UVR8-TCP4-LOX2 module regulates UV-B tolerance in Arabidopsis.

The phytohormone jasmonate (JA) coordinates stress and growth responses to increase plant survival in unfavorable environments. Although JA can enhance plant UV-B stress tolerance, the mechanisms underlying the interaction of UV-B and JA in this response remain unknown. In this study, we demonstrate that the UV RESISTANCE LOCUS 8 - TEOSINTE BRANCHED1, Cycloidea and PCF 4 - LIPOXYGENASE2 (UVR8-TCP4-LOX2) module regulates UV-B tolerance dependent on JA signaling pathway in Arabidopsis thaliana. We show that the nucleus-localized UVR8 physically interacts with TCP4 to increase the DNA-binding activity of TCP4 and upregulate the JA biosynthesis gene LOX2. Furthermore, UVR8 activates the expression of LOX2 in a TCP4-dependent manner. Our genetic analysis also provides evidence that TCP4 acts downstream of UVR8 and upstream of LOX2 to mediate plant responses to UV-B stress. Our results illustrate that the UV-B-dependent interaction of UVR8 and TCP4 serves as an important UVR8-TCP4-LOX2 module, which integrates UV-B radiation and JA signaling and represents a new UVR8 signaling mechanism in plants.

CONSTANS interacts with and antagonizes ABF transcription factors during salt stress under long-day conditions.

CONSTANS (CO) is a critical regulator of flowering that combines photoperiodic and circadian signals in Arabidopsis (Arabidopsis thaliana). CO is expressed in multiple tissues, including seedling roots and young leaves. However, the roles and underlying mechanisms of CO in modulating physiological processes outside of flowering remain obscure. Here, we show that the expression of CO responds to salinity treatment. CO negatively mediated salinity tolerance under long-day (LD) conditions. Seedlings from co-mutants were more tolerant to salinity stress, whereas overexpression of CO resulted in plants with reduced tolerance to salinity stress. Further genetic analyses revealed the negative involvement of GIGANTEA (GI) in salinity tolerance requires a functional CO. Mechanistic analysis demonstrated that CO physically interacts with 4 critical basic leucine zipper (bZIP) transcription factors; ABSCISIC ACID-RESPONSIVE ELEMENT BINDING FACTOR1 (ABF1), ABF2, ABF3, and ABF4. Disrupting these ABFs made plants hypersensitive to salinity stress, demonstrating that ABFs enhance salinity tolerance. Moreover, ABF mutations largely rescued the salinity-tolerant phenotype of co-mutants. CO suppresses the expression of several salinity-responsive genes and influences the transcriptional regulation function of ABF3. Collectively, our results show that the LD-induced CO works antagonistically with ABFs to modulate salinity responses, thus revealing how CO negatively regulates plant adaptation to salinity stress.

PHOSPHATE STARVATION RESPONSE1 (PHR1) interacts with JASMONATE ZIM-DOMAIN (JAZ) and MYC2 to modulate phosphate deficiency-induced jasmonate signaling in Arabidopsis.

Phosphorus (P) is a macronutrient necessary for plant growth and development. Inorganic phosphate (Pi) deficiency modulates the signaling pathway of the phytohormone jasmonate in Arabidopsis thaliana, but the underlying molecular mechanism currently remains elusive. Here, we confirmed that jasmonate signaling was enhanced under low Pi conditions, and the CORONATINE INSENSITIVE1 (COI1)-mediated pathway is critical for this process. A mechanistic investigation revealed that several JASMONATE ZIM-DOMAIN (JAZ) repressors physically interacted with the Pi signaling-related core transcription factors PHOSPHATE STARVATION RESPONSE1 (PHR1), PHR1-LIKE2 (PHL2), and PHL3. Phenotypic analyses showed that PHR1 and its homologs positively regulated jasmonate-induced anthocyanin accumulation and root growth inhibition. PHR1 stimulated the expression of several jasmonate-responsive genes, whereas JAZ proteins interfered with its transcriptional function. Furthermore, PHR1 physically associated with the basic helix-loop-helix (bHLH) transcription factors MYC2, MYC3, and MYC4. Genetic analyses and biochemical assays indicated that PHR1 and MYC2 synergistically increased the transcription of downstream jasmonate-responsive genes and enhanced the responses to jasmonate. Collectively, our study reveals the crucial regulatory roles of PHR1 in modulating jasmonate responses and provides a mechanistic understanding of how PHR1 functions together with JAZ and MYC2 to maintain the appropriate level of jasmonate signaling under conditions of Pi deficiency.

Auxin contributes to jasmonate-mediated regulation of abscisic acid signaling during seed germination in Arabidopsis.

Abscisic acid (ABA) represses seed germination and postgerminative growth in Arabidopsis thaliana. Auxin and jasmonic acid (JA) stimulate ABA function; however, the possible synergistic effects of auxin and JA on ABA signaling and the underlying molecular mechanisms remain elusive. Here, we show that exogenous auxin works synergistically with JA to enhance the ABA-induced delay of seed germination. Auxin biosynthesis, perception, and signaling are crucial for JA-promoted ABA responses. The auxin-dependent transcription factors AUXIN RESPONSE FACTOR10 (ARF10) and ARF16 interact with JASMONATE ZIM-DOMAIN (JAZ) repressors of JA signaling. ARF10 and ARF16 positively mediate JA-increased ABA responses, and overaccumulation of ARF16 partially restores the hyposensitive phenotype of JAZ-accumulating plants defective in JA signaling in response to combined ABA and JA treatment. Furthermore, ARF10 and ARF16 physically associate with ABSCISIC ACID INSENSITIVE5 (ABI5), a critical regulator of ABA signaling, and the ability of ARF16 to stimulate JA-mediated ABA responses is mainly dependent on ABI5. ARF10 and ARF16 activate the transcriptional function of ABI5, whereas JAZ repressors antagonize their effects. Collectively, our results demonstrate that auxin contributes to the synergetic modulation of JA on ABA signaling, and explain the mechanism by which ARF10/16 coordinate with JAZ and ABI5 to integrate the auxin, JA, and ABA signaling pathways.

CO interacts with JAZ repressors and bHLH subgroup IIId factors to negatively regulate jasmonate signaling in Arabidopsis seedlings.

CONSTANS (CO) is a master flowering-time regulator that integrates photoperiodic and circadian signals in Arabidopsis thaliana. CO is expressed in multiple tissues, including young leaves and seedling roots, but little is known about the roles and underlying mechanisms of CO in mediating physiological responses other than flowering. Here, we show that CO expression is responsive to jasmonate. CO negatively modulated jasmonate-imposed root-growth inhibition and anthocyanin accumulation. Seedlings from co mutants were more sensitive to jasmonate, whereas overexpression of CO resulted in plants with reduced sensitivity to jasmonate. Moreover, CO mediated the diurnal gating of several jasmonate-responsive genes under long-day conditions. We demonstrate that CO interacts with JASMONATE ZIM-DOMAIN (JAZ) repressors of jasmonate signaling. Genetic analyses indicated that CO functions in a CORONATINE INSENSITIVE1 (COI1)-dependent manner to modulate jasmonate responses. Furthermore, CO physically associated with the basic helix-loop-helix (bHLH) subgroup IIId transcription factors bHLH3 and bHLH17. CO acted cooperatively with bHLH17 in suppressing jasmonate signaling, but JAZ proteins interfered with their transcriptional functions and physical interaction. Collectively, our results reveal the crucial regulatory effects of CO on mediating jasmonate responses and explain the mechanism by which CO works together with JAZ and bHLH subgroup IIId factors to fine-tune jasmonate signaling.

Jasmonate-regulated root growth inhibition and root hair elongation.

The phytohormone jasmonate is an essential endogenous signal in the regulation of multiple plant processes for environmental adaptation, such as primary root growth inhibition and root hair elongation. Perception of environmental stresses promotes the accumulation of jasmonate, which is sensed by the CORONATINE INSENSITIVE1 (COI1)-JASMONATE ZIM-DOMAIN (JAZ) co-receptor, triggering the degradation of JAZ repressors and induction of transcriptional reprogramming. The basic helix-loop-helix (bHLH) subgroup IIIe transcription factors MYC2, MYC3, and MYC4 are the most extensively characterized JAZ-binding factors and together stimulate jasmonate-signaled primary root growth inhibition. Conversely, the bHLH subgroup IIId transcription factors (i.e. bHLH3 and bHLH17) physically associate with JAZ proteins and suppress jasmonate-induced root growth inhibition. For root hair development, JAZ proteins interact with and inhibit ROOT HAIR DEFECTIVE 6 (RHD6) and RHD6 LIKE1 (RSL1) transcription factors to modulate jasmonate-enhanced root hair elongation. Moreover, jasmonate also interacts with other signaling pathways (such as ethylene and auxin) to regulate primary root growth and/or root hair elongation. Here, we review recent progress into jasmonate-mediated primary root growth and root hair development.

The Arabidopsis transcription factor LBD15 mediates ABA signaling and tolerance of water-deficit stress by regulating ABI4 expression.

To survive, sessile plants must adapt to grow and develop when facing water-deficit stress. However, the molecular mechanisms underlying fine-tuning of the antagonistic action between stress response and growth remain to be determined. Here, plants overexpressing Lateral Organ Boundaries Domain 15 (LBD15) showed abscisic acid (ABA) hypersensitivity and tolerance of water-deficit stress, whereas the loss-of-function mutant lbd15 presented decreased sensitivity to ABA and increased sensitivity to water-deficit stress. Further analysis revealed that LBD15 directly binds to the promoter of the ABA signaling pathway gene ABSCISIC ACID INSENSITIVE4 (ABI4) to activate its expression, thereby forming an LBD15-ABI4 cascade to optimally regulate ABA signaling-mediated plant growth and tolerance of water-deficit stress. In addition, drought stress-induced ABA signaling promoted LBD15 expression, which directly activates expression of ABI4 to close stomata. As a result, water loss is reduced, and then water-deficit stress tolerance is increased. The results of this study reveal a molecular mechanism by which LBD15 coordinates and balances plant growth and resistance to water-deficit stress.

Genome-wide identification and characterization of the lateral organ boundaries domain gene family in Brassica rapa var. rapa.

The Lateral Organ Boundaries Domain (LBD) genes encode highly conserved plant-specific LOB domain proteins which regulate growth and development in various species. However, members of the LBD gene family have yet to be identified in Brassica rapa var. rapa. In the present study, fifty-nine LBD genes were identified and distributed on 10 chromosomes. The BrrLBD proteins are predicted to encode hydrophobic polypeptides between 118 and 394 amino acids in length and with molecular weights ranging from 13.31 to 44.24 kDa; the theoretical pI for these proteins varies from 4.83 to 9.68. There were 17 paralogous gene pairs in the BrrLBD family, suggesting that the amplification of the BrrLBD gene family involved large-scale gene duplication events. Members of the BrrLBD family were divided into 7 subclades (class I a to e, class II a and b). Analysis of gene structure and conserved domains revealed that most BrrLBD genes of the same subclade had similar gene structures and protein motifs. The expression profiles of 59 BrrLBD genes were determined through Quantitative Real-time fluorescent PCR (qRT-PCR). Most BrrLBD genes in the same subclade had similar gene expression profiles. However, the expression patterns of 7 genes differed from their duplicates, indicating that although the gene function of most BrrLBD genes has been conserved, some BrrLBD genes may have undergone evolutionary change.

A Splice Variant of BrrWSD1 in Turnip (Brassica rapa var. rapa) and Its Possible Role in Wax Ester Synthesis under Drought Stress.

Cuticular wax accumulation in plants contributes to drought tolerance. Here, we compared the drought levels on two varieties with different genotypes in turnip (Brassica rapa var. rapa) and found that the drought tolerance was higher in the waxy KTRG-B48a than in the wax-free KTRG-B48b. A combination of PacBio and Illumina sequencing analyses revealed that differential transcripts were mainly enriched in the wax synthesis pathway, and a splice variant (BrrWSD1-X2) was identified in the waxy KTRG-B48a. BrrWSD1-X2 had a stronger ability to synthesize wax esters than BrrWSD1-X1 using heterologous expression in yeast (Saccharomyces cerevisiae) mutant H1246a. Then, we speculated that the T to C transversion of the third intron and the higher number of TA repeats in the third intron of BrrWSD1 DNA in the waxy KTRG-B48a may result in a lower efficiency of splicing recognition of the third intron, resulting in the emergence of BrrWSD1-X2 in waxy varieties.

Genome-Wide Identification and Characterization of BrrTCP Transcription Factors in Brassica rapa ssp. rapa.

The teosinte branched1/cycloidea/proliferating cell factor (TCP) gene family is a plant-specific transcription factor that participates in the control of plant development by regulating cell proliferation. However, no report is currently available about this gene family in turnips (Brassica rapa ssp. rapa). In this study, a genome-wide analysis of TCP genes was performed in turnips. Thirty-nine TCP genes in turnip genome were identified and distributed on 10 chromosomes. Phylogenetic analysis clearly showed that the family was classified as two clades: class I and class II. Gene structure and conserved motif analysis showed that the same clade genes have similar gene structures and conserved motifs. The expression profiles of 39 TCP genes were determined through quantitative real-time PCR. Most CIN-type BrrTCP genes were highly expressed in leaf. The members of CYC/TB1 subclade are highly expressed in flower bud and weakly expressed in root. By contrast, class I clade showed more widespread but less tissue-specific expression patterns. Yeast two-hybrid data show that BrrTCP proteins preferentially formed heterodimers. The function of BrrTCP2 was confirmed through ectopic expression of BrrTCP2 in wild-type and loss-of-function ortholog mutant of Arabidopsis. Overexpression of BrrTCP2 in wild-type Arabidopsis resulted in the diminished leaf size. Overexpression of BrrTCP2 in triple mutants of tcp2/4/10 restored the leaf phenotype of tcp2/4/10 to the phenotype of wild type. The comprehensive analysis of turnip TCP gene family provided the foundation to further study the roles of TCP genes in turnips.

Activation of secondary cell wall biosynthesis by miR319-targeted TCP4 transcription factor.

The overexpression of miR319 in plants results in delayed senescence, and high levels of miR319-targeted TCP4 transcription factor cause premature onset of this process. However, the underlying mechanisms of this pathway remain elusive. Here, we found that miR319 overexpression results in a decrease in TCP4 abundance and secondary cell wall formation in the stem. Conversely, constitutive expression of miR319-resistant TCP4 promotes secondary cell wall formation, indicating that miR319-mediated TCP4 controls secondary cell wall formation during development. Further analysis revealed that TCP4 might directly bind the promoter of VND7 to activate its expression, which triggers the expression of a VND7 transcriptional network associated with secondary cell wall biosynthesis and programmed cell death and accelerates vessel formation. In addition, the development process gradually increased TCP4 expression. These results suggest that miR319 and its target TCP4 can act as switches that turn on secondary cell wall synthesis and programmed cell death.

A novel Ap2/ERF transcription factor from Stipa purpurea leads to enhanced drought tolerance in Arabidopsis thaliana.

KEY MESSAGE: SpERF1 acts as a positive regulator, contributing to drought stress tolerance in A. thaliana through activating DRE/CRT elements in the promoters of abiotic stress-responsive genes. Stipa purpurea is an endemic perennial grass species in alpine arid and semi-arid meadows on the Qinghai-Xizang Plateau, which is highly tolerant against drought and cold. ERF transcription factors are known to regulate gene expression under abiotic and biotic treatments. Herein, we isolated a full-length ERF gene CDS from S. purpurea named SpERF1, which was induced by drought, cold, and jasmonic acid stresses. Subcellular localization revealed that SpERF1 is a nuclear protein, consistent with its roles as a transcription factor. Overexpression of SpERF1 enhanced drought tolerance of transgenic Arabidopsis thaliana via the activation of DRE/CRT elements in the promoters of abiotic stress-responsive genes. Furthermore, increased accumulation of proline indicated that SpERF1 might be involved in proline synthesis in the transgenic lines, allowing them to have a better buffering capacity and membrane protection under drought stress. This study indicated that SpERF1 might be an attractive target in the genetic engineering for improving stress tolerance in other crops. Moreover, SpERF1 protein function analysis increased our understanding of S. purpurea's ability to adapt to the adverse conditions of the Qinghai-Xizang Plateau.