Characterization of the genes for two soybean aspartic proteinases and analysis of their different tissue-dependent expression.

We isolated and characterized two cDNAs for aspartic proteinases (APs; EC 3.4.23) in soybean [Glycine max (L.) Merr.]. The encoded enzymes, soyAP1 and soyAP2, share 55% amino acid sequence identity. Northern analysis demonstrated that soyAP1 is expressed specifically in seeds, especially in dry seeds, while the expression of soyAP2 takes place in various tissues such as roots, stems, leaves and flowers, but not in dry seeds. SoyAP1 is highly expressed even at an early stage of germination, with a subsequent decrease in expression intensity. In contrast, the soyAP2 mRNA level increases 48 h after imbibition. To elucidate the physiological functions of soyAPs, we investigated the localization of soyAP expression in seeds germinating for 48 h at 25 degrees C. SoyAP1 shows cell-type-specific expression in sieve tube cells of the hypocotyl. At the root tip, soyAP1 is expressed in immature tracheary elements and sieve tube cells, and its expression pattern changes with distance from the tip; strong signals observed throughout phloem converge gradually to sieve tube cells, whereas those observed in tracheary elements disappear while the elements are still immature. On the other hand, soyAP2 signals were detected broadly in the boundary region between the cortex and the central cylinder. These results suggest that soyAP1 and soyAP2 are functionally different from each other.