IMPACT OF AMPHIPHILE PARTIONING ON DESICCATION TOLERANCE

4. IMPACT OF AMPHIPHILE PARTIONING ON DESICCATION TOLERANCE

Sugars are reputed to protect membranes in dehydrated desiccation-tolerant organisms, such as seeds and pollens. They interact with the poar headgroups of the membrane phospholipids and control the gel-to-liquid crystalline transition temperature (Tm). Because the amount of sugar may be insufficient for full interaction in some organisms, another mechanism of membrane protection was sought. A mechanism is proposed that is based pm the partioning of ampjophilic compunds into membranes depending on the water available. This mechanism was tested with an amphi[hilic nitroide spin probe, using EPR spectroscopy. It aas found that, apart from the spin probe, endogenous amphiphiles may also partition into membranes during dehydration. The amphiphiles reduce the dehydration-induced increase of Tm and cause fluidization. The advantages and disadvantages of such a mechanism are discussed. The proposed mechanism is etremely efective at automatically inserting amphiphilic antioidants into membranes with dehydration, which could promote desiccation tolerance and extend storage longevity.

INTRODUCTION

Dehydrated, desiccation-sensitive organisms leak almost all of their cytoplasmic solutes when they are rehydrated. This points to the inability of their plasma membranes to cope with dehydration. Apperantly, some irreversible damage has occured in these membranes.

Under normal conditions of sufficient hydration, the hydrophobic effect of water forces phospholipid molecules into the bilayer structure, with the headgroups directed outward to the aqueous phase and the acyl chains inward (Tanford, 1978). On dehydration the bilayer structure may be lost as a result of the disappearance of this hydrophobic effect. In the case of liposomes made os phospholipids, all the entrapped solutes are released to the sorrounding water upon rehydration. In desiccation-tolerant organisms (anhydrobiotes) , membranes remain intact on dehydration and subsequent rehydration. Apperantly, there is a mechanism in sich organisms wich protect membrane structure.

ROLE OF SUGARS IN THE PROTECTION OF DEHYDRATED MEMBRANES

The ample occurance of non-reducing sugars in anhydrobiotes has been linked with the ability of membranes to retain the bilayer structure (Crowe et al., 1984). It has been demostrated that in the presence of trehalose or sucrose, liposomes are able to maintain their structure and size, and to retain trapped solutes inside upon rehydration (Crowe et al., 1986). Hydrogen bonding of the sugar OH-groups with the phosphate of the polar headgroups has been proposed as a likely mechanism for this protection (Crowe et al., 1987), which has been confirmed by different methods (review in Crowe et al., 1997). Due to this hydrogen bonding with the sugars, the phospholipid molecules in the membranes remain spaced. As a result, there is less oportunity for Van der Waals interactions between the cyl chains, and the dehydration-induced increase of Tm of as much as 70 dregrees celcius does not occur. Membranes thus remain in the lequid-crystalline phase (Crowe at al., 1987). Sugars can therefore be considered to replace water with regard to hydrogen bonding capability. Tm of dry membranes in the presence of sugars may be even lower than that of hydrated membranes, because sugars may space the headgroups more than water does.

This mechanism was hypothesized also to work in vivo to retain membrane structure and to prevent dehydration-induced phase transitions (see Crowe et al., 1992 for a review). Supportive data for this hypothesis have been presented for typha latifolia pollen using in situ fourier transform IR spectroscopy (FTIR) (Hoekstra et al., 1991). Whereas Tm of dry isolated membranes was close to 60 degrees celcius, the membranes inside ultra-dry pollen had Tm values of approximately 30 dregrees celcius. This reduction of the in situ Tm has been attributed

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