Abstract
The ACT domain (aspartate kinase, chorismate mutase and TyrA), an allosteric effector binding domain, is commonly found in amino acid metabolic enzymes. In addition to ACT domain-containing enzymes, plants have a novel family of ACT domain repeat (ACR) proteins, which do not contain any recognizable catalytic domain. Arabidopsis has 12 ACR proteins, whose functions are largely unknown. To study the functions of Arabidopsis ACR11, we have characterized two independent T-DNA insertion mutants, acr11-2 and acr11-3. RNA gel-blot analysis revealed that the expression of wild-type ACR11 transcripts was not detectable in the acr11 mutants. Interestingly, a lesion-mimic phenotype occurs in some rosette leaves of the acr11 mutants. In addition, high levels of reactive oxygen species (ROS), salicylic acid (SA), and callose accumulate in the mutant leaves when grown under normal conditions. The expression of several SA marker genes and the key SA biosynthetic gene ISOCHORISMATE SYNTHASE1 is up-regulated in the acr11 mutants. Furthermore, the acr11 mutants are more resistant to the infection of bacterial pathogen Pseudomonas syringae pathovar tomato DC3000. These results suggest that ACR11 may be directly or indirectly involved in the regulation of ROS and SA accumulation, which in turn modulates SA-associated defense responses and disease resistance in Arabidopsis.
Highlights
Amino acids are essential organic compounds for all life forms
The lesion-mimic phenotype accompanied by increased levels of reactive oxygen species (ROS) and salicylic acid (SA)-associated defense responses make the acr[11] mutants more resistant to the bacterial pathogen Pseudomonas syringae pathvar tomato DC3000 (Pst)
The Arabidopsis ACR11 protein is predicted to contain a transit peptide with the cleavage site located at the 52nd residue followed by two ACT domains (Fig. 1a)
Summary
Amino acids are essential organic compounds for all life forms. The synthesis of these important molecules is tightly regulated. It is well established that many key enzymes involved in amino acid biosynthesis are subject to feedback inhibition. These feedback-regulated enzymes are usually composed of catalytic domains and allosteric domains, which are responsible for catalyzing the reaction and allosteric regulation of the enzyme activity, respectively[1,2,3,4]. CM and PDH are involved in the biosynthesis of aromatic amino acids, which are feedback-regulated by phenylalanine and tyrosine through the regulatory ACT domain[1,8]. Feedback regulation of amino acid biosynthetic enzymes has been extensively studied in bacteria These enzymes are highly conserved from bacteria to plants. The functions of Arabidopsis ACR11 in the interconnections of GS/Fd-GOGAT cycle, glutamine homeostasis, redox balance, ROS accumulation, and SA-associated defense responses are discussed
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