The plant mitochondrial genome has acquired, during its evolution, several distinguishing features that are not well understood mechanistically or in evolutionary context. One of these is a process of asymmetric (non-reciprocal) recombination that appears to directly influence plant reproduction. Until now, our understanding of these processes has been hindered, in part, by the unavailability of effective model systems for investigating nuclear gene influence on the process. The effects of three nuclear genes, MSH1, OSB1 and RECA3, have recently been analyzed on mitochondrial genome stability. RECA3 and MSH1 function in distinct but overlapping pathways, while OSB1 cooperates with MSH1, to regulate the appearance of specific recombination products in Arabidopsis mitochondria. The principal investigators have recently shown that MSH1 functions similarly in tobacco, tomato, soybean and millet, but MSH1 RNAi knockdowns in these species also condition cytoplasmic male sterility. The principal investigators are now in a position to analyze the mitochondrial genome changes that occur when mutations in these nuclear genes permit normally rare recombination activity to occur at high frequency, and to compare the consequences of this recombination in multiple plant species. The central hypothesis is that MSH1, OSB1, and RECA3, together with specific breakage sites in the genome, represent components of an interacting network that operates in plant mitochondrial recombination-dependent replication initiation. The principal investigators propose that double-strand DNA breaks occur at specific locations within the mitochondrial genome to initiate recombination and replication, and that MSH1, OSB1 and RECA3 are involved in preventing recombination between repeats that are shorter than a few hundred bases. The principal investigators test these hypotheses by identifying double-strand breaks in repeated sequences, and in the Atp9, Atp6, CoxII and Rpl16 genes, in which there is evidence of recombination activity. This analysis is complemented by studies of inducible changes in tobacco, tomato, soybean and millet mitochondrial genomes that occur with MSH1 suppression and male sterility induction.

This project will develop a unifying model for recombination-dependent replication of plant mitochondrial genomes, and provide insights into the mechanisms for the induction and suppression of cytoplasmic male sterility, a phenomenon of agricultural importance that leads, in nature, to the gynodioecious reproductive strategies of approximately 5% of world plant species. The minority student training opportunities associated with this proposal will involve alignment with the Young Nebraska Scientist (YNS) Program, in which an undergraduate YNS student will participate in this research each summer.

Project Report

Mitochondrial serve as the energy generators for the cell, and these compartments contain their own genetic information. Mitochondria carry out essential functions for the cell that, in animals, can influence aging properties and predisposition to disease. In plants, mitochondria are involved in environmental sensing, stress responses, and conditioning changes in reproductive strategies. Plant mitochondrial genomes are unusually versatile, and undergo organizational rearrangements rapidly and frequently. The role and means of this genomic versatility were the focus of this research. In plants, mitochondrial genomic rearrangement occurs under the control of at least one nuclear gene, MSH1, that is responsive to the plant's environment. Environmental stresses like heat, cold, salt or high light can suppress MSH1 expression and result in adjustments in the mitochondrion. There are 47 sites within the mitochondrial genome of the model plant Arabidopsis that are responsive to MSH1, and when MSH1 is absent, these sites undergo recombination activity tochange the status of the genome. The changes that occur alter plant growth and development, resulting in male sterility, variegation, dwarfing, delays in flowering, and altered responses to biotic and abiotic stress. Analysis of the mitochondrial genomic rearrangements suggest that they participate in plant evolution, providing the plant with an accelerated genome response mechanism to environmental change. Understanding the nature of this response may allow us to develop plant breeding strategies that will enhance a plant's ability to adapt and accommodate environmental changes more readily. This research has resulted in the training of two scientists from underrepresented groups, two scientistsin bioinformatics, and three undergraduate trainees.

Agency
National Science Foundation (NSF)
Institute
Division of Molecular and Cellular Biosciences (MCB)
Application #
0744104
Program Officer
Karen C. Cone
Project Start
Project End
Budget Start
2008-03-15
Budget End
2011-02-28
Support Year
Fiscal Year
2007
Total Cost
$450,000
Indirect Cost
Name
University of Nebraska-Lincoln
Department
Type
DUNS #
City
Lincoln
State
NE
Country
United States
Zip Code
68588