Swelling and Contraction of Mitochondria from Cold-hardened and Nonhardened Wheat and Rye Seedlings

Abstract

A comparison of mitochondria isolated from 2 and 24 C grown winter wheat (Triticum aestivum L.) and winter rye (Secak cereak L.) seedlings revealed no correlation between changes in swelling and contraction characteristics and extent of cold hardiness. The swelling response changed markedly due to growth at low temperature, but the change was similar for the four cultivars examined. The swelling response was also observed to change rapidly during aging of isolated mitochondria, either at 2 or 24 C. Spontaneously swollen mitochondria, isolated from 24 C grown seedlings, contracted abruptly upon addition of certain oxidizable substrates, but this response was lost when seedlings were transferred from 24 to 2 C. Studies on the effect of various substrates and respiratory inhibitors on the swelling and contraction responses indicate that inhibitors which reduce or stop electron flow through the electron transport chain also inhibit substrate induced mitochondrial contraction.Although the swelling and contraction characteristics of isolated plant mitochondria have been thoroughly investigated, the mechanisms involved in these processes are not completely understood. Numerous studies (3,13,18,22) have demonstrated a passive, energy-independent swelling of plant mitochondria in the presence of isomolar salt solutions, or isomolar solutions of monosaccharide sugars. The existence of an energy-dependent (active) swelling component in plant mitochondria has also been demonstrated (2,5,19,21). Addition of certain oxidizable substrates to swollen mitochondria results in an immediate rapid contraction of the mitochondria while the addition of ATP initiates contraction of swollen corn mitochondria (18), but not swollen castor bean endosperm mitochondria (20). Earlier studies on the effect of specific inhibitors of electron transport and oxidative phosphorylation on swelling and contraction (18,20) postulated that high energy intermediates are associated with the active swelling and contraction observed in plant mitochondria. More recent investigations of energized swelling and contraction have led to interpretation of these phenomena in chemiosmotic terms, suggesting that the energy required is derived from changes in the electrochemical proton gradient (2).Recent studies (11,15) in our laboratories on the mechanism of cold hardening in plants have been directed to investigations of structural and functional properties of wheat and rye mitochondria in relation to growth at low temperature. These studies have revealed a marked increase in the level of fatty acid unsaturation during growth at low temperature, but no differences were observed among cultivars of contrasting cold hardiness. The functional properties of mitochondria from cold hardy cultivars were similar to those from less hardy cultivars. Electron spin resonance studies identified three possible temperature-dependent structural transitions in the mitochondrial membranes, and the shift in one of these appeared to be quantitatively greater in the cold hardy ...

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