Overexpression of mitochondrial genes is caused by interactions between the nucleus of Brassica rapa and the cytoplasm of Diplotaxis muralis in the leaves of alloplasmic lines of B. rapa.

In Brassica species, alloplasmic lines displaying cytoplasmic male sterility (CMS) are established by combining the nucleus from B. rapa with the cytoplasm from Diplotaxis muralis. The failure to observe restriction fragment length polymorphism (RFLP) patterns of mitochondrial genes ( coxII, coxIII, atpA, atp6, atp9, cob, nad3, nad6, and nad9) between alloplasmic lines of B. rapa and D. muralis indicates that introgression of the B. rapa nucleus into the cytoplasm of D. muralis does not cause any alterations in the structure of the mitochondrial genome. To investigate how the nucleus influences the cytoplasm, we examined the expression of mitochondrial genes in the leaves of euplasmic and alloplasmic lines of B. rapa and D. muralis. We detected higher levels of mitochondrial gene mRNAs in alloplasmic lines of B. rapa than in D. muralis. Patterns of mitochondrial gene transcription also differed among the alloplasmic lines of B. rapa. Thus, expression of mitochondrial genes in alloplasmic lines of B. rapa differed in the leaves compared to D. muralis. Overexpression of mitochondrial genes may be the result of novel interactions between the nucleus and the mitochondria in alloplasmic lines of B. rapa. Further study is necessary to clarify how these phenomena are involved in CMS.