Transformation of nad7 into the nuclear genome rescues the slow growth3 mutant in Arabidopsis.

Plant pentatricopeptide repeat (PPR) proteins are mostly involved in chloroplast or mitochondrial RNA metabolism. However, direct evidence that correction of the molecular defects in the organelles can restore the plant phenotypes has yet to be demonstrated in a ppr mutant. Arabidopsis slow growth3 (slo3), a ppr mutant, is impaired in the splicing of mitochondrial nad7 intron 2. Here, we have used slo3 as an example to demonstrate that transformation of correctly spliced nad7 into the nuclear genome and targeting the Nad7 subunit into mitochondria can restore complex I activity and plant phenotypes in the mutant. These results provide direct evidence that the strong growth and developmental phenotypes of the slo3 mutant are caused by defects in mitochondrial nad7.

ACR11 modulates levels of reactive oxygen species and salicylic acid-associated defense response in Arabidopsis.

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.