulating photomorphogenesis is generally regulated by sucrose, but ABI4 promotes hypocotyl elongation of Arabidopsis seedlings under blue (B) light under all tested sucrose concentrations. We further show that ABI4 physically interacts with PHYTOCHROME INTERACTING FACTOR 4 (PIF4), a well-characterized growth-promoting transcription factor, and post-translationally promotes PIF4 protein accumulation under B light. Further analyses indicate that ABI4 directly interacts with the B light photoreceptors cryptochromes (CRYs) and inhibits the interactions between CRYs and PIF4, thus relieving CRY-mediated repression of PIF4 protein accumulation. In addition, while ABI4 could directly activate its own expression, CRYs enhance, whereas PIF4 inhibits, ABI4-mediated activation of the ABI4 promoter. Together, our study demonstrates that the ABI4-PIF4 module plays an important role in mediating CRY-induced B light signaling in Arabidopsis.

Maize smart-canopy architecture enhances yield at high densities

Increasing planting density is a key strategy for enhancing maize yields1-3. An ideotype for dense planting requires a 'smart canopy' with leaf angles at different canopy layers differentially optimized to maximize light interception and photosynthesis4-6, among other features. Here we identified leaf angle architecture of smart canopy 1 (lac1), a natural mutant with upright upper leaves, less erect middle leaves and relatively flat lower leaves. lac1 has improved photosynthetic capacity and attenuated responses to shade under dense planting. lac1 encodes a brassinosteroid C-22 hydroxylase that predominantly regulates upper leaf angle. Phytochrome A photoreceptors accumulate in shade and interact with the transcription factor RAVL1 to promote its degradation via the 26S proteasome, thereby inhibiting activation of lac1 by RAVL1 and decreasing brassinosteroid levels. This ultimately decreases upper leaf angle in dense fields. Large-scale field trials demonstrate that lac1 boosts maize yields under high planting densities. To quickly introduce lac1 into breeding germplasm, we transformed a haploid inducer and recovered homozygous lac1 edits from 20 diverse inbred lines. The tested doubled haploids uniformly acquired smart-canopy-like plant architecture. We provide an important target and an accelerated strategy for developing high-density-tolerant cultivars, with lac1 serving as a genetic chassis for further engineering of a smart canopy in maize.

Mitochondrial ATP Synthase beta-Subunit Affects Plastid Retrograde Signaling in Arabidopsis

Plastid retrograde signaling plays a key role in coordinating the expression of plastid genes and photosynthesis-associated nuclear genes (PhANGs). Although plastid retrograde signaling can be substantially compromised by mitochondrial dysfunction, it is not yet clear whether specific mitochondrial factors are required to regulate plastid retrograde signaling. Here, we show that mitochondrial ATP synthase beta-subunit mutants with decreased ATP synthase activity are impaired in plastid retrograde signaling in Arabidopsis thaliana. Transcriptome analysis revealed that the expression levels of PhANGs were significantly higher in the mutants affected in the AT5G08670 gene encoding the mitochondrial ATP synthase beta-subunit, compared to wild-type (WT) seedlings when treated with lincomycin (LIN) or norflurazon (NF). Further studies indicated that the expression of nuclear genes involved in chloroplast and mitochondrial retrograde signaling was affected in the AT5G08670 mutant seedlings treated with LIN. These changes might be linked to the modulation of some transcription factors (TFs), such as LHY (Late Elongated Hypocotyl), PIF (Phytochrome-Interacting Factors), MYB, WRKY, and AP2/ERF (Ethylene Responsive Factors). These findings suggest that the activity of mitochondrial ATP synthase significantly influences plastid retrograde signaling.

Non-transcriptional regulatory activity of SMAX1 and SMXL2 mediates karrikin-regulated seedling response to red light in Arabidopsis

Karrikins and strigolactones govern plant development and environmental responses through closely related signaling pathways. The transcriptional repressor proteins SUPPRESSOR OF MAX2 1 (SMAX1), SMAX1-like2 (SMXL2), and D53-like SMXLs mediate karrikin and strigolactone signaling by directly binding downstream genes or by inhibiting the activities of transcription factors. In this study, we characterized the non-transcriptional regulatory activities of SMXL proteins in Arabidopsis. We discovered that SMAX1 and SMXL2 with mutations in their ethylene-response factor-associated amphiphilic repression (EAR) motif had undetectable or weak transcriptional repression activities but still partially rescued the hypocotyl elongation defects and fully reversed the cotyledon epinasty defects of the smax1 smxl2 mutant. SMAX1 and SMXL2 directly interact with PHYTOCHROME INTERACTION FACTOR 4 (PIF4) and PIF5 to enhance their protein stability by interacting with phytochrome B (phyB) and suppressing the association of phyB with PIF4 and PIF5. The karrikin-responsive genes were then identified by treatment with GR24ent-5DS, a GR24 analog showing karrikin activity. Interestingly, INDOLE-3-ACETIC ACID INDUCIBLE 29 (IAA29) expression was repressed by GR24ent-5DS treatment in a PIF4- and PIF5-dependent and EAR-independent manner, whereas KARRIKIN UPREGULATED F-BOX 1 (KUF1) expression was induced in a PIF4- and PIF5-independent and EAR-dependent manner. Furthermore, the non-transcriptional regulatory activity of SMAX1, which is independent of the EAR motif, had a global effect on gene expression. Taken together, these results indicate that non-transcriptional regulatory activities of SMAX1 and SMXL2 mediate karrikin-regulated seedling response to red light.

PHYTOCHROME A controls the DNA damage response and cell death tolerance within the Arabidopsis root meristem

The DNA damage response avoids mutations into dividing cells. Here, we analysed the role of photoreceptors on the restriction of root growth imposed by genotoxic agents and its relationship with cell viability and performance of meristems. Comparison of root growth of Arabidopsis WT, phyA-211, phyB-9, and phyA-211phyB-9 double mutants unveiled a critical role for phytochrome A (PhyA) in protecting roots from genotoxic stress, regeneration and cell replenishment in the meristematic zone. PhyA was located on primary root tips, where it influences genes related to the repair of DNA, including ERF115 and RAD51. Interestingly, phyA-211 mutants treated with zeocin failed to induce the expression of the repressor of cell cycle MYB3R3, which correlated with expression of the mitotic cyclin CycB1, suggesting that PhyA is required for safeguarding the DNA integrity during cell division. Moreover, the growth of the primary roots of PhyA downstream component HY5 and root growth analyses in darkness suggest that cell viability and DNA damage responses within root meristems may act independently from light and photomorphogenesis. These data support novel roles for PhyA as a key player for stem cell niche maintenance and DNA damage responses, which are critical for proper root growth.

PIF4 interacts with ABI4 to serve as a transcriptional activator complex to promote seed dormancy by enhancing ABA biosynthesis and signaling

Transcriptional regulation plays a key role in the control of seed dormancy, and many transcription factors (TFs) have been documented. However, the mechanisms underlying the interactions between different TFs within a transcriptional complex regulating seed dormancy remain largely unknown. Here, we showed that TF PHYTOCHROME-INTERACTING FACTOR4 (PIF4) physically interacted with the abscisic acid (ABA) signaling responsive TF ABSCISIC ACID INSENSITIVE4 (ABI4) to act as a transcriptional complex to promote ABA biosynthesis and signaling, finally deepening primary seed dormancy. Both pif4 and abi4 single mutants exhibited a decreased primary seed dormancy phenotype, with a synergistic effect in the pif4/abi4 double mutant. PIF4 binds to ABI4 to form a heterodimer, and ABI4 stabilizes PIF4 at the protein level, whereas PIF4 does not affect the protein stabilization of ABI4. Subsequently, both TFs independently and synergistically promoted the expression of ABI4 and NCED6, a key gene for ABA anabolism. The genetic evidence is also consistent with the phenotypic, physiological and biochemical analysis results. Altogether, this study revealed a transcriptional regulatory cascade in which the PIF4-ABI4 transcriptional activator complex synergistically enhanced seed dormancy by facilitating ABA biosynthesis and signaling.

Liquid-liquid phase separation of TZP promotes PPK-mediated phosphorylation of the phytochrome A photoreceptor

Phytochrome A (phyA) is the plant far-red (FR) light photoreceptor and plays an essential role in regulating photomorphogenic development in FR-rich conditions, such as canopy shade. It has long been observed that phyA is a phosphoprotein in vivo; however, the protein kinases that could phosphorylate phyA remain largely unknown. Here we show that a small protein kinase family, consisting of four members named PHOTOREGULATORY PROTEIN KINASES (PPKs) (also known as MUT9-LIKE KINASES), directly phosphorylate phyA in vitro and in vivo. In addition, TANDEM ZINC-FINGER/PLUS3 (TZP), a recently characterized phyA-interacting protein required for in vivo phosphorylation of phyA, is also directly phosphorylated by PPKs. We reveal that TZP contains two intrinsically disordered regions in its amino-terminal domain that undergo liquid-liquid phase separation (LLPS) upon light exposure. The LLPS of TZP promotes colocalization and interaction between PPKs and phyA, thus facilitating PPK-mediated phosphorylation of phyA in FR light. Our study identifies PPKs as a class of protein kinases mediating the phosphorylation of phyA and demonstrates that the LLPS of TZP contributes significantly to more production of the phosphorylated phyA form in FR light.

RHA2b-mediated MYB30 degradation facilitates MYB75-regulated, sucrose-induced anthocyanin biosynthesis in Arabidopsis seedlings

Anthocyanins play diverse roles in plant physiology and stress adaptation. In Arabidopsis, the MYB-bHLH-WD40 (MBW) complex has a crucial role in the regulation of anthocyanin synthesis. Here, we report that the R2R3-MYB transcription factor MYB30 and the ubiquitin E3 ligase RHA2b participate in anthocyanin biosynthesis through regulation of the MBW complex. MYB30 was found to negatively regulate sucrose-induced anthocyanin biosynthesis in Arabidopsis seedlings. Expression of multiple genes involved in flavonoid or anthocyanin biosynthesis was affected in the myb30 mutant, and MYB30 directly repressed the expression of MYB75, which encodes a core component of the MBW complex, by binding to its promoter. Moreover, MYB30 physically interacted with MYB75 to inhibit its activity by repressing MBW complex assembly. In addition, sucrose treatment significantly promoted MYB30 degradation via the action of RHA2b. The ubiquitination and degradation of MYB30 were significantly attenuated in the rha2b mutant under high-sucrose treatment, and further analysis showed that MYB75 directly promoted RHA2b expression in response to high sucrose. Our work thus reveals an anthocyanin biosynthetic regulatory module, RHA2b-MYB30, that controls the function of the MBW complex via MYB75. The repression of MYB75 by MYB30 is released by MYB75-induced RHA2b expression, thus ensuring the self-activation of MYB75 when anthocyanin synthesis is needed.

Histone methylation readers MRG1/2 interact with PIF4 to promote thermomorphogenesis in Arabidopsis

Warm ambient conditions induce thermomorphogenesis and affect plant growth and development. However, the chromatin regulatory mechanisms involved in thermomorphogenesis remain largely obscure. In this study, we show that the histone methylation readers MORF-related gene 1 and 2 (MRG1/2) are required to promote hypocotyl elongation in response to warm ambient conditions. A transcriptome sequencing analysis indicates that MRG1/2 and phytochrome interacting factor 4 (PIF4) coactivate a number of thermoresponsive genes, including YUCCA8, which encodes a rate-limiting enzyme in the auxin biosynthesis pathway. Additionally, MRG2 physically interacts with PIF4 to bind to thermoresponsive genes and enhances the H4K5 acetylation of the chromatin of target genes in a PIF4-dependent manner. Furthermore, MRG2 competes with phyB for binding to PIF4 and stabilizes PIF4 in planta. Our study indicates that MRG1/2 activate thermoresponsive genes by inducing histone acetylation and stabilizing PIF4 in Arabidopsis.

Light controls mesophyll-specific post-transcriptional splicing of photoregulatory genes by AtPRMT5

Light plays a central role in plant growth and development, providing an energy source and governing various aspects of plant morphology. Previous study showed that many polyadenylated full-length RNA molecules within the nucleus contain unspliced introns (post-transcriptionally spliced introns, PTS introns), which may play a role in rapidly responding to changes in environmental signals. However, the mechanism underlying post-transcriptional regulation during initial light exposure of young, etiolated seedlings remains elusive. In this study, we used FLEP-seq2, a Nanopore-based sequencing technique, to analyze nuclear RNAs in Arabidopsis (Arabidopsis thaliana) seedlings under different light conditions and found numerous light-responsive PTS introns. We also used single-nucleus RNA sequencing (snRNA-seq) to profile transcripts in single nucleus and investigate the distribution of light-responsive PTS introns across distinct cell types. We established that light-induced PTS introns are predominant in mesophyll cells during seedling de-etiolation following exposure of etiolated seedlings to light. We further demonstrated the involvement of the splicing-related factor A. thaliana PROTEIN ARGININE METHYLTRANSFERASE 5 (AtPRMT5), working in concert with the E3 ubiquitin ligase CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1), a critical repressor of light signaling pathways. We showed that these two proteins orchestrate light-induced PTS events in mesophyll cells and facilitate chloroplast development, photosynthesis, and morphogenesis in response to ever-changing light conditions. These findings provide crucial insights into the intricate mechanisms underlying plant acclimation to light at the cell-type level.

Genome-wide identification, phylogeny and expression analysis of the R2R3-MYB gene family in quinoa (Chenopodium quinoa) under abiotic stress

The MYB transcription factor (TF) are among the largest gene families of plants being responsible for several biological processes. The R2R3-MYB gene family are integral player regulating plant primary and secondary metabolism, growth and development, and responses to hormones and stresses. The phylogenetic analysis combined with gene structure analysis and motif determination resulted in division of R2R3-MYB gene family into 27 subgroups. Evidence generated from synteny analyses indicated that CqR2R3-MYBs gene family is featured by tandem and segmental duplication events. On the basis of RNA-Seq data, the expression patterns of different tissues under salt treatment were investigated resulting CqR2R3-MYB genes high expression both in roots and stem of quinoa (Chenopodium quinoa ) plants. More than half of CqR2R3-MYB genes showed expression under salt stress. Based on this result, CqR2R3-MYB s may regulate quinoa plant growth development and resistance to abiotic stresses. These findings provided comprehensive insights on role of CqR2R3-MYBs gene family members in quinoa and candidate MYB gene family members can be further studies on their role for abiotic stress tolerance in crop plants.

S-nitrosylation of the transcription factor MYB30 facilitates nitric oxide-promoted seed germination in Arabidopsis

The gaseous signaling molecule nitric oxide (NO) plays an important role in breaking seed dormancy. NO induces a decrease in abscisic acid (ABA) content by transcriptionally activating its catabolic enzyme, the ABA 8'-hydroxylase CYP707A2. However, the underlying mechanism of this process remains unclear. Here, we report that the transcription factor MYB30 plays a critical role in NO-induced seed germination in Arabidopsis (Arabidopsis thaliana). MYB30 loss-of-function attenuates NO-mediated seed dormancy breaking. MYB30 triggers a NO-induced decrease in ABA content during germination by directly promoting CYP707A2 expression. NO induces S-nitrosylation at Cys-49 of MYB30 and enhances its transcriptional activity. Conversely, the ABA receptors PYRABACTIN RESISTANCE1 (PYR1)/PYR1-LIKE (PYL)/REGULATORY COMPONENTS OF ABA RECEPTORS (RCAR) interact with MYB30 and repress its transcriptional activity. ABA promotes the interaction between PYL4 and MYB30, whereas S-nitrosylation releases the PYL4-mediated inhibition of MYB30 by interfering with the PYL4-MYB30 interaction. Genetic analysis showed that MYB30 functions downstream of PYLs during seed dormancy and germination in response to NO. Furthermore, MYB30 mutation significantly represses the reduced dormancy phenotype and the enhanced CYP707A2 expression of the pyr1 pyl1 pyl2 pyl4 quadruple mutant. Our findings reveal that S-nitrosylation of MYB30 precisely regulates the balance of seed dormancy and germination, providing insights into the underlying mechanism of NO-promoted seed germination.

Genome-wide identification and comprehensive analysis of the phytochrome-interacting factor (PIF) gene family in wheat

Phytochrome-interacting factors (PIFs) are essential transcription factors for plant growth, development, and stress responses. Although PIF genes have been extensively studied in many plant species, they have not been thoroughly investigated in wheat. Here, we identified 18 PIF genes in cultivated hexaploid wheat (Triticum aestivum L). Phylogenetic analysis, exon-intron structures, and motif compositions revealed the presence of four distinct groups of TaPIFs. Genome-wide collinearity analysis of PIF genes revealed the evolutionary history of PIFs in wheat, Oryza sativa, and Brachypodium distachyon. Cis-regulatory element analysis suggested that TaPIF genes indicated participated in plant development and stress responses. Subcellular localization assays indicated that TaPIF2-1B and TaPIF4-5B were transcriptionally active. Both were found to be localized to the nucleus. Gene expression analyses demonstrated that TaPIFs were primarily expressed in the leaves and were induced by various biotic and abiotic stresses and phytohormone treatments. This study provides new insights into PIF-mediated stress responses and lays a strong foundation for future investigation of PIF genes in wheat.

Arabidopsis transcription factor TCP13 promotes shade avoidance syndrome-like responses by directly targeting a subset of shade-responsive gene promoters

TCP13 belongs to a subgroup of TCP transcription factors implicated in the shade avoidance syndrome (SAS), but its exact role remains unclear. Here, we show that TCP13 promotes the SAS-like response by enhancing hypocotyl elongation and suppressing flavonoid biosynthesis as a part of the incoherent feed-forward loop in light signaling. Shade is known to promote the SAS by activating PHYTOCHROME-INTERACTING FACTOR (PIF)-auxin signaling in plants, but we found no evidence in a transcriptome analysis that TCP13 activates PIF-auxin signaling. Instead, TCP13 mimics shade by activating the expression of a subset of shade-inducible and cell elongation-promoting SAUR genes including SAUR19, by direct targeting of their promoters. We also found that TCP13 and PIF4, a molecular proxy for shade, repress the expression of flavonoid biosynthetic genes by directly targeting both shared and distinct sets of biosynthetic gene promoters. Together, our results indicate that TCP13 promotes the SAS-like response by directly targeting a subset of shade-responsive genes without activating the PIF-auxin signaling pathway.

Assessing the Function of CBF1 in Modulating the Interaction Between Phytochrome B and PIF4

Phytochromes are red (R) and far-red (FR) light photoreceptors in plants. Upon light exposure, photoactivated phytochromes translocate into the nucleus, where they interact with their partner proteins to transduce light signals. The yeast two-hybrid (Y2H) system is a powerful technique for rapidly identifying and verifying protein-protein interactions, and PHYTOCHROME-INTERACTING FACTOR3 (PIF3), the founding member of the PIF proteins, was initially identified in a Y2H screen for phytochrome B (phyB)-interacting proteins. Recently, we developed a yeast three-hybrid (Y3H) system by introducing an additional vector into this Y2H system, and thus a new regulator could be co-expressed and its role in modulating the interactions between phytochromes and their signaling partners could be examined. By employing this Y3H system, we recently showed that both MYB30 and CBF1, two negative regulators of seedlings photomorphogenesis, act to inhibit the interactions between phyB and PIF4/PIF5. In this chapter, we will use the CBF1-phyB-PIF4 module as an example and describe the detailed procedure for performing this Y3H assay. It will be intriguing and exciting to explore the potential usage of this Y3H system in future research.

Epicuticular wax accumulation and regulation of wax pathway gene expression during bioenergy Sorghum stem development

Bioenergy sorghum is a drought-tolerant high-biomass C4 grass targeted for production on annual cropland marginal for food crops due primarily to abiotic constraints. To better understand the overall contribution of stem wax to bioenergy sorghum's resilience, the current study characterized sorghum stem cuticular wax loads, composition, morphometrics, wax pathway gene expression and regulation using vegetative phase Wray, R07020, and TX08001 genotypes. Wax loads on sorghum stems (~103-215 µg/cm2) were much higher than Arabidopsis stem and leaf wax loads. Wax on developing sorghum stem internodes was enriched in C28/30 primary alcohols (~65%) while stem wax on fully developed stems was enriched in C28/30 aldehydes (~80%). Scanning Electron Microscopy showed minimal wax on internodes prior to the onset of elongation and that wax tubules first appear associated with cork-silica cell complexes when internode cell elongation is complete. Sorghum homologs of genes involved in wax biosynthesis/transport were differentially expressed in the stem epidermis. Expression of many wax pathway genes (i.e., SbKCS6, SbCER3-1, SbWSD1, SbABCG12, SbABCG11) is low in immature apical internodes then increases at the onset of stem wax accumulation. SbCER4 is expressed relatively early in stem development consistent with accumulation of C28/30 primary alcohols on developing apical internodes. High expression of two SbCER3 homologs in fully elongated internodes is consistent with a role in production of C28/30 aldehydes. Gene regulatory network analysis aided the identification of sorghum homologs of transcription factors that regulate wax biosynthesis (i.e., SbSHN1, SbWRI1/3, SbMYB94/96/30/60, MYS1) and other transcription factors that could regulate and specify expression of the wax pathway in epidermal cells during cuticle development.

Light signaling as cellular integrator of multiple environmental cues in plants

Plants being sessile need to rapidly adapt to the constantly changing environment through modifications in their internal clock, metabolism, and gene expression. They have evolved an intricate system to perceive and transfer the signals from the primary environmental factors namely light, temperature and water to regulate their growth development and survival. Over past few decades rigorous research using molecular genetics approaches, especially in model plant Arabidopsis, has resulted in substantial progress in discovering various photoreceptor systems and light signaling components. In parallel several molecular pathways operating in response to other environmental cues have also been elucidated. Interestingly, the studies have shown that expression profiles of genes involved in photomorphogenesis can undergo modulation in response to other cues from the environment. Recently, the photoreceptor, PHYB, has been shown to function as a thermosensor. Downstream components of light signaling pathway like COP1 and PIF have also emerged as integrating hubs for various kinds of signals. All these findings indicate that light signaling components may act as central integrator of various environmental cues to regulate plant growth and development processes. In this review, we present a perspective on cross talk of signaling mechanisms induced in response to myriad array of signals and their integration with the light signaling components. By putting light signals on the central stage, we propose the possibilities of enhancing plant resilience to the changing environment by fine-tuning the genetic manipulation of its signaling components in the future.

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.

PUB25 and PUB26 dynamically modulate ICE1 stability via differential ubiquitination during cold stress in Arabidopsis

Ubiquitination modulates protein turnover or activity depending on the number and location of attached ubiquitin (Ub) moieties. Proteins marked by a lysine 48 (K48)-linked polyubiquitin chain are usually targeted to the 26S proteasome for degradation; however, other polyubiquitin chains, such as those attached to K63, usually regulate other protein properties. Here, we show that 2 PLANT U-BOX E3 ligases, PUB25 and PUB26, facilitate both K48- and K63-linked ubiquitination of the transcriptional regulator INDUCER OF C-REPEAT BINDING FACTOR (CBF) EXPRESSION1 (ICE1) during different periods of cold stress in Arabidopsis (Arabidopsis thaliana), thus dynamically modulating ICE1 stability. Moreover, PUB25 and PUB26 attach both K48- and K63-linked Ub chains to MYB15 in response to cold stress. However, the ubiquitination patterns of ICE1 and MYB15 mediated by PUB25 and PUB26 differ, thus modulating their protein stability and abundance during different stages of cold stress. Furthermore, ICE1 interacts with and inhibits the DNA-binding activity of MYB15, resulting in an upregulation of CBF expression. This study unravels a mechanism by which PUB25 and PUB26 add different polyubiquitin chains to ICE1 and MYB15 to modulate their stability, thereby regulating the timing and degree of cold stress responses in plants.

Deciphering transcriptional mechanisms of maize internodal elongation by regulatory network analysis

The lengths of the basal internodes is an important factor for lodging resistance of maize (Zea mays). In this study, foliar application of coronatine (COR) to 10 cultivars at the V8 growth stage had different suppression effects on the length of the eighth internode, with three being categorized as strong-inhibition cultivars (SC), five as moderate (MC), and two as weak (WC). RNA-sequencing of the eighth internode of the cultivars revealed a total of 7895 internode elongation-regulating genes, including 777 transcription factors (TFs). Genes related to the hormones cytokinin, gibberellin, auxin, and ethylene in the SC group were significantly down-regulated compared to WC, and more cell-cycle regulatory factors and cell wall-related genes showed significant changes, which severely inhibited internode elongation. In addition, we used EMSAs to explore the direct regulatory relationship between two important TFs, ZmABI7 and ZmMYB117, which regulate the cell cycle and cell wall modification by directly binding to the promoters of their target genes ZmCYC1, ZmCYC3, ZmCYC7, and ZmCPP1. The transcriptome reported in this study will provide a useful resource for studying maize internode development, with potential use for targeted genetic control of internode length to improve the lodging resistance of maize.

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.

Functions of Phytochrome-Interacting Factors (PIFs) in the regulation of plant growth and development: A comprehensive review

Transcription factors play important roles in governing plant responses upon changes in their ambient conditions. Any fluctuation in the supply of critical requirements for plants, such as optimum light, temperature, and water leads to the reprogramming of gene-signaling pathways. At the same time, plants also evaluate and shift their metabolism according to the various stages of development. Phytochrome-Interacting Factors are one of the most important classes of transcription factors that regulate both developmental and external stimuli-based growth of plants. This review focuses on the identification of PIFs in various organisms, regulation of PIFs by various proteins, functions of PIFs of Arabidopsis in diverse developmental pathways such as seed germination, photomorphogenesis, flowering, senescence, seed and fruit development, and external stimuli-induced plant responses such as shade avoidance response, thermomorphogenesis, and various abiotic stress responses. Recent advances related to the functional characterization of PIFs of crops such as rice, maize, and tomato have also been incorporated in this review, to ascertain the potential of PIFs as key regulators to enhance the agronomic traits of these crops. Thus, an attempt has been made to provide a holistic view of the function of PIFs in various processes in plants.

MIW1 participates in ABA signaling through the regulation of MYB30 in Arabidopsis

Seed germination and seedling establishment are critical biological processes, and their underlying molecular mechanisms have practical implications. The ABA signaling during seed germination and early seedling development is negatively regulated by transcription factor MYB30, but its interaction partners and downstream targets are not fully understood. In this study, we identified MIW1 (MYB30-interacting WD40 protein 1), a WD40 protein that could interact with MYB30 and promote its degradation. In the miw1 mutant, the MYB30 protein became more stable. MIW1 enhanced the ABA-mediated inhibition of postgerminative development. The miw1 mutants became hyposensitive to exogenous ABA, and this effect was suppressed by mutations in MYB30. Furthermore, we found that MYB30 negatively regulated the expression of the ABA receptor genes PYR1/PYL/RCARs. The changes in PYLs expression during early seedling development or under ABA treatment became more pronounced in the myb30 mutant. ChIP-qPCR analyses showed MYB30 could directly bind to the promoters of PYL11 and PYL12. Our study reveals that the WD40 protein MIW1 promotes the expression of PYLs by destabilizing MYB30, thus positively regulating the ABA signaling during postgermination in Arabidopsis.

Screening Candidate Genes at the Co Locus Conferring to the Columnar Growth Habit in Apple (Malus × Domestica Borkh.)

The columnar growth trait of apple (Malus × domestica Borkh.) is genetically controlled by the Columnar (Co) locus on 10 chromosomes, including several candidate genes. Except for MdCo31, other candidate genes at the Co locus are less elucidated. In this study, a strategy of step-by-step screening was adopted to select 11 candidate genes by experimental cloning, transient expression, and genetic transformation. There existed several SNPs in four genes by sequence alignment in columnar and non-columnar apples. Two genes were detected in the nucleus and three genes in the cell membrane, other genes were located in multiple cellular structures by subcellular location. Ectopic expression demonstrated that more branching occurred in MdCo38-OE by upregulating NtPIN1 and NtGA2ox and enlarged leaves in MdCo41-OE tobaccos by upregulating NtCCDs. Transcripts of MdCo38 and MdCo41 were associated with the Co genotypes in apples. The results indicate that MdCo38 and MdCo41 are involved in the columnar growth phenotype in apple, probably through altering polar auxin transport, active gibberellin levels, and strigolactone biosynthesis.

A 5.5-kb LTR-retrotransposon insertion inside phytochrome B gene (CsPHYB) results in long hypocotyl and early flowering in cucumber (Cucumis sativus L.)

The novel spontaneous long hypocotyl and early flowering (lhef) mutation in cucumber is due to a 5551-bp LTR-retrotransposon insertion in CsPHYB gene encoding PHYTOCHROME B, which plays a major role in regulating photomorphogenic hypocotyl growth and flowering. Hypocotyl length and flowering time are important for establishing high-quality seedlings in modern cucumber production, but little is known for the underlying molecular mechanisms of these two traits. In this study, a spontaneous cucumber long hypocotyl and early flowering mutant was identified and characterized. Based on multiple lines of evidence, we show that cucumber phytochrome B (CsPHYB) is the candidate gene for this mutation, and a 5551-bp LTR-retrotransposon insertion in the first exon of CsPHYB was responsible for the mutant phenotypes. Uniqueness of the mutant allele at CsPHYB was verified in 114 natural cucumber lines. Ectopic expression of the CsPHYB in Arabidopsis phyB mutant rescued the long hypocotyl and early flowering phenotype of phyB-9 mutant. The wild-type CsPHYB protein was localized on the membrane and cytoplasm under white light condition, whereas in the nucleus under red light, it is consistent with its roles as a red-light photoreceptor in Arabidopsis. However, the mutant csphyb protein was localized on the membrane and cytoplasm under both white and red-light conditions. Expression dynamics of CsPHYB and several cell elongation-related genes were positively correlated with hypocotyl elongation; the transcription levels of key positive and negative regulators for flowering time were also consistent with the anthesis dates in the mutant and wild-type plants. Yeast two hybrid and bimolecular fluorescence complementation assays identified physical interactions between CsPHYB and phytochrome interacting factor 3/4 (CsPIF3/4). These findings will provide new insights into the roles of the CsPHYB in cucumber hypocotyl growth and flowering time.

Actin Depolymerization Factor ADF1 Regulated by MYB30 Plays an Important Role in Plant Thermal Adaptation

Actin filaments are essential for plant adaptation to high temperatures. However, the molecular mechanisms of actin filaments in plant thermal adaptation remain unclear. Here, we found that the expression of Arabidopsis actin depolymerization factor 1 (AtADF1) was repressed by high temperatures. Compared with wild-type seedlings (WT), the mutation of AtADF1 and the overexpression of AtADF1 led to promoted and inhibited plant growth under high temperature conditions, respectively. Further, high temperatures induced the stability of actin filaments in plants. Compared with WT, Atadf1-1 mutant seedlings showed more stability of actin filaments under normal and high temperature conditions, while the AtADF1 overexpression seedlings showed the opposite results. Additionally, AtMYB30 directly bound to the promoter of AtADF1 at a known AtMYB30 binding site, AACAAAC, and promoted the transcription of AtADF1 under high temperature treatments. Genetic analysis further indicated that AtMYB30 regulated AtADF1 under high temperature treatments. Chinese cabbage ADF1 (BrADF1) was highly homologous with AtADF1. The expression of BrADF1 was inhibited by high temperatures. BrADF1 overexpression inhibited plant growth and reduced the percentage of actin cable and the average length of actin filaments in Arabidopsis, which were similar to those of AtADF1 overexpression seedlings. AtADF1 and BrADF1 also affected the expression of some key heat response genes. In conclusion, our results indicate that ADF1 plays an important role in plant thermal adaptation by blocking the high-temperature-induced stability of actin filaments and is directly regulated by MYB30.

Phytochrome B regulates reactive oxygen signaling during abiotic and biotic stress in plants

Reactive oxygen species (ROS) and the photoreceptor protein phytochrome B (phyB) play a key role in plant acclimation to stress. However, how phyB that primarily functions in the nuclei impacts ROS signaling mediated by respiratory burst oxidase homolog (RBOH) proteins that reside on the plasma membrane, during stress, is unknown. Arabidopsis thaliana and Oryza sativa mutants, RNA-Seq, bioinformatics, biochemistry, molecular biology, and whole-plant ROS imaging were used to address this question. Here, we reveal that phyB and RBOHs function as part of a key regulatory module that controls apoplastic ROS production, stress-response transcript expression, and plant acclimation in response to excess light stress. We further show that phyB can regulate ROS production during stress even if it is restricted to the cytosol and that phyB, respiratory burst oxidase protein D (RBOHD), and respiratory burst oxidase protein F (RBOHF) coregulate thousands of transcripts in response to light stress. Surprisingly, we found that phyB is also required for ROS accumulation in response to heat, wounding, cold, and bacterial infection. Our findings reveal that phyB plays a canonical role in plant responses to biotic and abiotic stresses, regulating apoplastic ROS production, possibly while at the cytosol, and that phyB and RBOHD/RBOHF function in the same regulatory pathway.

MYB30 integrates light signals with antioxidant biosynthesis to regulate plant responses during postsubmergence recovery

Submergence is an abiotic stress that limits agricultural production world-wide. Plants sense oxygen levels during submergence and postsubmergence reoxygenation and modulate their responses. Increasing evidence suggests that completely submerged plants are often exposed to low-light stress, owing to the depth and turbidity of the surrounding water; however, how light availability affects submergence tolerance remains largely unknown. Here, we showed that Arabidopsis thaliana MYB DOMAIN PROTEIN30 (MYB30) is an important transcription factor that integrates light signaling and postsubmergence stress responses. MYB DOMAIN PROTEIN30 protein abundance decreased upon submergence and accumulated during reoxygenation. Under submergence conditions, CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1), a central regulator of light signaling, caused the ubiquitination and degradation of MYB30. In response to desubmergence, however, light-induced MYB30 interacted with MYC2, a master transcription factor involved in jasmonate signaling, and activated the expression of the VITAMIN C DEFECTIVE1 (VTC1) and GLUTATHIONE SYNTHETASE1 (GSH1) gene families to enhance antioxidant biosynthesis. Consistent with this, the myb30 knockout mutant showed increased sensitivity to submergence, which was partially rescued by overexpression of VTC1 or GSH1. Thus, our findings uncover the mechanism by which the COP1-MYB30 module integrates light signals with cellular oxidative homeostasis to coordinate plant responses to postsubmergence stress.

Phytochrome B regulates reactive oxygen signaling during abiotic and biotic stress in plants

Reactive oxygen species (ROS) and the photoreceptor protein phytochrome B (phyB) play a key role in plant acclimation to stress. However, how phyB that primarily functions in the nuclei impacts ROS signaling mediated by respiratory burst oxidase homolog (RBOH) proteins that reside on the plasma membrane, during stress, is unknown. Arabidopsis thaliana and Oryza sativa mutants, RNA-Seq, bioinformatics, biochemistry, molecular biology, and whole-plant ROS imaging were used to address this question. Here, we reveal that phyB and RBOHs function as part of a key regulatory module that controls apoplastic ROS production, stress-response transcript expression, and plant acclimation in response to excess light stress. We further show that phyB can regulate ROS production during stress even if it is restricted to the cytosol and that phyB, respiratory burst oxidase protein D (RBOHD), and respiratory burst oxidase protein F (RBOHF) coregulate thousands of transcripts in response to light stress. Surprisingly, we found that phyB is also required for ROS accumulation in response to heat, wounding, cold, and bacterial infection. Our findings reveal that phyB plays a canonical role in plant responses to biotic and abiotic stresses, regulating apoplastic ROS production, possibly while at the cytosol, and that phyB and RBOHD/RBOHF function in the same regulatory pathway.

A MYB Transcription Factor Atlas Provides Insights into the Evolution of Environmental Adaptations in Plants

The MYB transcription factor superfamily includes key regulators of plant development and responses to environmental changes. The diversity of lifestyles and morphological characteristics exhibited by plants are potentially associated with the genomic dynamics of the MYB superfamily. With the release of the plant genomes, a comprehensive phylogenomic analysis of the MYB superfamily across Viridiplantae is allowed. The present study performed phylogenetic, phylogenomic, syntenic, horizontal gene transfer, and neo/sub-functionalization analysis of the MYB superfamily to explore the evolutionary contributions of MYB members to species diversification, trait formation, and environmental adaptation in 437 different plant species. We identified major changes in copy number variation and genomic context within subclades across lineages. Multiple MYB subclades showed highly conserved copy number patterns and synteny across flowering plants, whereas others were more dynamic and showed lineage-specific patterns. As examples of lineage-specific morphological divergence, we hypothesize that the gain of a MYB orthogroup associated with flower development and environmental responses and an orthogroup associated with auxin and wax biosynthesis in angiosperms were correlated with the emergence of flowering plants, unbiased neo-/sub-functionalization of gene duplicates contributed to environmental adaptation, and species-specific neo-/sub-functionalization contributed to phenotype divergence between species. Transposable element insertion in promoter regions may have facilitated the sub-/neo-functionalization of MYB genes and likely played a tissue-specific role contributing to sub-/neo-functionalization in plant root tissues. This study provides new insights into the evolutionary divergence of the MYB superfamily across major flowering and non-flowering lineages and emphasizes the need for lineage-/tissue-specific characterization to further understand trait variability and environmental adaptation.

Co-regulation of mitochondrial and chloroplast function: Molecular components and mechanisms

The metabolic interdependence, interactions, and coordination of functions between chloroplasts and mitochondria are established and intensively studied. However, less is known about the regulatory components that control these interactions and their responses to external stimuli. Here, we outline how chloroplastic and mitochondrial activities are coordinated via common components involved in signal transduction pathways, gene regulatory events, and post-transcriptional processes. The endoplasmic reticulum emerges as a point of convergence for both transcriptional and post-transcriptional pathways that coordinate chloroplast and mitochondrial functions. Although the identification of molecular components and mechanisms of chloroplast and mitochondrial signaling increasingly suggests common players, this raises the question of how these allow for distinct organelle-specific downstream pathways. Outstanding questions with respect to the regulation of post-transcriptional pathways and the cell and/or tissue specificity of organelle signaling are crucial for understanding how these pathways are integrated at a whole-plant level to optimize plant growth and its response to changing environmental conditions.

MYB30 and MYB14 form a repressor-activator module with WRKY8 that controls stilbene biosynthesis in grapevine

When exposed to pathogen infection or ultraviolet (UV) radiation, grapevine (Vitis vinifera) plants rapidly accumulate the stilbenoid resveratrol (Res) with concomitant increase of stilbene synthase (STS), the key enzyme in stilbene biosynthesis. Although a few transcription factors have been shown to regulate STSs, the molecular mechanism governing the regulation of STSs is not well elucidated. Our previous work showed that a VvMYB14-VvWRKY8 regulatory loop fine-tunes stilbene biosynthesis in grapevine through protein-protein interaction; overexpression of VvWRKY8 down-regulates VvMYB14 and VvSTS15/21; and application of exogenous Res up-regulates WRKY8 expression. Here, we identified an R2R3-MYB repressor, VvMYB30, which competes with the activator VvMYB14 for binding to the common binding sites in the VvSTS15/21 promoter. Similar to VvMYB14, VvMYB30 physically interacts with VvWRKY8 through their N-termini, forming a complex that does not bind DNA. Exposure to UV-B/C stress induces VvMYB14, VvWRKY8, and VvSTS15/21, but represses VvMYB30 in grapevine leaves. In addition, MYB30 expression is up-regulated by VvWRKY8-overexpression or exogenous Res. These findings suggest that the VvMYB14-VvWRKY8-VvMYB30 regulatory circuit allows grapevine to respond to UV stress by producing Res and prevents over-accumulation of Res to balance metabolic costs. Our work highlights the stress-mediated induction and feedback inhibition of stilbene biosynthesis through a complex regulatory network involving multiple positive and negative transcriptional regulators.

14-3-3 proteins regulate photomorphogenesis by facilitating light-induced degradation of PIF3

14-3-3s are highly conserved phosphopeptide-binding proteins that play important roles in various developmental and signaling pathways in plants. However, although protein phosphorylation has been proven to be a key mechanism for regulating many pivotal components of the light signaling pathway, the role of 14-3-3 proteins in photomorphogenesis remains largely obscure. PHYTOCHROME-INTERACTING FACTOR3 (PIF3) is an extensively studied transcription factor repressing photomorphogenesis, and it is well-established that upon red (R) light exposure, photo-activated phytochrome B (phyB) interacts with PIF3 and induces its rapid phosphorylation and degradation. PHOTOREGULATORY PROTEIN KINASES (PPKs), a family of nuclear protein kinases, interact with phyB and PIF3 in R light and mediate multisite phosphorylation of PIF3 in vivo. Here, we report that two members of the 14-3-3 protein family, 14-3-3λ and κ, bind to a serine residue in the bHLH domain of PIF3 that can be phosphorylated by PPKs, and act as key positive regulators of R light-induced photomorphogenesis. Moreover, 14-3-3λ and κ preferentially interact with photo-activated phyB and promote the phyB-PIF3-PPK complex formation, thereby facilitating phyB-induced phosphorylation and degradation of PIF3 upon R light exposure. Together, our data demonstrate that 14-3-3λ and κ work in close concert with the phyB-PIF3 module to regulate light signaling in Arabidopsis.

COP1 SUPPRESSOR 6 represses the PIF4 and PIF5 action to promote light-inhibited hypocotyl growth

Light signaling precisely controls photomorphogenic development in plants. PHYTOCHROME INTERACTING FACTOR 4 and 5 (PIF4 and PIF5) play critical roles in the regulation of this developmental process. In this study, we report CONSTITUTIVELY PHOTOMORPHOGENIC 1 SUPPRESSOR 6 (CSU6) functions as a key regulator of light signaling. Loss of CSU6 function largely rescues the cop1-6 constitutively photomorphogenic phenotype. CSU6 promotes hypocotyl growth in the dark, but inhibits hypocotyl elongation in the light. CSU6 not only associates with the promoter regions of PIF4 and PIF5 to inhibit their expression in the morning, but also directly interacts with both PIF4 and PIF5 to repress their transcriptional activation activity. CSU6 negatively controls a group of PIF4- and PIF5-regulated gene expressions. Mutations in PIF4 and/or PIF5 are epistatic to the loss of CSU6, suggesting that CSU6 acts upstream of PIF4 and PIF5. Taken together, CSU6 promotes light-inhibited hypocotyl elongation by negatively regulating PIF4 and PIF5 transcription and biochemical activity.

Enhanced lignin synthesis and ecotypic variation in defense-related gene expression in response to shade in Norway spruce

During the growth season, northern forests in Sweden daily receive more hours of far-red (FR)-enriched light or twilight (shade) as compared to southern forests. Norway spruce (shade-tolerant) are adapted to latitudinal variation in twilight characterized by a northward increase in FR requirement to maintain growth. Shade is a stressful condition that affects plant growth and increases plant's susceptibility to pathogen attack. Lignin plays a central role in plant defense and its metabolism is regulated by light wavelength composition (light quality). In the current work, we studied regulation of lignin synthesis and defense-related genes (growth-defense trade-offs) in response to shade in Norway spruce. In most angiosperms, light promotes lignin synthesis, whereas shade decreases lignin production leading to weaker stem, which may make plants more disease susceptible. In contrast, enhanced lignin synthesis was detected in response to shade in Norway spruce. We detected a higher number of immunity/defense-related genes up-regulated in northern populations as compared to south ones in response to shade. Enhanced lignin synthesis coupled with higher defense-related gene expression can be interpreted as an adaptive strategy for better survival in northern populations. Findings will contribute to ensuring deployment of well-adapted genetic material and identifying tree families with enhanced disease resistance.

The retrograde signaling regulator ANAC017 recruits the MKK9-MPK3/6, ethylene, and auxin signaling pathways to balance mitochondrial dysfunction with growth

In plant cells, mitochondria are ideally positioned to sense and balance changes in energy metabolism in response to changing environmental conditions. Retrograde signaling from mitochondria to the nucleus is crucial for adjusting the required transcriptional responses. We show that ANAC017, the master regulator of mitochondrial stress, directly recruits a signaling cascade involving the plant hormones ethylene and auxin as well as the MAP KINASE KINASE (MKK) 9-MAP KINASE (MPK) 3/6 pathway in Arabidopsis thaliana. Chromatin immunoprecipitation followed by sequencing and overexpression demonstrated that ANAC017 directly regulates several genes of the ethylene and auxin pathways, including MKK9, 1-AMINO-CYCLOPROPANE-1-CARBOXYLATE SYNTHASE 2, and YUCCA 5, in addition to genes encoding transcription factors regulating plant growth and stress responses such as BASIC REGION/LEUCINE ZIPPER MOTIF (bZIP) 60, bZIP53, ANAC081/ATAF2, and RADICAL-INDUCED CELL DEATH1. A time-resolved RNA-seq experiment established that ethylene signaling precedes the stimulation of auxin signaling in the mitochondrial stress response, with a large part of the transcriptional regulation dependent on ETHYLENE-INSENSITIVE 3. These results were confirmed by mutant analyses. Our findings identify the molecular components controlled by ANAC017, which integrates the primary stress responses to mitochondrial dysfunction with whole plant growth via the activation of regulatory and partly antagonistic feedback loops.

Integration of ABA, GA, and light signaling in seed germination through the regulation of ABI5

Seed germination is precisely controlled by a variety of signals, among which light signals and the phytohormones abscisic acid (ABA) and gibberellin (GA) play crucial roles. New findings have greatly increased our understanding of the mechanisms by which these three signals regulate seed germination and the close connections between them. Although much work has been devoted to ABA, GA, and light signal interactions, there is still no systematic description of their combination, especially in seed germination. In this review, we integrate ABA, GA, and light signaling in seed germination through the direct and indirect regulation of ABSCISIC ACID INSENSITIVE5 (ABI5), the core transcription factor that represses seed germination in ABA signaling, into our current understanding of the regulatory mechanism of seed germination.

COP1 positively regulates ABA signaling during Arabidopsis seedling growth in darkness by mediating ABA-induced ABI5 accumulation

CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1), a well-characterized E3 ubiquitin ligase, is a central repressor of seedling photomorphogenic development in darkness. However, whether COP1 is involved in modulating abscisic acid (ABA) signaling in darkness remains largely obscure. Here, we report that COP1 is a positive regulator of ABA signaling during Arabidopsis seedling growth in the dark. COP1 mediates ABA-induced accumulation of ABI5, a transcription factor playing a key role in ABA signaling, through transcriptional and post-translational regulatory mechanisms. We further show that COP1 physically interacts with ABA-hypersensitive DCAF1 (ABD1), a substrate receptor of the CUL4-DDB1 E3 ligase targeting ABI5 for degradation. Accordingly, COP1 directly ubiquitinates ABD1 in vitro, and negatively regulates ABD1 protein abundance in vivo in the dark but not in the light. Therefore, COP1 promotes ABI5 protein stability post-translationally in darkness by destabilizing ABD1 in response to ABA. Interestingly, we reveal that ABA induces the nuclear accumulation of COP1 in darkness, thus enhancing its activity in propagating the ABA signal. Together, our study uncovers that COP1 modulates ABA signaling during seedling growth in darkness by mediating ABA-induced ABI5 accumulation, demonstrating that plants adjust their ABA signaling mechanisms according to their light environment.

Identification and Analysis of MYB Gene Family for Discovering Potential Regulators Responding to Abiotic Stresses in Curcuma wenyujin

MYB superfamily is one of the most abundant families in plants, and plays critical role in plant growth, development, metabolism regulation, and stress response. Curcuma wenyujin is the main source plant of three traditional Chinese medicines, which are widely used in clinical treatment due to its diverse pharmacological activities. In present study, 88 CwMYBs were identified and analyzed in C. wenyujin, including 43 MYB-related genes, 42 R2R3-MYB genes, two 3R-MYB genes, and one 4R-MYB gene. Forty-three MYB-related proteins were classified into several types based on conserved domains and specific motifs, including CCA1-like type, R-R type, Myb-CC type, GARP-like type, and TBR-like type. The analysis of motifs in MYB DBD and no-MYB regions revealed the relevance of protein structure and function. Comparative phylogeny analysis divided 42 R2R3-MYB proteins into 19 subgroups and provided a reference for understanding the functions of some CwMYBs based on orthologs of previously characterized MYBs. Expression profile analysis of CwMYB genes revealed the differentially expressed genes responding to various abiotic stresses. Four candidate MYB genes were identified by combining the results of phylogeny analysis and expression analysis. CwMYB10, CwMYB18, CwMYB39, and CwMYB41 were significantly induced by cold, NaCl, and MeJA stress treatments. CwMYB18 and CwMYB41 were proved as regulators with activity of transcriptional activation, whereas CwMYB39 and CwMYB10 were not. They may participate in the response to abiotic stresses through different mechanisms in C. wenyujin. This study was the first step toward understanding the CwMYB family and the response to abiotic stresses in C. wenyujin.

Evolution and functional diversification of R2R3-MYB transcription factors in plants

R2R3-MYB genes (R2R3-MYBs) form one of the largest transcription factor gene families in the plant kingdom, with substantial structural and functional diversity. However, the evolutionary processes leading to this amazing functional diversity have not yet been clearly established. Recently developed genomic and classical molecular technologies have provided detailed insights into the evolutionary relationships and functions of plant R2R3-MYBs. Here, we review recent genome-level and functional analyses of plant R2R3-MYBs, with an emphasis on their evolution and functional diversification. In land plants, this gene family underwent a large expansion by whole genome duplications and small-scale duplications. Along with this population explosion, a series of functionally conserved or lineage-specific subfamilies/groups arose with roles in three major plant-specific biological processes: development and cell differentiation, specialized metabolism, and biotic and abiotic stresses. The rapid expansion and functional diversification of plant R2R3-MYBs are highly consistent with the increasing complexity of angiosperms. In particular, recently derived R2R3-MYBs with three highly homologous intron patterns (a, b, and c) are disproportionately related to specialized metabolism and have become the predominant subfamilies in land plant genomes. The evolution of plant R2R3-MYBs is an active area of research, and further studies are expected to improve our understanding of the evolution and functional diversification of this gene family.

The role of phytochrome-mediated gibberellic acid signaling in the modulation of seed germination under low light stress in rice (O. sativa L.)

Seed germination plays cardinal roles in seedling establishment and their successive growth. However, seed germination is retarded by far-red (FR) enrichment under low light stress, and the inhibitory signalling mechanism remains ambiguous. Our results indicated that low light treatment, both in the open and growth chamber conditions, inhibits rice seed germination by decreasing the gibberellin (GA) contents. To explore the mechanism of GA-deficiency under low light stress, differential expression profiling of GA-anabolic, -catabolic, ABA -anabolic, -catabolic, and SLR1 was investigated, revealing that expression of ABA- anabolic, GA-catabolic genes and SLR1 was upregulated with a simultaneous downregulation of ABA-catabolic and GA-anabolic genes under low light treatment. These results suggested that FR-induced GA inadequacy is resulted by upregulation of SLR1 and GA-catabolism genes consequently increase DELLA that further subsided GA-responses in the germinating rice seeds. Moreover, we provided evidence that FR-induced GA inadequacy demotes rice seed germination by decreasing amylase activity, eventually decreasing the carbohydrate solubilization in the germinating seeds. Finally, we suggest that under low light stress, due to a retarded conversion of phytochrome A to their bioactive form, the ABA-catabolic genes were eventually upregulated with a simultaneous downregulation of GA-anabolic genes. Consequently, a lower GA pool fails to leverage the GA-dependent DELLA degradation, further shutting down the expected GA responses that reduce germination efficiency under FR-enriched light.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-022-01167-7.

Mutual upregulation of HY5 and TZP in mediating phytochrome A signaling

Phytochrome A (phyA) is the far-red (FR) light photoreceptor in plants that is essential for seedling de-etiolation under FR-rich environments, such as canopy shade. TANDEM ZINC-FINGER/PLUS3 (TZP) was recently identified as a key component of phyA signal transduction in Arabidopsis thaliana; however, how TZP is integrated into the phyA signaling networks remains largely obscure. Here, we demonstrate that ELONGATED HYPOCOTYL5 (HY5), a well-characterized transcription factor promoting photomorphogenesis, mediates FR light induction of TZP expression by directly binding to a G-box motif in the TZP promoter. Furthermore, TZP physically interacts with CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1), an E3 ubiquitin ligase targeting HY5 for 26S proteasome-mediated degradation, and this interaction inhibits COP1 interaction with HY5. Consistent with those results, TZP post-translationally promotes HY5 protein stability in FR light, and in turn, TZP protein itself is destabilized by COP1 in both dark and FR light conditions. Moreover, tzp hy5 double mutants display an additive phenotype relative to their respective single mutants under high FR light intensities, indicating that TZP and HY5 also function in largely independent pathways. Together, our data demonstrate that HY5 and TZP mutually upregulate each other in transmitting the FR light signal, thus providing insights into the complicated but delicate control of phyA signaling networks.

Series-Spatial Transcriptome Profiling of Leafy Head Reveals the Key Transition Leaves for Head Formation in Chinese Cabbage

Chinese cabbage is an important leaf heading vegetable crop. At the heading stage, its leaves across inner to outer show significant morphological differentiation. However, the genetic control of this complex leaf morphological differentiation remains unclear. Here, we reported the transcriptome profiling of Chinese cabbage plant at the heading stage using 24 spatially dissected tissues representing different regions of the inner to outer leaves. Genome-wide transcriptome analysis clearly separated the inner leaf tissues from the outer leaf tissues. In particular, we identified the key transition leaf by the spatial expression analysis of key genes for leaf development and sugar metabolism. We observed that the key transition leaves were the first inwardly curved ones. Surprisingly, most of the heading candidate genes identified by domestication selection analysis obviously showed a corresponding expression transition, supporting that key transition leaves are related to leafy head formation. The key transition leaves were controlled by a complex signal network, including not only internal hormones and protein kinases but also external light and other stimuli. Our findings provide new insights and the rich resource to unravel the genetic control of heading traits.

Combined analyses of translatome and transcriptome in Arabidopsis reveal new players responding to magnesium deficiency

Translational control of gene expression, including recruitment of ribosomes to messenger RNA (mRNA), is particularly important during the response to stress. Purification of ribosome-associated mRNAs using translating ribosome affinity purification (TRAP) followed by RNA-sequencing facilitates the study of mRNAs undergoing active transcription and better proxies the translatome, or protein response, to stimuli. To identify plant responses to Magnesium (Mg) deficiency at the translational level, we combined transcriptome and translatome analyses. Excitingly, we found 26 previously unreported Mg-responsive genes that were only regulated at the translational level and not the transcriptional level, during the early response to Mg deficiency. In addition, mutants of the transcription factor ELONGATED HYPOCOTYL 5 (HY5), the H⁺ /CATION EXCHANGER 1 and 3 (CAX1 and CAX3), and UBIQUITIN 11 (UBQ11) exhibited early chlorosis phenotype under Mg deficiency, supporting their functional involvement in ion homeostasis. Overall, our study strongly supports that TRAP-seq combined with RNA-seq followed by phenotype screening could facilitate the identification of novel players during stress responses.

Integration of Light and Brassinosteroid Signaling during Seedling Establishment

Light and brassinosteroid (BR) are external stimuli and internal cue respectively, that both play critical roles in a wide range of developmental and physiological process. Seedlings grown in the light exhibit photomorphogenesis, while BR promotes seedling etiolation. Light and BR oppositely control the development switch from shotomorphogenesis in the dark to photomorphogenesis in the light. Recent progress report that substantial components have been identified as hubs to integrate light and BR signals. Photomorphogenic repressors including COP1, PIFs, and AGB1 have been reported to elevate BR response, while photomorphogenesis-promoting factors such as HY5, BZS1, and NF-YCs have been proven to repress BR signal. In addition, BR components also modulate light signal. Here, we review the current research on signaling network associated with light and brassinosteroids, with a focus on the integration of light and BR signals enabling plants to thrive in the changeable environment.

PIF4 and PIF4-Interacting Proteins: At the Nexus of Plant Light, Temperature and Hormone Signal Integrations

Basic helix-loop-helix (bHLH) family transcription factor PHYTOCHROME INTERACTING FACTOR 4 (PIF4) is necessary for plant adaption to light or high ambient temperature. PIF4 directly associates with plenty of its target genes and modulates the global transcriptome to induce or reduce gene expression levels. However, PIF4 activity is tightly controlled by its interacting proteins. Until now, twenty-five individual proteins have been reported to physically interact with PIF4. These PIF4-interacting proteins act together with PIF4 and form a unique nexus for plant adaption to light or temperature change. In this review, we will discuss the different categories of PIF4-interacting proteins, including photoreceptors, circadian clock regulators, hormone signaling components, and transcription factors. These distinct PIF4-interacting proteins either integrate light and/or temperature cues with endogenous hormone signaling, or control PIF4 abundances and transcriptional activities. Taken together, PIF4 and PIF4-interacting proteins play major roles for exogenous and endogenous signal integrations, and therefore establish a robust network for plants to cope with their surrounding environmental alterations.