Activity and protein level of AO isoforms in pea plants (Pisum sativum L.) during vegetative development and in response to stress conditions.

Among three AO isoforms detected in pea plants, the activity of PAO-1 was dominant in leaves of seedlings and young leaves of mature plants, while PAO-3 revealed the highest band intensity in old leaves and roots. PAO-1 and PAO-3 are homodimers consisting of 145 kDa and 140 kDa subunits, respectively, while PAO-2 is a heterodimer of one 145 kDa and one 140 kDa subunit. In leaves, the activity of PAO-1 disappeared gradually with leaf ageing, while in roots it was present only in seedlings but not in mature pea plants. PAO-3 could oxidize abscisic aldehyde, a precursor of abscisic acid, indicating the possible involvement of this isoform in ABA synthesis in pea. The ability of PAO-3 to oxidize abscisic aldehyde was higher in old leaves than in young ones and increased significantly both in roots and leaves of plants exposed to salinity and ammonium treatments. A marked increase of the AO protein level was observed after ammonium application but not under salinity. Interestingly, the activity of PAO isoforms may be transcriptionally and post-transcriptionally regulated during vegetative growth and in response to stress conditions, and such a regulation might be particularly important to adjust ABA levels to the recent requirements of the plant. The observations suggest that the AO isoforms have different metabolic roles and that the activity and protein level of each isoform is regulated not only by environmental conditions but also through plant developmental stages.