Daoxin Xie

The SCF(COI1) ubiquitin-ligase complexes are required for jasmonate response in Arabidopsis.

Xie and colleagues previously isolated the Arabidopsis COI1 gene that is required for response to jasmonates (JAs), which regulate root growth, pollen fertility, wound healing, and defense against insects and pathogens. In this study, we demonstrate that COI1 associates physically with AtCUL1, AtRbx1, and either of the Arabidopsis Skp1-like proteins ASK1 or ASK2 to assemble ubiquitin-ligase complexes, which we have designated SCFCOI1. COI1E22A, a single amino acid substitution in the F-box motif of COI1, abolishes the formation of the SCFCOI1 complexes and results in loss of the JA response. AtRbx1 double-stranded RNA-mediated genetic interference reduces AtRbx1 expression and affects JA-inducible gene expression. Furthermore, we show that the AtCUL1 component of SCFCOI1 complexes is modified in planta, where mutations in AXR1 decrease the abundance of the modified AtCUL1 of SCFCOI1 and lead to a reduction in JA response. Finally, we demonstrate that the axr1 and coi1 mutations display a synergistic genetic interaction in the double mutant. These results suggest that the COI1-mediated JA response is dependent on the SCFCOI1 complexes in Arabidopsis and that the AXR1-dependent modification of the AtCUL1 subunit of SCFCOI1 complexes is important for JA signaling.

The Arabidopsis CORONATINE INSENSITIVE1 Protein Is a Jasmonate Receptor

Jasmonates play a number of diverse roles in plant defense and development. CORONATINE INSENSITIVE1 (COI1), an F-box protein essential for all the jasmonate responses, interacts with multiple proteins to form the SCFCOI1 E3 ubiquitin ligase complex and recruits jasmonate ZIM-domain (JAZ) proteins for degradation by the 26S proteasome. To determine which protein directly binds to jasmonoyl-isoleucine (JA-Ile)/coronatine (COR) and serves as a receptor for jasmonate, we built a high-quality structural model of COI1 and performed molecular modeling of COI1–jasmonate interactions. Our results imply that COI1 has the structural traits for binding JA-Ile or COR. The direct binding of these molecules with COI1 was further examined using a combination of molecular and biochemical approaches. First, we used the immobilized jasmonate approach to show that the COI1 protein in crude leaf extracts can bind to the jasmonate moiety of JA-Ile. Second, we employed surface plasmon resonance technology with purified COI1 and JAZ1 protein to reveal the interaction among COI1, JA-Ile, and JAZ1. Finally, we used the photoaffinity labeling technology to show the direct binding of COR with purified insect-expressed COI1. Taken together, these results demonstrate that COI1 directly binds to JA-Ile and COR and serves as a receptor for jasmonate.

The Jasmonate-ZIM-Domain Proteins Interact with the WD-Repeat/bHLH/MYB Complexes to Regulate Jasmonate-Mediated Anthocyanin Accumulation and Trichome Initiation in Arabidopsis thaliana

This work examines the molecular mechanism of jasmonate regulation of anthocyanin biosynthesis and trichome initiation. It identifies three bHLH transcription factors and two MYB transcription factors as new targets of JAZ proteins, showing that JAZ proteins attenuate the transcriptional function of WD-repeat/bHLH/MYB complexes to regulate anthocyanin accumulation and trichome. Jasmonates (JAs) mediate plant responses to insect attack, wounding, pathogen infection, stress, and UV damage and regulate plant fertility, anthocyanin accumulation, trichome formation, and many other plant developmental processes. Arabidopsis thaliana Jasmonate ZIM-domain (JAZ) proteins, substrates of the CORONATINE INSENSITIVE1 (COI1)–based SCFCOI1 complex, negatively regulate these plant responses. Little is known about the molecular mechanism for JA regulation of anthocyanin accumulation and trichome initiation. In this study, we revealed that JAZ proteins interact with bHLH (Transparent Testa8, Glabra3 [GL3], and Enhancer of Glabra3 [EGL3]) and R2R3 MYB transcription factors (MYB75 and Glabra1), essential components of WD-repeat/bHLH/MYB transcriptional complexes, to repress JA-regulated anthocyanin accumulation and trichome initiation. Genetic and physiological evidence showed that JA regulates WD-repeat/bHLH/MYB complex-mediated anthocyanin accumulation and trichome initiation in a COI1-dependent manner. Overexpression of the MYB transcription factor MYB75 and bHLH factors (GL3 and EGL3) restored anthocyanin accumulation and trichome initiation in the coi1 mutant, respectively. We speculate that the JA-induced degradation of JAZ proteins abolishes the interactions of JAZ proteins with bHLH and MYB factors, allowing the transcriptional function of WD-repeat/bHLH/MYB complexes, which subsequently activate respective downstream signal cascades to modulate anthocyanin accumulation and trichome initiation.

The Jasmonate-ZIM Domain Proteins Interact with the R2R3-MYB Transcription Factors MYB21 and MYB24 to Affect Jasmonate-Regulated Stamen Development in Arabidopsis

Jasmonate is essential for diverse biological processes, including male fertility and plant defense in Arabidopsis. This work shows that the R2R3-MYB transcription factors MYB21 and MYB24 function as direct targets of JAZ proteins to mediate jasmonate-regulated stamen development. The Arabidopsis thaliana F-box protein CORONATINE INSENSITIVE1 (COI1) perceives jasmonate (JA) signals and subsequently targets the Jasmonate-ZIM domain proteins (JAZs) for degradation by the SCFCOI1-26S proteasome pathway to mediate various jasmonate-regulated processes, including fertility, root growth, anthocyanin accumulation, senescence, and defense. In this study, we screened JAZ-interacting proteins from an Arabidopsis cDNA library in the yeast two-hybrid system. MYB21 and MYB24, two R2R3-MYB transcription factors, were found to interact with JAZ1, JAZ8, and JAZ11 in yeast and in planta. Genetic and physiological experiments showed that the myb21 myb24 double mutant exhibited defects specifically in pollen maturation, anther dehiscence, and filament elongation leading to male sterility. Transgenic expression of MYB21 in the coi1-1 mutant was able to rescue male fertility partially but unable to recover JA-regulated root growth inhibition, anthocyanin accumulation, and plant defense. These results demonstrate that the R2R3-MYB transcription factors MYB21 and MYB24 function as direct targets of JAZs to regulate male fertility specifically. We speculate that JAZs interact with MYB21 and MYB24 to attenuate their transcriptional function; upon perception of JA signal, COI1 recruits JAZs to the SCFCOI1 complex for ubiquitination and degradation through the 26S proteasome; MYB21 and MYB24 are then released to activate expression of various genes essential for JA-regulated anther development and filament elongation.

Viral Virulence Protein Suppresses RNA Silencing–Mediated Defense but Upregulates the Role of MicroRNA in Host Gene Expression

Small interfering RNAs (siRNAs) and microRNAs (miRNAs) are processed by the ribonuclease Dicer from distinct precursors, double-stranded RNA (dsRNA) and hairpin RNAs, respectively, although either may guide RNA silencing via a similar complex. The siRNA pathway is antiviral, whereas an emerging role for miRNAs is in the control of development. Here, we describe a virulence factor encoded by turnip yellow mosaic virus, p69, which suppresses the siRNA pathway but promotes the miRNA pathway in Arabidopsis thaliana. p69 suppression of the siRNA pathway is upstream of dsRNA and is as effective as genetic mutations in A. thaliana genes involved in dsRNA production. Possibly as a consequence of p69 suppression, p69-expressing plants contained elevated levels of a Dicer mRNA and of miRNAs as well as a correspondingly enhanced miRNA-guided cleavage of two host mRNAs. Because p69-expressing plants exhibited disease-like symptoms in the absence of viral infection, our findings suggest a novel mechanism for viral virulence by promoting the miRNA-guided inhibition of host gene expression.

Control of a key transition from prostrate to erect growth in rice domestication.

The transition from the prostrate growth of ancestral wild rice (O. rufipogon Griff.) to the erect growth of Oryza sativa cultivars was one of the most critical events in rice domestication. This evolutionary step importantly improved plant architecture and increased grain yield. Here we find that prostrate growth of wild rice from Yuanjiang County in China is controlled by a semi-dominant gene, PROG1 (PROSTRATE GROWTH 1), on chromosome 7 that encodes a single Cys2-His2 zinc-finger protein. prog1 variants identified in O. sativa disrupt the prog1 function and inactivate prog1 expression, leading to erect growth, greater grain number and higher grain yield in cultivated rice. Sequence comparison shows that 182 varieties of cultivated rice, including 87 indica and 95 japonica cultivars from 17 countries, carry identical mutations in the prog1 coding region that may have become fixed during rice domestication.

Gibberellin Acts through Jasmonate to Control the Expression of MYB21, MYB24, and MYB57 to Promote Stamen Filament Growth in Arabidopsis

Precise coordination between stamen and pistil development is essential to make a fertile flower. Mutations impairing stamen filament elongation, pollen maturation, or anther dehiscence will cause male sterility. Deficiency in plant hormone gibberellin (GA) causes male sterility due to accumulation of DELLA proteins, and GA triggers DELLA degradation to promote stamen development. Deficiency in plant hormone jasmonate (JA) also causes male sterility. However, little is known about the relationship between GA and JA in controlling stamen development. Here, we show that MYB21, MYB24, and MYB57 are GA-dependent stamen-enriched genes. Loss-of-function of two DELLAs RGA and RGL2 restores the expression of these three MYB genes together with restoration of stamen filament growth in GA-deficient plants. Genetic analysis showed that the myb21-t1 myb24-t1 myb57-t1 triple mutant confers a short stamen phenotype leading to male sterility. Further genetic and molecular studies demonstrate that GA suppresses DELLAs to mobilize the expression of the key JA biosynthesis gene DAD1, and this is consistent with the observation that the JA content in the young flower buds of the GA-deficient quadruple mutant ga1-3 gai-t6 rga-t2 rgl1-1 is much lower than that in the WT. We conclude that GA promotes JA biosynthesis to control the expression of MYB21, MYB24, and MYB57. Therefore, we have established a hierarchical relationship between GA and JA in that modulation of JA pathway by GA is one of the prerequisites for GA to regulate the normal stamen development in Arabidopsis.

The COP9 Signalosome Interacts Physically with SCF COI1 and Modulates Jasmonate Responses

The COP9 signalosome (CSN) is an evolutionarily conserved, nucleus-enriched multiprotein complex. CSN plays roles in photomorphogenesis, auxin response, and floral organ formation, possibly via the regulation of ubiquitin-proteasome–mediated protein degradation. COI1 encodes an F-box protein, which is a subunit of SCFCOI1 E3 ubiquitin ligase, and is required for jasmonate (JA) responses. Here, we demonstrate using coimmunoprecipitation and gel-filtration analyses that endogenous as well as epitope-tagged COI1 forms SCFCOI1 and associates directly with CSN in vivo. Like the coi1-1 mutant, CSN reduction-of-function plants exhibited a JA-insensitive root elongation phenotype and an absence of JA-induced–specific gene expression. Genome expression profile analyses indicated that JA-triggered genome expression is critically dependent on COI1 dosage. More importantly, most of the COI1-dependent JA-responsive genes also required CSN function, and CSN abundance was shown to be important for JA responses. Furthermore, we showed that both COI1 and CSN are essential for modulating the expression of genes in most cellular pathways responsive to JA. Thus, CSN and SCFCOI1 work together to control genome expression and promote JA responses.

The bHLH Transcription Factor MYC3 Interacts with the Jasmonate ZIM-Domain Proteins to Mediate Jasmonate Response in Arabidopsis

The Arabidopsis Jasmonate ZIM-domain proteins (JAZs) act as substrates of SCF(COI1) complex to repress their downstream targets, which are essential for JA-regulated plant development and defense. The bHLH transcription factor MYC2 was found to interact with JAZs and mediate JA responses including JA-inhibitory root growth. Here, we identified another bHLH transcription factor MYC3 which directly interacted with JAZs by virtue of its N-terminal region to regulate JA responses. The transgenic plants with overexpression of MYC3 exhibited hypersensitivity in JA-inhibitory root elongation and seedling development. The JAZ-interacting pattern and the JA-induced expression pattern of MYC3 were distinguishable from those of MYC2. We speculate that MYC3 and MYC2 may have redundant but also distinguishable functions in regulation of JA responses.

COS1: An Arabidopsis coronatine insensitive1 Suppressor Essential for Regulation of Jasmonate-Mediated Plant Defense and Senescence

The Arabidopsis thaliana CORONATINE INSENSITIVE1 (COI1) gene encodes an F-box protein to assemble SCFCOI1 complexes essential for response to jasmonates (JAs), which are a family of plant signaling molecules required for many essential functions, including plant defense and reproduction. To better understand the molecular basis of JA action, we screened for suppressors of coi1 and isolated a coi1 suppressor1 (cos1) mutant. The cos1 mutation restores the coi1-related phenotypes, including defects in JA sensitivity, senescence, and plant defense responses. The COS1 gene was cloned through a map-based approach and found to encode lumazine synthase, a key component in the riboflavin pathway that is essential for diverse yet critical cellular processes. We demonstrated a novel function for the riboflavin pathway that acts downstream of COI1 in the JA signaling pathway and is required for suppression of the COI1-mediated root growth, senescence, and plant defense.