cDNA cloning of abscisic aldehyde oxidase in Arabidopsis and Nicotiana

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Aldehyde oxidase (AO) has been extensively investigated in animals and microorganisms. In plants, however, there was only a limited information on this enzyme. A great interest in this enzyme has been in its possible involvement in abscisic acid (ABA) biosynthesis. In the present study, we cloned four full-length AO cDNAs from Arabidopsis thaliana and investigate their physiological functions, and we purified and characterized a leaf AO of Nicotiana plumbaginifolia. The summary of the results are as follows ;(1) Four different putative AO cDNA clones were obtained from A.thaliana. Their products were analyzed by using antibodies each specific to four AOs in A.thaliana.(2) The four AO cDNAs were introduced into yeast Pichia pastoris and the products were functionally expressed in the yeast cells. The result revealed that two AO genes encoded three AOs in A.thaliana seedlings and another AO gene produced a leaf AO.Biochemical studies using AOs expressed in yeast indicated that leaf AO is a most possible candidate of an AO that catalyzes oxidation of abscisic aldehyde to form ABA.(3) However, partially purified N.plumbaginifolia leaf AO showed a very weak preference to abscisic aldehyde. This result with combination of the results from A.thaliana suggests that there might be present another AO in N.plumbaginifolia leaves, which involves in ABA biosynthesis.