Short Hypocotyl Phenotype Research Articles

Alternative localization of HEME OXYGENASE 1 in plant cells regulates cytosolic heme catabolism.

Heme, an organometallic tetrapyrrole, is widely engaged in oxygen transport, electron delivery, enzymatic reactions, and signal transduction. In plants, it is also involved in photomorphogenesis and photosynthesis. HEME OXYGENASE 1 (HO1) initiates the first committed step in heme catabolism, and it has generally been thought that this reaction takes place in chloroplasts. Here, we show that HO1 in both Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa) has 2 transcription start sites (TSSs), producing long (HO1L) and short (HO1S) transcripts. Their products localize to the chloroplast and the cytosol, respectively. During early development or de-etiolation, the HO1L/HO1S ratio gradually increases. Light perception via phytochromes (Phys) and cryptochromes elevates the HO1L/HO1S ratio in the whole seedling through the functions of ELONGATED HYPOCOTYL 5 (HY5) and HY5 HOMOLOG and through the suppression of DE-ETIOLATED 1, CONSTITUTIVE PHOTOMORPHOGENESIS 1, and PHYTOCHROME INTERACTING FACTORs. HO1L introduction complements the HO1-deficient mutant; surprisingly, HO1S expression also restores the short hypocotyl phenotype and high pigment content and helps the mutant recover from the genomes uncoupled (gun) phenotype. This indicates the assembly of functional Phys within these lines. Furthermore, our findings support the hypothesis that a mobile heme signal is involved in retrograde signaling from the chloroplast. Altogether, our work clarifies the molecular mechanism of HO1 TSS regulation and highlights the presence of a cytosolic bypass for heme catabolism in plant cells.

Arabidopsis hypocotyl growth in darkness requires the phosphorylation of a microtubule-associated protein.

Rapid hypocotyl elongation allows buried seedlings to emerge, where light triggers de-etiolation and inhibits hypocotyl growth mainly by photoreceptors. Phosphorylation/dephosphorylation events regulate many aspects of plant development. Only recently we have begun to uncover the earliest phospho-signaling responders to light. Here, we reported a large-scale phosphoproteomic analysis and identified 20 proteins that changed their phosphorylation pattern following a 20 min light pulse compared to darkness. Microtubule-associated proteins were highly overrepresented in this group. Among them, we studied CIP7 (COP1-INTERACTING-PROTEIN 7), which presented microtubule (MT) localization in contrast to the previous description. An isoform of CIP7 phosphorylated at Serine915 was detected in etiolated seedlings but was undetectable after a light pulse in the presence of photoreceptors, while CIP7 transcript expression decays with long light exposure. The short hypocotyl phenotype and rearrangement of MTs in etiolated cip7 mutants are complemented by CIP7-YFP and the phospho-mimetic CIP7S915D-YFP, but not the phospho-null CIP7S915A-YFP suggesting that the phosphorylated S915CIP7 isoform promotes hypocotyl elongation through MT reorganization in darkness. Our evidence on Serine915 of CIP7 unveils phospho-regulation of MT-based processes during skotomorphogenic hypocotyl growth.

Protein interactome of 3',5'-cyclic adenosine monophosphate reveals its role in regulating the actin cytoskeleton.

Identification of protein interactors is ideally suited for the functional characterization of small molecules. Cyclic adenosine monophosphate (3',5'-cAMP) is an evolutionary ancient signaling metabolite largely uncharacterized in plants. To tap into the physiological roles of 3',5'-cAMP, we used a novel chemo-proteomics approach, thermal proteome profiling (TPP), for the unbiased identification of 3',5'-cAMP protein targets. TPP measures shifts in the protein thermal stability upon ligand binding. Comprehensive proteomics analysis yielded a list of 51 proteins significantly altered in their thermal stability upon incubation with 3',5'-cAMP. The list contained metabolic enzymes, ribosomal subunits, translation initiation factors, and proteins associated with the regulation of plant growth such as CELL DIVISION CYCLE 48. To functionally validate obtained results, we focused on the role of 3',5'-cAMP in regulating the actin cytoskeleton suggested by the presence of actin among the 51 identified proteins. 3',5'-cAMP supplementation affected actin organization by inducing actin-bundling. Consistent with these results, the increase in 3',5'-cAMP levels, obtained either by feeding or by chemical modulation of 3',5'-cAMP metabolism was sufficient to partially rescue the short hypocotyl phenotype of the actin2 actin7 mutant, severely compromised in actin level. The observed rescue was specific to 3',5'-cAMP, as demonstrated using a positional isomer 2',3'-cAMP, and true for the nanomolar 3',5'-cAMP concentrations reported for plant cells. In vitro characterization of the 3',5'-cAMP - actin pairing argues against a direct interaction between actin and 3',5'-cAMP. Alternative mechanisms by which 3',5'-cAMP would affect actin dynamics, such as by interfering with calcium signaling, are discussed. In summary, our work provides a novel resource, 3',5'-cAMP interactome, and a new functional insight into the 3',5'-cAMP-mediated regulation in plants.

Assessment of Biological Activity of 28-Homobrassinolide via a Multi-Level Comparative Analysis.

Brassinosteroids (BRs) play vital roles in the plant life cycle and synthetic BRs are widely used to increase crop yield and plant stress tolerance. Among them are 24R-methyl-epibrassinolide (24-EBL) and 24S-ethyl-28-homobrassinolide (28-HBL), which differ from brassinolide (BL, the most active BR) at the C-24 position. Although it is well known that 24-EBL is 10% active as BL, there is no consensus on the bioactivity of 28-HBL. A recent outpouring of research interest in 28-HBL on major crops accompanied with a surge of industrial-scale synthesis that produces mixtures of active (22R,23R)-28-HBL and inactive (22S,23S)-28HBL, demands a standardized assay system capable of analyzing different synthetic "28-HBL" products. In this study, the relative bioactivity of 28-HBL to BL and 24-EBL, including its capacity to induce the well-established BR responses at molecular, biochemical, and physiological levels, was systematically analyzed using the whole seedlings of the wild-type and BR-deficient mutant of Arabidopsis thaliana. These multi-level bioassays consistently showed that 28-HBL exhibits a much stronger bioactivity than 24-EBL and is almost as active as BL in rescuing the short hypocotyl phenotype of the dark-grown det2 mutant. These results are consistent with the previously established structure-activity relationship of BRs, proving that this multi-level whole seedling bioassay system could be used to analyze different batches of industrially produced 28-HBL or other BL analogs to ensure the full potential of BRs in modern agriculture.

AtHDA15 attenuates COP1 via transcriptional quiescence, direct binding, and sub-compartmentalization during photomorphogenesis

Light is an essential environmental cue that determines the overall growth and development of plants. However, the molecular mechanisms underpinning the light signaling network are obscured by the epigenetic machinery where reversible acetylation and deacetylation play crucial roles in modulating light-regulated gene expression. In this paper, we demonstrate that HDA15 represses COP1, the master switch in the light signaling network, by deacetylation, protein interaction, and sub-compartmentalization. hda15 T-DNA mutant lines exhibited light hyposensitivity with significantly reduced HY5 and PIF3 transcript levels leading to long-hypocotyl phenotypes in the dark while its overexpression exhibited elevated HY5 transcripts and short hypocotyl phenotypes. In vivo and in vitro binding assays further show that HDA15 directly interacts with COP1 inside the nucleus modulating COP1’s repressive activities. Crossing hda15-t27 with cop1-4 mutants resulted in short-hypocotyl and dwarfed phenotypes, reminiscent of cop1-4 mutants suggesting COP1 is epistatic to HDA15. Although light signals the nucleocytoplasmic shuttling of HDA15, the presence of COP1 triggers its nuclear localization. A working model is presented elucidating the concerted interplay between HDA15 and COP1 under light and dark conditions.

The blue light receptor CRY1 interacts with FIP37 to promote N6 -methyladenosine RNA modification and photomorphogenesis in Arabidopsis.

Light is a particularly important environmental cue that regulates a variety of diverse plant developmental processes, such as photomorphogenesis. Blue light promotes photomorphogenesis mainly through the activation of the photoreceptor cryptochrome 1 (CRY1). However, the mechanism underlying the CRY1-mediated regulation of growth is not fully understood. Here, we found that blue light induced N6 -methyladenosine (m6 A) RNA modification during photomorphogenesis partially via CRY1. Cryptochrome 1 mediates blue light-induced expression of FKBP12-interacting protein 37 (FIP37), which is a component of m6 A writer. Moreover, we showed that CRY1 physically interacted with FIP37 invitro and invivo, and mediated blue light activation of FIP37 binding to RNA. Furthermore, CRY1 and FIP37 modulated m6 A on photomorphogenesis-related genes PIF3, PIF4, and PIF5, thereby accelerating the decay of their transcripts. Genetically, FIP37 repressed hypocotyl elongation under blue light, and fip37 mutation could partially rescue the short-hypocotyl phenotype of CRY1-overexpressing plants. Together, our results provide a new insight into CRY1 signal in modulating m6 A methylation and stability of PIFs, and establish an essential molecular link between m6 A modification and determination of photomorphogenesis in plants.

WRKY-ing in the light.

This article is a Commentary on Zhou et al. (2022), 235: 111–125.

The E3 ligase XBAT35 mediates thermoresponsive hypocotyl growth by targeting ELF3 for degradation in Arabidopsis.

Plants are capable of coordination of their growth and development with ambient temperatures. EARLY FLOWERING3 (ELF3), an essential component of the plant circadian clock, is also involved in ambient temperature sensing, as well as in inhibiting the expression and protein activity of the thermoresponsive regulator phytochrome interacting factor 4 (PIF4). The ELF3 activity is subjected to attenuation in response to warm temperature; however, how the protein level of ELF3 is regulated at warm temperature remains less understood. Here, we report that the E3 ligase XB3 ORTHOLOG 5 IN ARABIDOPSIS THALIANA, XBAT35, mediates ELF3 degradation. XBAT35 interacts with ELF3 and ubiquitinates ELF3. Loss-of-function mutation of XBAT35 increases the protein level of ELF3 and confers a short-hypocotyl phenotype under warm temperature conditions. Thus, our findings establish that XBAT35 mediates ELF3 degradation to lift the inhibition of ELF3 on PIF4 for promoting thermoresponsive hypocotyl growth in plants.

Mapping and identification of CsSh5.1, a gene encoding a xyloglucan galactosyltransferase required for hypocotyl elongation in cucumber (Cucumis sativus L.).

CsSh5.1, which controls hypocotyl elongation under high temperature conditions in cucumber, was mapped to a 57.1 kb region on chromosome 5 containing a candidate gene encoding a xyloglucan galactosyltransferase. Hypocotyl growth is a vital process in seedling establishment. Hypocotyl elongation after germination relies more on longitudinal cell elongation than cell division. Cell elongation is largely determined by the extensibility of the cell wall. Here, we identified a spontaneous mutant in cucumber (Cucumis sativus L.), sh5.1, which exhibits a temperature-insensitive short hypocotyl phenotype. Genetic analysis showed that the phenotype of sh5.1 was controlled by a recessive nuclear gene. CsSh5.1 was mapped to a 57.1kb interval on chromosome 5, containing eight predicted genes. Sequencing analysis revealed that the Csa5G171710 is the candidate gene of CsSh5.1, which was further confirmed via co-segregation analysis and genomic DNA sequencing in natural cucumber variations. The result indicated that hypocotyl elongation might be controlled by this gene. CsSh5.1 encodes a xyloglucan galactosyltransferase that specifically adds galactose to xyloglucan and forms galactosylated xyloglucans, which determine the strength and extensibility of the cell walls. CsSh5.1 expression in wild-type (WT) hypocotyl was significantly higher than that in sh5.1 hypocotyl under high temperature, suggesting its important role in hypocotyl cell elongation under high temperature. The identification of CsSh5.1 is helpful for elucidating the function of xyloglucan galactosyltransferase in cell wall expansion and understanding the mechanism of hypocotyl elongation in cucumber.

AtHDA15 binds directly to COP1 positively regulating photomorphogenesis

Reversible histone acetylation and deacetylation play crucial roles in modulating light-regulated gene expression during seedling development. However, it remains largely unknown how histone-modifying enzymes interpose within the molecular framework of light signaling network. In this study, we show that AtHDA15 positively regulates photomorphogenesis by directly binding to COP1, a master regulator in the repression of photomorphogenesis. hda15 T-DNA knock-out and RNAi lines exhibited light hyposensitivity with reduced HY5 and PIF3 protein levels leading to long hypocotyl phenotypes in the dark while its overexpression leads to increased HY5 concentrations and short hypocotyl phenotypes. In vivo and in vitro binding assays show that HDA15 directly interacts with COP1 inside the nucleus modulating COP1’s repressive activities. As COP1 is established to act within the nucleus to regulate specific transcription factors associated with growth and development in skotomorphogenesis, the direct binding by HDA15 is predicted to abrogate activities of COP1 in the presence of light and modulate its repressive activities in the dark. Our results append the mounting evidence for the role of HDACs in post-translational regulation in addition to their well-known histone modifying functions.

The apple RING-H2 protein MdCIP8 regulates anthocyanin accumulation and hypocotyl elongation by interacting with MdCOP1

COP1, an important RING ubiquitin ligase E3, is a molecular switch for light regulation in plant development. As an interacting protein of COP1, CIP8 contains a RING-H2 domain, but its biological function is unclear. Here, the apple MdCIP8 was identified based on its homology with AtCIP8 in Arabidopsis. MdCIP8 was constitutively expressed at different levels in various apple tissues, and the expression level of MdCIP8 was not affected by light and dark conditions. MdCIP8 reversed the short hypocotyl phenotype of the cip8 mutant under light conditions. Furthermore, the yeast two-hybrid experiment showed that MdCIP8 interacted with the RING domain of MdCOP1 through its RING-H2 domain. MdCIP8-OX/cop1-4 exhibited the phenotype of the cop1-4 mutant, indicating that CIP8 acts upstream of COP1. In addition, an apple transient injection experiment showed that MdCIP8 inhibited anthocyanin accumulation in an MdCOP1-dependent pathway. Overall, our findings reveal that CIP8 plays an inhibitory role in the light-regulation responses of plants.

Overexpression of blue light receptor AaCRY1 improves artemisinin content in Artemisia annua L. .

Artemisinin, an effective antimalarial compound, is isolated from the medicinal plant Artemisia annua L. However, because of the low content of artemisinin in A. annua, the demand of artemisinin exceeds supply. Previous studies show that the artemisinin biosynthesis is promoted by light in A. annua. Cryptochrome1 (CRY1) is involved in many processes in the light response. In this study, AaCRY1 was cloned from A. annua. Overexpressing AaCRY1 in Arabidopsis thaliana cry1 mutant resulted in blue-light-dependent short hypocotyl phenotype and short coleoptile under blue light. Yeast two-hybrid and subcellular colocalization showed that AaCRY1 interacted with AtCOP1 (ubiquitin E3 ligase CONSTITUTIVE PHOTOMORPHOGENIC1). Overexpression of AaCRY1 in transgenic A. annua increased the artemisinin content. When AaCRY1 was overexpressed in A. annua driven by the CYP71AV1 (cytochrome P450 dependent amorpha-4,11-diene 12-hydroxylase) promoter, the artemisinin content was 1.6 times higher than that of the control. Furthermore, we expressed the C terminal of AaCRY1(CCT) involved a GUS-CCT fusion protein in A. annua. The results showed that the artemisinin content was increased to 1.7- to 2.4-fold in GUS-CCT transgenic A. annua plants. These results demonstrate that overexpression of GUS-CCT is an effective strategy to increase artemisinin production in A. annua.

The cold response regulator CBF1 promotes Arabidopsis hypocotyl growth at ambient temperatures.

Light and temperature are two core environmental factors that coordinately regulate plant growth and survival throughout their entire life cycle. However, the mechanisms integrating light and temperature signaling pathways in plants remain poorly understood. Here, we report that CBF1, an AP2/ERF-family transcription factor essential for plant cold acclimation, promotes hypocotyl growth under ambient temperatures in Arabidopsis. We show that CBF1 increases the protein abundance of PIF4 and PIF5, two phytochrome-interacting bHLH-family transcription factors that play pivotal roles in modulating plant growth and development, by directly binding to their promoters to induce their gene expression, and by inhibiting their interaction with phyB in the light. Moreover, our data demonstrate that CBF1 promotes PIF4/PIF5 protein accumulation and hypocotyl growth at both 22°C and 17°C, but not at 4°C, with a more prominent role at 17°C than at 22°C. Together, our study reveals that CBF1 integrates light and temperature control of hypocotyl growth by promoting PIF4 and PIF5 protein abundance in the light, thus providing insights into the integration mechanisms of light and temperature signaling pathways in plants.

Degrading to inhibit: a role for Phytochrome Interacting Factors in determining hypocotyl growth under UV-B.

Plants perceive light through specific photoreceptors for red/far-red, blue, and UV-B wavelengths of the electromagnetic spectrum, regulating physiological responses such as hypocotyl elongation, in a process known as photomorphogenesis. Over the last two decades, a diverse panel of mutants exhibiting defects in light-regulated hypocotyl elongation has been used to elucidate the molecular links between photoreceptor signalling and its downstream physiological responses (de Wit et al., 2016a). At the molecular level, this link involves the interplay of three transcription factor families: EIN3, PIFs, and HY5 (Shi et al., 2018). The latter is a positive regulator of photomorphogenesis whilst the former two are repressors and, as such, promote hypocotyl elongation. The UVR8 photoreceptor and its signalling pathway also serve as a regulatory signal for plant morphogenesis, and its interaction with the E3 ubiquitin ligase COP1 induces HY5 induction and stability (Podolec and Ulm, 2018). However, the mechanisms by which UVR8 triggers hypocotyl growth inhibition have only just begun to be clarified. In their study in this issue of The Plant Journal, (Tavridou et al., 2019) show that UVR8-mediated inhibition of PIF4 and PIF5 is an important and intrinsic part of UVR8 photomorphogenic signalling. The laboratory of Roman Ulm, Professor at the Department of Botany and Plant Biology at the University of Geneva (Geneva, Switzerland), has been carrying out seminal work on the plant responses to UV-B radiation for several years. Roman joined the University of Geneva in 2010 and has been the Director of the department since 2017, previously having been an independent group leader at the University of Freiburg (Freiburg, Germany). The work described in this article took over 3 years for the joint co-authors, postdocs Eleni Tavridou and Marie Pireyre, who joined the project in 2017. The major goal of their work was to investigate if there is a role for UVR8 photoreceptor-induced destabilization of PIF4 and PIF5 in UV-B-induced gene repression and hypocotyl growth inhibition. Previous work described an effect of UV-B on PIF4 and PIF5 stability, particularly when plants are grown in shade conditions (high FR; very low R:FR ratio of 0.05) or under elevated temperature (28°C), and associated shade and thermomorphogenic responses were antagonized by UV-B (Hayes et al., 2014, 2017). However, a role for PIF4 and PIF5 destabilization in a standard hypocotyl growth assay and UV-B-induced gene repression had not been described before. Their work thus reveals a novel role for PIF4 and PIF5 degradation as part of the core UVR8 signalling pathway. Tavridou and colleagues achieved this by discovering of a role for UVR8 photoreceptor-induced destabilization of PIF4 and PIF5 in UV-B-induced gene repression and hypocotyl growth inhibition (Tavridou et al., 2019). PIF4 and PIF5 are bHLH transcription factors that play crucial roles in the regulation of genes associated with hypocotyl elongation. UV-B-induced inhibition of hypocotyl growth is a broadly used phenotypic assay to study the activity of the UVR8 photoreceptor and its signalling pathway. UV-B exposure results in monomerization of UVR8, leading to its interaction with COP1 (Rizzini et al., 2011), which affects the targeting of PIF4 and PIF5 to the proteasome via an unknown mechanism (see Figure). The results provide evidence for a mechanism underlying hypocotyl growth inhibition that involves UVR8-induced degradation of PIF4 and PIF5. In agreement, binding of PIF4 and PIF5 to promoters of elongation marker genes is reduced under UV-B exposure, concurring with the transcriptional downregulation of these genes under UV-B. Moreover, pif4pif5 mutants have a short hypocotyl phenotype, in agreement with the conclusion that UVR8-induced PIF4 and PIF5 degradation contributes to hypocotyl growth inhibition under UV-B. Not only are these conclusions immensely informative in their own right, they also raise a number of important questions, for example: Which E3 ubiquitin ligase(s) is responsible for the degradation of PIF4 and PIF5 in response to UV-B? and What mechanistically links UVR8 photoreceptor activation to PIF4 and PIF5 degradation? Regarding the second question, there is currently no evidence that UVR8 interacts directly with PIFs in a manner analogous to that of phytochromes and cryptochromes (Ma et al., 2016; de Wit et al., 2016b). However, a recent publication (Sharma et al., 2019) suggests that PIF5 is stabilized by direct interaction with COP1, and that UVR8 disrupts this stabilization through its UV-B-activated interaction with COP1. The above questions notwithstanding, the highlighted paper provides compelling evidence linking PIF4 and PIF5 to UV-dependent hypocotyl inhibition and, as such, adds this function to the vast range of roles displayed by these enigmatic proteins.

The ABA-induced soybean ERF transcription factor gene GmERF75 plays a role in enhancing osmotic stress tolerance in Arabidopsis and soybean

BackgroundEthylene-responsive factors (ERFs) play important roles in plant growth and development and the response to adverse environmental factors, including abiotic and biotic stresses.ResultsIn the present study, we identified 160 soybean ERF genes distributed across 20 chromosomes that could be clustered into eight groups based on phylogenetic relationships. A highly ABA-responsive ERF gene, GmERF75, belonging to Group VII was further characterized. Subcellular localization analysis showed that the GmERF75 protein is localized in the nucleus, and qRT-PCR results showed that GmERF75 is responsive to multiple abiotic stresses and exogenous hormones. GmERF75-overexpressing Arabidopsis lines showed higher chlorophyll content compared to WT and mutants under osmotic stress. Two independent Arabidopsis mutations of AtERF71, a gene homologous to GmERF75, displayed shorter hypocotyls, and overexpression of GmERF75 in these mutants could rescue the short hypocotyl phenotypes. Overexpressing GmERF75 in soybean hairy roots improved root growth under exogenous ABA and salt stress.ConclusionsThese results suggested that GmERF75 is an important plant transcription factor that plays a critical role in enhancing osmotic tolerance in both Arabidopsis and soybean.

MYC2 regulates ARR16, a component of cytokinin signaling pathways, in Arabidopsis seedling development.

MYC2 is a basic helix‐loop‐helix transcription factor that acts as a repressor of blue light‐mediated photomorphogenic growth; however, it promotes lateral root formation. MYC2 also regulates different phytohormone‐signaling pathways in crucial manner. Arabidopsis response regulator 16 (ARR16) is a negative regulator of cytokinin signaling pathways. Here, we show that MYC2 directly binds to the E‐box of ARR16 minimal promoter and negatively regulates its expression in a cytokinin‐dependent manner. While ARR16 and MYC2 influence jasmonic acid and cytokinin signaling, the expression of ARR16 is regulated by cry1, GBF1, and HYH, the components of light signaling pathways. The transgenic studies show that the expression of ARR16 is regulated by MYC2 at various stages of development. The mutational studies reveal that ARR16 positively regulates the hypocotyl growth in blue light, and phenotypic analysis of atmyc2 arr16 double mutant further reveals that arr16 can suppress the short hypocotyl phenotype of atmyc2. Altogether, this work highlights MYC2‐mediated transcriptional repression of ARR16 in Arabidopsis seedling development.

Correction: FIN219/JAR1 and cryptochrome1 antagonize each other to modulate photomorphogenesis under blue light in Arabidopsis.

[This corrects the article DOI: 10.1371/journal.pgen.1007248.].

FIN219/JAR1 and cryptochrome1 antagonize each other to modulate photomorphogenesis under blue light in Arabidopsis.

Plant development is affected by the integration of light and phytohormones, including jasmonates (JAs). To address the molecular mechanisms of possible interactions between blue light and JA signaling in Arabidopsis thaliana, we used molecular and transgenic approaches to understand the regulatory relationships between FAR-RED INSENSITIVE 219 (FIN219)/JASMONATE RESISTANT1 (JAR1) and the blue-light photoreceptor cryptochrome1 (CRY1). FIN219 overexpression in the wild type resulted in a short-hypocotyl phenotype under blue light. However, FIN219 overexpression in cry1, cry2 and cry1cry2 double mutant backgrounds resulted in phenotypes similar to their respective mutant backgrounds, which suggests that FIN219 function may require blue light photoreceptors. Intriguingly, FIN219 overexpression in transgenic plants harboring ectopic expression of the C terminus of CRY1 (GUS-CCT1), which exhibits a hypersensitive short-hypocotyl phenotype in all light conditions including darkness, led to a rescued phenotype under all light conditions except red light. Further expression studies showed mutual suppression between FIN219 and CRY1 under blue light. Strikingly, FIN219 overexpression in GUS-CCT1 transgenic lines (FIN219-OE/GUS-CCT1) abolished GUS-CCT1 fusion protein under blue light, whereas GUS-CCT1 fusion protein was stable in the fin219-2 mutant background (fin219-2/GUS-CCT1). Moreover, FIN219 strongly interacted with COP1 under blue light, and methyl JA (MeJA) treatment enhanced the interaction between FIN219 and GUS-CCT1 under blue light. Furthermore, FIN219 level affected GUS-CCT1 seedling responses such as anthocyanin accumulation and bacterial resistance under various light conditions and MeJA treatment. Thus, FIN219/JAR1 and CRY1 antagonize each other to modulate photomorphogenic development of seedlings and stress responses in Arabidopsis.

COP1 mediates dark-specific degradation of microtubule-associated protein WDL3 in regulating Arabidopsis hypocotyl elongation

CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1), a well-known E3 ubiquitin ligase, functions as a central regulator of plant growth and photomorphogenic development in plants, including hypocotyl elongation. It has been well-established that, in darkness, COP1 targets many photomorphogenesis-promoting factors for ubiquitination and degradation in the nucleus. However, increasing evidence has shown that a proportion of COP1 is also localized outside the nucleus in dark-grown seedlings, but the physiological function of this localization remains largely unclear. In this study, we demonstrate that COP1 directly targets and mediates the degradation of WAVE-DAMPENED 2-LIKE 3 (WDL3) protein, a member of the microtubule-associated protein (MAP) WVD2/WDL family involved in regulating hypocotyl cell elongation of Arabidopsis seedlings. We show that COP1 interacts with WDL3 in vivo in a dark-dependent manner at cortical microtubules. Moreover, our data indicate that COP1 directly ubiquitinates WDL3 in vitro and that WDL3 protein is degraded in WT seedlings but is abundant in the cop1 mutant in the dark. Consistently, introduction of the wdl3 mutation weakened, whereas overexpression of WDL3 enhanced, the short-hypocotyl phenotype of cop1 mutant in darkness. Together, this study reveals a function of COP1 in regulating the protein turnover of a cytosol-localized MAP in etiolated hypocotyls, thus providing insights into COP1-mediated degradation of downstream factors to control seedling photomorphogenesis.

ERECTA Regulates Cell Elongation by Activating Auxin Biosynthesis in Arabidopsis thaliana.

The ERECTA family genes, ERECTA (ER), ERECTA-LIKE1 (ERL1), and ERECTA-LIKE2 (ERL2), encode leucine-rich repeat receptor-like kinases in Arabidopsis thaliana. Knocking out these three genes can cause severe phenotypes, which indicates that they play significant roles in plant growth and development. However, the molecular mechanism within remains unclear. Here we show that the short hypocotyl phenotypes of er erl1 erl2 mutants are mainly due to the defects of cell elongation rather than the cell division. In contrast, in the ERECTA overexpression transgenic plants, the hypocotyl length is increased with elongated cells. Moreover, we show that the er erl1 erl2 triple mutant contains a low level of auxin, and the expression levels of the key auxin biosynthesis genes are significantly reduced. Consistent with this observation, increasing exogenous or endogenous auxin levels could partially rescue the cell elongation defects of the er erl1 erl2 triple mutant. Therefore, our results provide a molecular basis for auxin mediated ERECTA control of the hypocotyl length in Arabidopsis thaliana.