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BRInging light to hormone receptor activation
trends in plant science Journal Club
BRInging light to hormone receptor activation Brassinosteroid (BR), a plant hormone of growing interest and importance, is involved in a range of developmental processes, such as stem elongation, root growth and light signaling. The model organism, Arabidopsis, has been useful in dissecting the role of BR in plant development and photomorphogenesis. Mutant plants that are defective in BR biosynthesis develop in the dark as if they were grown in the light. One interesting mutant, bri1, resembles BR biosynthetic mutants but is insensitive to exogenous BR. The BRI gene encodes a leucine-rich repeat receptor-like kinase (LRR-RLK), the hypothesized BR receptor, the LRR being extracellular. There are .100 LRR-RLKclass genes in Arabidopsis, and so understanding how these potential receptors function is of fundamental importance. Now, Zuhua He and colleagues1 report important insight into the mechanism of receptor activation in plants. The group constructed a chimeric receptor and uncoupled
two domains that are necessary for specific signaling. The XA21 LRR-RLK from rice confers bacterial resistance, and resembles BRI1, the LRR and RLK domains flanking a transmembranal domain. He et al. demonstrated that a chimeric BRI1 LRR-Xa21 RLK gene activated defense responses in rice upon treatment with BR. The defense-response markers included cell death, oxidative burst and genes upregulated in disease response. One defense-response gene was induced in a BR-dose-dependent manner. Both functional BRI1 LRR and Xa21 kinase domains were necessary for the activation, as missense mutants in either domain abolished activation. The results suggest that BR binds specifically to the BRI1 LRR and activates the kinase domain, which then signals downstream responses. This study gives a clearer understanding of the primary events of BR signaling, but also paves the way for a more general mechanism of LRR-RLK activation in plants.
ORCAstrating plant primary and secondary metabolism Primary metabolic pathways contribute the necessary components for plant life, but they also produce precursors that are consumed for secondary metabolite biosynthesis. Secondary metabolites are non-essential but confer predator and stress protection, increasing plantsÕ fitness; the regulation of their biosynthesis is not well understood, but some are accumulated by (methyl)jasmonate (MeJA), a stress response plant hormone. One class of secondary metabolites, the terpenoid indole alkaloids (TIAs), originates from the tryptophan biosynthetic pathway and is activated by exogenous MeJA. Tryptophan acts as a primary metabolite precursor in TIA secondary metabolite biosynthesis, but little is understood about how the primary and secondary metabolism pathways are coordinated. Now, Leslie van der Fits and Johan Memelink1 provide a link between these primary and secondary metabolic pathways. A metabolism activation screen identified a key metabolic regulator, ORCA3, in periwinkle (Vinca). Constitutive expression of Orca3 activates primary and secondary metabolism pathways; overexpression induces tryptophan and TIA biosynthetic gene expression. When fed a limiting TIA precursor, Orca3 overex366
September 2000, Vol. 5, No. 9
pression lines also have increased tryptophan levels and TIA, demonstrating that both pathways are activated. ORCA3 has a DNA-binding motif that is present in the AP2/EREBP class of plant transcription factors. ORCA3 transcriptionally activates at least three genes that involve primary (tryptophan) and secondary (TIA) metabolism pathways by binding directly to their promoters. Exogenous MeJA induces Orca3 expression and downstream primary and secondary metabolism genes, demonstrating the role of ORCA3 in stress response. The current research has identified ORCA3 as a transcription factor that coordinately regulates primary and secondary metabolism pathways in plants. This suggests a model where MeJA-mediated stress response induces ORCA3-dependent metabolic activation, increasing the primary metabolic pathway that feeds tryptophan into the TIA secondary metabolism pathway, increasing TIA-mediated protection. It will be important to uncover the range of stress responses involving ORCA3. Do master genes such as Orca3 regulate other metabolic pathways? How does the stress hormone, MeJA, activate Orca3 expression? Does Orca3 overexpres-
Research suggests a model where BR interacts with the LRR domain, activating the RLK, which then transduces the signal inside the plant cell. This opens a new path of receptor research in plants, with many questions remaining. Does BR binding activate kinase activity? What is the topography of the Bri1 protein? What are downstream components of the signaling pathway? He et al. have devised a powerful approach to dissect the molecular mechanism of receptors in plants and their research points to exciting directions for the future. Trevor Stokes (e-mail [email protected])
Reference 1 He, Z. et al. (2000) Perception of Brassinosteroids by the extracellular domain of the receptor kinase BRI1. Science 288, 2360Ð2363
sion affect plant fitness? van der Fits and Memelink identify a master regulator that connects primary and secondary metabolite biosyntheses, an important step in understanding how plants orchestrate metabolic pathways. Trevor Stokes (e-mail [email protected]) Reference 1 van der Fits, L. and Memelink, J. (2000) ORCA3, a jasmonate-responsive transcriptional regulator of plant primary and secondary metabolism. Science 289, 295Ð297
Letters Correspondence in Trends in Plant Science may address topics raised in very recent issues of the magazine, or other matters of general current interest to plant scientists. Letters should be no more than 700Ð800 words long with a maximum of 12 references and one small figure. Letters should be sent, together with an electronic copy on disc to the Editor (or e-mail your text to [email protected]). The decision to publish rests with the Editor, and the author(s) of any article highlighted in Correspondence will normally be invited to reply.
1360 - 1385/00/$ Ð see front matter © 2000 Elsevier Science Ltd. All rights reserved.
BRInging light to hormone receptor activation Brassinosteroid (BR), a plant hormone of growing interest and importance, is involved in a range of developmental processes, such as stem elongation, root growth and light signaling. The model organism, Arabidopsis, has been useful in dissecting the role of BR in plant development and photomorphogenesis. Mutant plants that are defective in BR biosynthesis develop in the dark as if they were grown in the light. One interesting mutant, bri1, resembles BR biosynthetic mutants but is insensitive to exogenous BR. The BRI gene encodes a leucine-rich repeat receptor-like kinase (LRR-RLK), the hypothesized BR receptor, the LRR being extracellular. There are .100 LRR-RLKclass genes in Arabidopsis, and so understanding how these potential receptors function is of fundamental importance. Now, Zuhua He and colleagues1 report important insight into the mechanism of receptor activation in plants. The group constructed a chimeric receptor and uncoupled
two domains that are necessary for specific signaling. The XA21 LRR-RLK from rice confers bacterial resistance, and resembles BRI1, the LRR and RLK domains flanking a transmembranal domain. He et al. demonstrated that a chimeric BRI1 LRR-Xa21 RLK gene activated defense responses in rice upon treatment with BR. The defense-response markers included cell death, oxidative burst and genes upregulated in disease response. One defense-response gene was induced in a BR-dose-dependent manner. Both functional BRI1 LRR and Xa21 kinase domains were necessary for the activation, as missense mutants in either domain abolished activation. The results suggest that BR binds specifically to the BRI1 LRR and activates the kinase domain, which then signals downstream responses. This study gives a clearer understanding of the primary events of BR signaling, but also paves the way for a more general mechanism of LRR-RLK activation in plants.
ORCAstrating plant primary and secondary metabolism Primary metabolic pathways contribute the necessary components for plant life, but they also produce precursors that are consumed for secondary metabolite biosynthesis. Secondary metabolites are non-essential but confer predator and stress protection, increasing plantsÕ fitness; the regulation of their biosynthesis is not well understood, but some are accumulated by (methyl)jasmonate (MeJA), a stress response plant hormone. One class of secondary metabolites, the terpenoid indole alkaloids (TIAs), originates from the tryptophan biosynthetic pathway and is activated by exogenous MeJA. Tryptophan acts as a primary metabolite precursor in TIA secondary metabolite biosynthesis, but little is understood about how the primary and secondary metabolism pathways are coordinated. Now, Leslie van der Fits and Johan Memelink1 provide a link between these primary and secondary metabolic pathways. A metabolism activation screen identified a key metabolic regulator, ORCA3, in periwinkle (Vinca). Constitutive expression of Orca3 activates primary and secondary metabolism pathways; overexpression induces tryptophan and TIA biosynthetic gene expression. When fed a limiting TIA precursor, Orca3 overex366
September 2000, Vol. 5, No. 9
pression lines also have increased tryptophan levels and TIA, demonstrating that both pathways are activated. ORCA3 has a DNA-binding motif that is present in the AP2/EREBP class of plant transcription factors. ORCA3 transcriptionally activates at least three genes that involve primary (tryptophan) and secondary (TIA) metabolism pathways by binding directly to their promoters. Exogenous MeJA induces Orca3 expression and downstream primary and secondary metabolism genes, demonstrating the role of ORCA3 in stress response. The current research has identified ORCA3 as a transcription factor that coordinately regulates primary and secondary metabolism pathways in plants. This suggests a model where MeJA-mediated stress response induces ORCA3-dependent metabolic activation, increasing the primary metabolic pathway that feeds tryptophan into the TIA secondary metabolism pathway, increasing TIA-mediated protection. It will be important to uncover the range of stress responses involving ORCA3. Do master genes such as Orca3 regulate other metabolic pathways? How does the stress hormone, MeJA, activate Orca3 expression? Does Orca3 overexpres-
Research suggests a model where BR interacts with the LRR domain, activating the RLK, which then transduces the signal inside the plant cell. This opens a new path of receptor research in plants, with many questions remaining. Does BR binding activate kinase activity? What is the topography of the Bri1 protein? What are downstream components of the signaling pathway? He et al. have devised a powerful approach to dissect the molecular mechanism of receptors in plants and their research points to exciting directions for the future. Trevor Stokes (e-mail [email protected])
Reference 1 He, Z. et al. (2000) Perception of Brassinosteroids by the extracellular domain of the receptor kinase BRI1. Science 288, 2360Ð2363
sion affect plant fitness? van der Fits and Memelink identify a master regulator that connects primary and secondary metabolite biosyntheses, an important step in understanding how plants orchestrate metabolic pathways. Trevor Stokes (e-mail [email protected]) Reference 1 van der Fits, L. and Memelink, J. (2000) ORCA3, a jasmonate-responsive transcriptional regulator of plant primary and secondary metabolism. Science 289, 295Ð297
Letters Correspondence in Trends in Plant Science may address topics raised in very recent issues of the magazine, or other matters of general current interest to plant scientists. Letters should be no more than 700Ð800 words long with a maximum of 12 references and one small figure. Letters should be sent, together with an electronic copy on disc to the Editor (or e-mail your text to [email protected]). The decision to publish rests with the Editor, and the author(s) of any article highlighted in Correspondence will normally be invited to reply.
1360 - 1385/00/$ Ð see front matter © 2000 Elsevier Science Ltd. All rights reserved.