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Plants contain multiple forms of catalase (CAT) and their specific functions remain uncertain. We cloned two potato cDNAs corresponding to CAT1 and CAT2 genes, analysed their phylogenetic relationship, and studied their expression and activity in different organs to gain clues to their functions. Phylogenetic trees and the alignment of CAT cDNA sequences provided evidence that CAT1 and CAT2 genes have high identity to catalases of other solanaceous species, but are not phylogenetically closely related to one another, which contradicts the phylogenetic closeness ascribed to these genes. Northern blot analyses revealed that expression of CAT genes is controlled by leaf developmental phase. CAT2 expression was higher in both very young and senescent leaves, whereas CAT1 mRNA accumulated mainly in mature leaf, where the lowest CAT2 expression occurred. CAT1 and CAT2 are also differentially expressed in root, sprout and petal. Expression and activity patterns are consistent with different physiological roles for CAT1 and CAT2 isoforms. CAT1 is considered to be associated with photorespiration whereas CAT2 would fulfill physiological roles unrelated to this process. CAT2 appears to be a multifunctional isoform, associated with glyoxysomal activity in leaf senescence, other processes in non-photosynthetic organs and defence, functions that in other solanaceous species are fulfilled by two different isoforms.

The effect of hydrogen peroxide (H2O2) on catalase (CAT) isoform activities and amounts and on mRNA levels was studied in leaves from potato plants untreated and treated with homobrassinolide (HBR). Northern blot analysis revealed that 100 mm H2O2 supplied through the leaf petiole for 4 h did not induce CAT expression. In contrast, CAT1 and CAT2 responded differently to longer treatment, as CAT2 transcript levels increased markedly whereas CAT1 transcript levels remained unchanged. Western blot analysis showed disparity between the level of CAT1 transcript and CAT1 amount, which actually decreased after 28 h. CAT2 amount correlated well with transcript accumulation and CAT2 activity as visualised by zymogram analysis. H2O2 modified the relative importance of CAT isoforms. After 4 h, CAT1 was prevalent in untreated and H2O2-treated leaves. After 28 h, CAT2 was prevalent in H2O2-treated leaves; therefore, the quantified increase in total CAT activity in these leaves was due to the rise in CAT2. HBR pre-treatment increased CAT2 basal level not changing the pattern of CAT responses to H2O2, only lowering its amplitude. Even so, ultrastructural studies showed that HBR significantly reduced H2O2 negative effects on cellular sub-structures, allowing better recovery of affected structures and reducing the macroscopic injury symptoms on leaves, thus data point to a HBR protective role.