The cuticle is the major barrier which prevents excessive loss of water from primary tissues. Secondary tissues and wounded tissues generally develop periderms in which suberized cells assume the function of a cuticle. Suberin also functions as a barrier in a variety of other anatomical regions of plants. The Casparian band of the root endodermis is well known to be suberized (9, 14) and
Effect of abscisic acid (ABA) on suberization of potato (Solanum tuberosum var. Russet-Burbank) tuber tissue culture was studied by measuring deposition of suberin components and the level of certain key enzymes postulated to be involved in suberization. ABA treatment resulted in a 3-fold increase in the polymeric aliphatic components of suberin and a 4-fold increase in the polymeric aromatic components. Hydrocarbons and fatty alcohols, two components characteristic of waxes associated with potato suberin, increased 9-and 5-fold, respectively, as a result of ABA treatment.Thus, the deposition of the polymeric aliphatics and aromatics as well as waxes, all of which have been postulated to be components of suberized cell walls, was markedly stimulated by ABA. w-Hydroxy-fatty acid dehydrogenase which showed a rather high initial level of activity increased only 60% due to ABA treatment. Phenylalanine ammonia-lyase activity reached a maximum at a 5-fold level after 4 days in the ABA medium, whereas the control showed only a fold increase. ABA treatment also resulted in a dramatic (7-fold) increase in an isozyme of peroxidase which has been specifically associated with suberization. Thus, ABA appears to induce certain key enzymes which are most probably involved in suberization.ABA is generally regarded as a depressor or inhibitor of physiological and biochemical events in plants (24). In contrast to the large number of reports on inhibitory effects of ABA, there are few reports concerned with stimulatory effects. Recently, it was shown that ABA promoted cell division and DNA synthesis in Jerusalem artichoke tuber tissue culture (18).We concluded that ABA stimulated suberization in potato tuber tissue based on the observation that ABA was released into the washing medium during the first day after c.t'ting and that ABA at least partially reversed the inhibition of suberization caused by washing (22). In support of this conclusion, ABA was found to induce the deposition of polymeric aliphatic components characteristic of suberin in potato tissue culture. More recently, it was realized that suberin is probably composed of polymeric materials containing aromatic and aliphatic components and that waxes associated with suberin probably provide the water vapor diffusion barrier to suberized cell walls (23). If the previously observed 2Author to whom inquiries should be made.ABA stimulation of deposition of polymeric aliphatic material in potato tissue culture represented induction of suberization, the synthesis of other components which are thought to be part of suberized cell walls should also be induced by ABA. In this paper we show that ABA in the medium of potato tuber tissue culture stimulates the deposition of aliphatic and aromatic polymeric materials and waxes, all of which are characteristic of suberized cell walls. Effects of ABA on the levels of certain enzymes thought to be involved in suberization are also reported. MATERIALS AND METHODSTissue Cultures. Callus cultures were prepared from tuber parenchy...
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