Association of mitochondrial plasmids with rejuvenation of the coastal redwood, Sequoia sempervirens (D. Don) Endl.

Repeated grafting of shoot apices from mature Sequoia sempervirens (D. Don) Endl. trees onto young seedling shoots in vitro causes emergence of rejuvenated, or phase-reversed, scion shoots. The phase reversal is indicated by a restored rooting competence and renewed vigor of roots and shoots. We earlier reported of restriction fragment length polymorphism between juvenile or rejuvenated and adult shoot mtDNA (mitochondrial DNA). This investigation of undigested mtDNA disclosed that S. sempervirens shoots contained at least six small mtDNA molecules, four of which were uniquely associated with juvenile and rejuvenated shoots. The small molecules remained observable in continuously subcultured shoots. Results of cloning and sequencing indicated they are circular in form; we thus, called them plasmids. Significant sequence homology, 38 to 56%, was found among the molecules, indicating highly conserved regions and possibly common origin. No similar phase-associated relationships were observed for unrestricted cpDNA (chloroplast DNA) or nucDNA (nuclear DNA). The plasmids hybridized with the larger, electrophoretically less mobile mtDNA, but not with cpDNA or nucDNA, suggesting their origin in the master circle mtDNA. The distinctness of these plasmids has remained unchanged after more than 20 years of subculturing.