Cell Wall Amine Oxidases: New Players in Root Xylem Differentiation under Stress Conditions
The inhibition of K+ uptake through the plasma membrane resulting from injury caused by cutting, or from application of polyamines (PAs), has been investigated in root segments of maize (Zea mays L.) and pea (Pisum sativum L.). It was found, for both treatments, that K+ uptake recovered if the segments were washed for 2 h. The K+ uptake inhibited by cutting and that inhibited by spermidine treatment were stimulated to the same extent by fusicoccin. In addition, there was a correlation between the extent of the recovery of K+ uptake caused by washing and the distribution, along the root axis, of both PAs and the activities of enzymes responsible for PA degradation. In apical segments of maize, where the PA content and the activity of the degradative enzyme polyamine oxidase (EC 22.214.171.124) were higher than in the more distal segments, the recovery of K+ uptake caused by washing was also higher. On the other hand, the opposite trend was observed in root segments of pea, where the PA content and the activity of the degradative enzyme diamine oxidase (EC 126.96.36.199) were higher in distal segments in which K+ uptake was greatly stimulated by washing. The effect of the amine-oxidase inhibitor, aminoguanidine, indicates that the degradation products of PAs are involved in the mechanism of inhibition of K+ uptake by PAs. The data also seem to indicate that PAs and their degradation products are responsible for the inhibition of K+ uptake occurring as a result of injury sustained by cutting roots into segments.