Natural variation in Miniature5 determines mitochondrial nad1 splicing and seed development in maize

Area of Science:

• Plant genetics
• Molecular biology
• Mitochondrial gene expression

Background:

• Seed size is vital for cereal grain yield, but natural genetic variations affecting it are underutilized in maize breeding.
• Defective-kernel genes, particularly those impacting mitochondrial function, play a significant role in seed development.

Purpose of the Study:

• To identify and characterize the maize Miniature5 (Mn5) gene and its role in seed size determination.
• To investigate the molecular mechanism by which Mn5 influences mitochondrial gene splicing and respiratory complex assembly.

Main Methods:

• Positional cloning of the Mn5 gene.
• Analysis of Mn5 protein function using mutant lines.
• RNA binding assays and co-immunoprecipitation to study Mn5 interactions.
• Mitochondrial respiratory complex activity assays and electron microscopy.

Main Results:

• The maize Miniature5 (Mn5) gene encodes a mitochondrial PPR protein. A natural missense mutation (Mn5^Val109) in maize populations correlates with reduced seed size.
• The Mn5^Val109 variant impairs the trans-splicing of mitochondrial nad1 intron1, leading to reduced respiratory complex I abundance and activity, and disorganized mitochondrial cristae.
• Mn5 binds to the pre-nad1.1 transcript downstream of the MSP1 binding site, redefining the 3'-end of the pre-RNA, and interacts with multiple maturases and PPR proteins involved in intron splicing.

Conclusions:

• Mn5 is essential for the proper splicing of mitochondrial nad1 intron1, likely by recruiting other splicing factors to form spliceosomal complexes.
• Natural variations in Mn5 provide a valuable genetic resource for improving seed size and yield in maize breeding programs.