Differences in salicylic acid glucose conjugations by UGT74F1 and UGT74F2 from Arabidopsis thaliana
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Abstract Salicylic acid (SA) is a signaling molecule utilized by plants in response to various stresses. Through conjugation with small organic molecules such as glucose, an inactive form of SA is generated which can be transported into and stored in plant vacuoles. In the model organism Arabidopsis thaliana, SA glucose conjugates are formed by two homologous enzymes (UGT74F1 and UGT74F2) that transfer glucose from UDP-glucose to SA. Despite being 77% identical and with conserved active site residues, these enzymes catalyze the formation of different products: UGT74F1 forms salicylic acid glucoside (SAG), while UGT74F2 forms primarily salicylic acid glucose ester (SGE). The position of the glucose on the aglycone determines how SA is stored, further metabolized, and contributes to a defense response. We determined the crystal structures of the UGT74F2 wild-type and T15S mutant enzymes, in different substrate/product complexes. On the basis of the crystal structures and the effect on enzyme activity of mutations in the SA binding site, we propose the catalytic mechanism of SGE and SAG formation and that SA binds to the active site in two conformations, with each enzyme selecting a certain binding mode of SA. Additionally, we show that two threonines are key determinants of product specificity.Keywords:
Aglycone
Product inhibition
Jasmonic acid
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Seeds of rape (Brassica napus L.) were cultured in the presence of exogenous salicylic acid ranging from 0.08 to 10.00 μmol/L (0、 0.08、 0.40、 2.00 and 10.00 μmol/L)to see the effect of salicylic acid on the formation of lateral roots in rape. The results show that 0.40 μmol/L salicylic acid treatment enhanced the formation of lateral roots significantly, in which the number of lateral roots was increased by 47.8% compared with the control. More auxin and lower abscisic acid were detected both in leafs and roots of the treatment of 0.40 μmol/L salicylic acid. It showed that salicylic acid might be involved in the regulation of lateral root formation through influencing the content of endogenous auxin and abscisic acid in rape.
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Salicylic acid is an important endogenous signal molecule in the activation of plant defense responses. The basic properties and the function of salicylic acid involved in plant disease resistance are introduced. The operation mechanisms of salicylic acid inducing plant disease resistance have been discussed in the interaction of salicylic acid with salicylic acid_binding protein and salicylic acid_dependent signalling pathway with salicylic acid_independent signalling pathway. Finally, significance of the study on mechanisms of salicylic acid action are summarized.
Systemic Acquired Resistance
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Salicylic acid (SA) is a new resistance inducer, and has been a focus in development of SA-induced resistance of plants to diseases and pests. The basic properties and the function of salicylic acid involved in plant resistance to diseases and pests are introduced. The mechanism of salicylic acid in inducing resistance of plants to pest insects and diseases is discussed in terms of interaction of salicylic acid with salicylic acid-binding protein, and salicylic acid-dependent signaling pathway. The prospects for ap plication of SA in inducing resistance of plants to pests and diseases are also made.
Systemic Acquired Resistance
Inducer
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It has been proposed that salicylic acid acts as an endogenous signal responsible for inducing systemic acquired resistance in plants. The contribution of salicylic acid to systemic acquired resistance was investigated in transgenic tobacco plants harboring a bacterial gene encoding salicylate hydroxylase, which converts salicylic acid to catechol. Transgenic plants that express salicylate hydroxylase accumulated little or no salicylic acid and were defective in their ability to induce acquired resistance against tobacco mosaic virus. Thus, salicylic acid is essential for the development of systemic acquired resistance in tobacco.
Systemic Acquired Resistance
Methyl salicylate
Catechol
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Exogenous application of salicylic acid may improve tolerance to salinity. To investigate whether exogenous salicylic acid application had similar protective effects when applied as a priming agent or concomitantly with NaCl, tomato seedlings primed or not with 10 µM salicylic acid were further treated with 125 mM NaCl, 10 µM salicylic acid or combined treatments. Both priming and concomitant application of salicylic acid increased plant growth of salt-stressed plants but their positive impact was not additive. The endogenous salicylic acid concentration increased in the leaves after concomitant application but not in response to priming, suggesting that salicylic acid accumulated during priming was metabolized subsequently. Priming increased Na+ and K+ accumulation in leaves of salt-treated plants while concomitant application had no impact on shoot Na+ and K+ accumulation. Both priming and concomitant salicylic acid decreased osmotic potential values in salt-treated plants. Carbon isotope discrimination showed that combination of both salicylic acid application methods were required to maintain a good water use efficiency in salt-treated plants. Our work demonstrated that both procedures of salicylic acid application have positive impact on salt resistance but that the underlying properties sustaining these adaptations differ according to application methods.
Priming (agriculture)
Systemic Acquired Resistance
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Systemic Acquired Resistance
Plant hormone
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The seedlings of rice at one leave stage were treated with 0,1.0,1.5,2.0,2.5,3.0,3.5 mmol/L of salicylic acid(SA).The effects of salicylic acid on resistance of rice seedlings to cold stress were studied.The results showed that salicylic acid could enhance the resistance of rice to cold stress and the concentration of salicylic acid at 1.5 mmol/L showed the most effective.The salicylic acid increased the content of protine and chlorophyll as well as the activity of SOD,while the malodiadehyde content decreased in the leave of treated plants.However,the effects of salicylic acid on resistance to cold stress of rice seedlings showed the characters low to promote and high to suppress .
Cold stress
Rice plant
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This article gave an review of the recent research progresses on salicylic acid and heat stress resistance in plants,which aimed mainly at the relationship between salicylic acid and active oxygen,antioxident systerm,heat shock protein,photosynthesis,abscisic acid,calcium ion.The contents of H2O2 in plants could be reduced by a certain concentration of exogenous salicylic acid.Exogenous salicylic acid can improve the activity of superoxide dismutase,but there was still a debate on its influence on the activities of peroxidase and catalase.The exogenous salicylic acid and heat acclimation may show common mechanism of inducing thermotolerance,and the salicylic acid may introduce the heat shock protein.A proper concentration of salicylic acid could increase the photosynthesis ability of plant leaves.Salicylic acid and abscisic acid(ABA) were both the important signal molecule to heat acclimation.Salicylic acid might conduct the disease resistance signal through the calcium messenger system.Further research ideas for this aspect were suggested at the end of the paper.
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The role of a malate binding site in a concavity external to the more deeply situated active site has been a major mystery of the fumarase reaction. The malate, within 12 Å of the active site, was bound by hydrogen bonds to two main-chain amides and to two basic residues, H129 and R126. Mutation of the His of this so-called B site of Escherichia coli fumarase had little effect on the overall initial rate kinetics of the enzyme, which has obscured an understanding of the critical role of the site. Contrary to the WT enzyme, which is rate-limited in the recycling of free enzyme isoforms that follows product release, the enzyme with both basic residues modified is rate-limited in the product release step itself. A loss of complexity in the mutated, but still functional, step is indicated by a greatly reduced sensitivity of its rate to changes in temperature. Unlike the inhibition by glycerol shown with normal enzyme and attributed to a viscogenic effect on the recycling rate, the product-release step of the B-site mutants is accelerated by glycerol, suggestive of a structural effect on the 12-Å space between the A and B sites. It is proposed that the “extra” malate represents a stage in the transfer of substrate and product between the solvent and the “buried” active site of the enzyme.
Fumarase
Product inhibition
Malate dehydrogenase
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