The role of two isoenzymes of α-amylase of Araucaria araucana (Araucariaceae) on the digestion of starch granules during germination

Starch is the principal reserve of Araucaria araucana seeds, and it is hydrolysed during germination mainly by α-amylase. There are several α-amylase isoenzymes whose patterns change in the embryo and in the megagametophyte from the one observed in quiescent seeds (T₀) to a different one observed 90 h after imbibition (T₉₀). The objective of this research was to study the roles of two purified α-amylase isoenzymes by in vitro digestion of starch granules extracted from the tissues at two times of imbibition: one is abundant in quiescent seeds and the other is abundant after 90 h of imbibition. The isoenzymes digested the starch granules of their own stage of germination better, since the isoenzyme T₀ digested starch granules mainly from quiescent seeds, while the isoenzyme T₀ digested starch mainly at 90 h of imbibition. The sizes of the starch granule and the tissue from which these granules originated make a difference to digestion by the isoenzymes. Embryonic isoenzyme T₀ digested large embryonic starch granules better than small and medium-sized granules, and better than those isolated from megagametophytes. Similarly isoenzyme T₀ digested small embryonic starch granules better than medium-sized and large granules, and better than those isolated from megagametophytes. However, a mixture of partially purified megagametophytic isoenzymes T₀ and T₉₀ digested the megagametophytic granules better than those isolated from embryos. Studies of in vitro sequential digestion of starch granules with these isoenzymes corroborated their specificity. The isoenzyme T₉₀ digested starch granules previously digested by the isoenzyme T₀. This suggests that in vivo these two isoenzymes may act sequentially in starch granule digestion.