Mutation of Oryza sativa CORONATINE INSENSITIVE 1b (OsCOI1b) delays leaf senescence

Jasmonic acid (JA) functions in plant development, including senescence and immunity. Arabidopsis thaliana CORONATINE INSENSITIVE 1 encodes a JA receptor and functions in the JA‐responsive signaling pathway. The Arabidopsis genome harbors a single COI gene, but the rice (Oryza sativa) genome harbors three COI homologs, OsCOI1a, OsCOI1b, and OsCOI2. Thus, it remains unclear whether each OsCOI has distinct, additive, synergistic, or redundant functions in development. Here, we use the oscoi1b‐1 knockout mutants to show that OsCOI1b mainly affects leaf senescence under senescence‐promoting conditions. oscoi1b‐1 mutants stayed green during dark‐induced and natural senescence, with substantial retention of chlorophylls and photosynthetic capacity. Furthermore, several senescence‐associated genes were downregulated in oscoi1b‐1 mutants, including homologs of Arabidopsis thaliana ETHYLENE INSENSITIVE 3 and ORESARA 1, important regulators of leaf senescence. These results suggest that crosstalk between JA signaling and ethylene signaling affects leaf senescence. The Arabidopsis coi1‐1 plants containing 35S:OsCOI1a or 35S:OsCOI1b rescued the delayed leaf senescence during dark incubation, suggesting that both OsCOI1a and OsCOI1b are required for promoting leaf senescence in rice. oscoi1b‐1 mutants showed significant decreases in spikelet fertility and grain weight, leading to severe reduction of grain yield, indicating that OsCOI1‐mediated JA signaling affects spikelet fertility and grain filling.     

Design and synthesis of biotin-tagged photoaffinity probes of jasmonates

Jasmonates (JAs) are a class of oxylipin compounds that play diverse roles in plant defense and development. The F-box protein coronatine insensitive1 (COI1) plays a crucial role in the JA signaling pathway. To determine whether COI1 binds directly to jasmonates, three biotin-tagged photoaffinity probes for JAs, a jasmonic acid photoaffinity probe (PAJA), a JAIle photoaffinity probe (PAJAIle), and a coronatine photoaffinity probe (PACOR), were designed and synthesized based on analysis of JA structure–activity relationships and molecular modeling of the interaction between COI1 and JAs. Among them, PACOR exhibited the most significant biological activity in inhibiting root growth, promoting accumulation of JA-responsive proteins, and triggering COI1–JAZ1 interaction in Arabidopsis seedlings. PACOR is an effective tool for elucidating the interaction between COI1 and JA.     

Amino acid substitutions of GLY98, LEU245 and GLU543 in COI1 distinctively affect jasmonate-regulated male fertility in Arabidopsis

Jasmonate (JA) regulates various plant defense and developmental processes. The F-box protein CORONATINE INSENSITIVE 1 (COI1) perceives JA signals to mediate diverse plant responses including male fertility, root growth, anthocyanin accumulation, and defense against abiotic and biotic stresses. In this study, we carried out genetic, physiological and biochemical analysis on a series of coi1 mutant alleles, and found that different amino acid mutations in COI1 distinctively affect JA-regulated male fertility in Arabidopsis. All the JA responses are disrupted by the COI1 mutations W467* in coi1-1, Q343* (coi1-6), G369E (coi1-4), G98D (coi1-5), G155E (coi1-7), D452A (coi1-9) and L490A (coi1-10), though the coi1-5 mutant (COI1_G98D) contains adequate COI1 protein (～60% of wild-type). Interestingly, the low basal level of COI1^E543K in the coi1-8 mutant (～10% of wild-type COI1 level) is sufficient for maintaining male fertility (～50% of wild-type fertility); the coi1-2 mutant with low level of COI1^L245F (～10% of wild-type) is male sterile under normal growth condition (22°C) but male fertile (～80% of wild-type fertility) at low temperature (16°C); however, both coi1-2 and coi1-8 are defective in the other JA responses (root growth, anthocyanin accumulation, and plant response to the pathogen Pst DC3000 infection).     

Jasmonic acid perception by COI1 involves inositol polyphosphates in Arabidopsis thaliana

Plant responses to wounding are part of their defense responses against insects, and are tightly regulated. The isoleucin conjugate of jasmonic acid (JA‐Ile) is a major regulatory molecule. We have previously shown that inositol polyphosphate signals are required for defense responses in Arabidopsis; however, the way in which inositol polyphosphates contribute to plant responses to wounding has so far remained unclear. Arabidopsis F‐box proteins involved in the perception of JA‐Ile (COI1) and auxin (TIR1) are structurally similar. Because TIR1 has recently been shown to contain inositol hexakisphosphate (InsP₆) as a co‐factor of unknown function, here we explored the possibility that InsP₆ or another inositol polyphosphate is required for COI1 function. In support of this hypothesis, COI1 variants with changes in putative inositol polyphosphate coordinating residues exhibited a reduced interaction with the COI1 target, JAZ9, in yeast two‐hybrid tests. The equivalent COI1 variants displayed a reduced capability to rescue jasmonate‐mediated root growth inhibition or silique development in Arabidopsis coi1 mutants. Yeast two‐hybrid tests using wild‐type COI1 in an ipk1Δ yeast strain exhibiting increased levels of inositol pentakisphosphate (InsP₅) and reduced levels of InsP₆ indicate an enhanced COI1/JAZ9 interaction. Consistent with these findings, Arabidopsis ipk1‐1 mutants, also with increased InsP₅ and reduced InsP₆ levels, showed increased defensive capabilities via COI1‐mediated processes, including wound‐induced gene expression, defense against caterpillars or root growth inhibition by jasmonate. The combined data from experiments using mutated COI1 variants, as well as yeast and Arabidopsis backgrounds altered in inositol polyphosphate metabolism, indicate that an inositol polyphosphate, and probably InsP₅, contributes to COI1 function.     

OsCOI1，水稻中一个受茉莉酸甲酯和脱落酸诱导表达的F-box家族基因

利用Blast检索、EST分析和RT-PCR, 在水稻中分离到一个与拟南芥COI1同源的新基因,命名为OsCOI1. OsCOI1编码595个氨基酸. 推测的OsCOI1编码蛋白有一个F-box motif和16个富含亮氨酸的重复序列,这与拟南芥COI1相似. OsCOI1在氨基酸水平上和COI1有很高的同源性(74%). 经半定量RT-PCR法和RNA印迹分析,表明水稻中OsCOI1表达水平在经茉莉酸甲酯和脱落酸处理后呈明显变化,但不受水杨酸和乙烯的影响,说明OsCOI1可能在茉莉酸信号途径和脱落酸途径中具有特定功能.     

Specific Missense Alleles of the Arabidopsis Jasmonic Acid Co-Receptor COI1 Regulate Innate Immune Receptor Accumulation and Function

Plants utilize proteins containing nucleotide binding site (NB) and leucine-rich repeat (LRR) domains as intracellular innate immune receptors to recognize pathogens and initiate defense responses. Since mis-activation of defense responses can lead to tissue damage and even developmental arrest, proper regulation of NB–LRR protein signaling is critical. RAR1, SGT1, and HSP90 act as regulatory chaperones of pre-activation NB–LRR steady-state proteins. We extended our analysis of mutants derived from a rar1 suppressor screen and present two allelic rar1 suppressor (rsp) mutations of Arabidopsis COI1. Like all other coi1 mutations, coi1^rsp missense mutations impair Jasmonic Acid (JA) signaling resulting in JA–insensitivity. However, unlike previously identified coi1 alleles, both coi1^rsp alleles lack a male sterile phenotype. The coi1^rsp mutants express two sets of disease resistance phenotypes. The first, also observed in coi1-1 null allele, includes enhanced basal defense against the virulent bacterial pathogen Pto DC3000 and enhanced effector-triggered immunity (ETI) mediated by the NB–LRR RPM1 protein in both rar1 and wild-type backgrounds. These enhanced disease resistance phenotypes depend on the JA signaling function of COI1. Additionally, the coi1^rsp mutants showed a unique inability to properly regulate RPM1 accumulation and HR, exhibited increased RPM1 levels in rar1, and weakened RPM1-mediated HR in RAR1. Importantly, there was no change in the steady-state levels or HR function of RPM1 in coi1-1. These results suggest that the coi1^rsp proteins regulate NB–LRR protein accumulation independent of JA signaling. Based on the phenotypic similarities and genetic interactions among coi1^rsp, sgt1b, and hsp90.2^rsp mutants, our data suggest that COI1 affects NB–LRR accumulation via two NB–LRR co-chaperones, SGT1b and HSP90. Together, our data demonstrate a role for COI1 in disease resistance independent of JA signaling and provide a molecular link between the JA and NB–LRR signaling pathways.     

A small molecule antagonizes jasmonic acid perception and auxin responses in vascular and nonvascular plants

The phytohormone jasmonoyl-L-isoleucine (JA-Ile) regulates many stress responses and developmental processes in plants. A co-receptor complex formed by the F-box protein Coronatine Insensitive 1 (COI1) and a Jasmonate (JA) ZIM-domain (JAZ) repressor perceives the hormone. JA-Ile antagonists are invaluable tools for exploring the role of JA-Ile in specific tissues and developmental stages, and for identifying regulatory processes of the signaling pathway. Using two complementary chemical screens, we identified three compounds that exhibit a robust inhibitory effect on both the hormone-mediated COI–JAZ interaction and degradation of JAZ1 and JAZ9 in vivo. One molecule, J4, also restrains specific JA-induced physiological responses in different angiosperm plants, including JA-mediated gene expression, growth inhibition, chlorophyll degradation, and anthocyanin accumulation. Interaction experiments with purified proteins indicate that J4 directly interferes with the formation of the Arabidopsis (Arabidopsis thaliana) COI1–JAZ complex otherwise induced by JA. The antagonistic effect of J4 on COI1–JAZ also occurs in the liverwort Marchantia polymorpha, suggesting the mode of action is conserved in land plants. Besides JA signaling, J4 works as an antagonist of the closely related auxin signaling pathway, preventing Transport Inhibitor Response1/Aux–indole-3-acetic acid interaction and auxin responses in planta, including hormone-mediated degradation of an auxin repressor, gene expression, and gravitropic response. However, J4 does not affect other hormonal pathways. Altogether, our results show that this dual antagonist competes with JA-Ile and auxin, preventing the formation of phylogenetically related receptor complexes. J4 may be a useful tool to dissect both the JA-Ile and auxin pathways in particular tissues and developmental stages since it reversibly inhibits these pathways.One-sentence summary: A chemical screen identified a molecule that antagonizes jasmonate perception by directly interfering with receptor complex formation in phylogenetically distant vascular and nonvascular plants.     

Fusarium oxysporum hijacks COI1-mediated jasmonate signaling to promote disease development in Arabidopsis

Although defense responses mediated by the plant oxylipin jasmonic acid (JA) are often necessary for resistance against pathogens with necrotrophic lifestyles, in this report we demonstrate that jasmonate signaling mediated through COI1 in Arabidopsis thaliana is responsible for susceptibility to wilt disease caused by the root-infecting fungal pathogen Fusarium oxysporum . Despite compromised JA-dependent defense responses, the JA perception mutant coronatine insensitive 1 ( coi1 ), but not JA biosynthesis mutants, exhibited a high level of resistance to wilt disease caused by F. oxysporum . This response was independent from salicylic acid-dependent defenses, as coi1/NahG plants showed similar disease resistance to coi1 plants. Inoculation of reciprocal grafts made between coi1 and wild-type plants revealed that coi1 -mediated resistance occurred primarily through the coi1 rootstock tissues. Furthermore, microscopy and quantification of fungal DNA during infection indicated that coi1 -mediated resistance was not associated with reduced fungal penetration and colonization until a late stage of infection, when leaf necrosis was highly developed in wild-type plants. In contrast to wild-type leaves, coi1 leaves showed no necrosis following the application of F. oxysporum culture filtrate, and showed reduced expression of senescence-associated genes during disease development, suggesting that coi1 resistance is most likely achieved through the inhibition of F. oxysporum -incited lesion development and plant senescence. Together, our results indicate that F. oxysporum hijacks non-defensive aspects of the JA-signaling pathway to cause wilt-disease symptoms that lead to plant death in Arabidopsis.     

OsCOI1,水稻中一个受茉莉酸甲酯和脱落酸诱导表达的F-box家族基因

利用Blast检索、EST分析和RT-PCR,在水稻中分离到一个与拟南芥COI1同源的新基因,命名为OsCOI1.OsCOI1编码595个氨基酸.推测的OsCOI1编码蛋白有一个F-boxmotif和16个富含亮氨酸的重复序列,这与拟南芥COI1相似.OsCOI1在氨基酸水平上和COI1有很高的同源性(74%).经半定量RT-PCR法和RNA印迹分析,表明水稻中OsCOI1表达水平在经茉莉酸甲酯和脱落酸处理后呈明显变化,但不受水杨酸和乙烯的影响,说明OsCOI1可能在茉莉酸信号途径和脱落酸途径中具有特定功能.