Organellar gene transcription and early seedling development are affected in the rpoT;2 mutant of Arabidopsis

Summary An Arabidopsis mutant that exhibited reduced root length was isolated from a population of activation‐tagged T‐DNA insertion lines in a screen for aberrant root growth. This mutant also exhibited reduced hypocotyl length as well as a delay in greening and altered leaf shape. Molecular genetic analysis of the mutant indicated a single T‐DNA insertion in the gene RpoT;2 encoding a homolog of the phage‐type RNA polymerase (RNAP), that is targeted to both mitochondria and plastids. A second T‐DNA‐tagged allele also showed a similar phenotype. The mutation in RpoT;2 affected the light‐induced accumulation of several plastid mRNAs and proteins and resulted in a lower photosynthetic efficiency. In contrast to the alterations in the plastid gene expression, no major effect of the rpoT;2 mutation on the accumulation of examined mitochondrial gene transcripts and proteins was observed. The rpoT;2 mutant exhibited tissue‐specific alterations in the transcript levels of two other organelle‐directed nuclear‐encoded RNAPs, RpoT;1 and RpoT;3. This suggests the existence of cross‐talk between the regulatory pathways of the three RNAPs through organelle to nucleus communication. These data provide an important information on a role of RpoT;2 in plastid gene expression and early plant development.