Autonomously replicating RNA in mitochondria of maize plants with S-type cytoplasm

Area of Science:

• Mitochondrial genetics
• Plant molecular biology
• Organelle genomics

Background:

• Mitochondria possess their own genetic material, distinct from nuclear DNA.
• Male-sterile cytoplasms in plants can harbor unique genetic elements affecting mitochondrial function.
• Understanding extranuclear RNA synthesis is crucial for organelle inheritance studies.

Purpose of the Study:

• To investigate RNA synthesis in maize mitochondria with S-type male-sterile cytoplasm.
• To characterize actinomycin D-resistant RNA species and their origin.
• To determine if these RNAs represent a novel inheritable element.

Main Methods:

• Isolation of mitochondria from maize with S-type and normal (N) cytoplasms.
• RNA synthesis assays in the presence of actinomycin D and RNase.
• Analysis of RNA size, strandedness, and homology to mitochondrial DNA (mtDNA).
• Mitochondrial lysis experiments to assess RNA sensitivity to RNase.

Main Results:

• Mitochondria from S-cytoplasm maize synthesize four RNA species resistant to actinomycin D.
• This synthesis is intrinsic to mitochondria and independent of mtDNA.
• Two double-stranded RNAs (2850 and 900 bp) and two single-stranded RNAs (2150 and 850 bp) were identified.
• These RNAs show homology to S-cytoplasm mitochondrial RNA but not N-cytoplasm RNA.

Conclusions:

• S-cytoplasm maize mitochondria harbor a unique RNA synthesis capability.
• The characterized RNAs are likely replicated independently of mtDNA.
• These findings suggest the existence of a novel inheritable organellar element, an RNA plasmid.