Oxidative stress in horseradish (Armoracia lapathifolia Gilib.) tissues grown in vitro

In a previous study it was reported that transformed tissue of horseradish (Armoracia lapathifolia Gilib.), obtained by infection of leaf explants with A. tumefaciens, developed two tumour lines with different morphology. One line grew as a completely unorganized tissue (TN - tumour tissue), while the other line grew as a partially organized teratogenous tumour with malformed hyperhydric shoots (TM - teratoma tissue), but did not regenerate the whole plant of normal morphology. The factor responsible for this problem could be the increased production of reactive oxygen species (ROS). Therefore, in this study a possible involvement of activated oxygen metabolism in dedifferentiation and hyperhydricity in TM and TN tissues is investigated. Elevated values of malondialdehyde and protein carbonyl contents found in TM and TN, in comparison with plantlet leaf, confirm the presence of oxidative stress. However, lower H₂O₂ content was measured in TM and TN. Lipoxygenase (LOX) activity was more pronounced in TM and especially in TN compared to leaf, which suggests that the LOX-dependent peroxidation of fatty acids might be one of the causes of oxidative damage. Moreover, significantly higher peroxidase (PRX) and as-corbate peroxidase (APX) activity as well as the increased number of their isoforms was found in transformed TM and TN in comparison with leaf. On the other hand, significantly lower superoxide dismutase (SOD) activity was found in TM and TN, which correlates with lower H₂O₂ content. High catalase (CAT) activity measured in leaf and partially organized TM is consistent with the role of CAT in growth and differentiation. In conclusion, in horseradish transformed tissues that underwent dedifferentiation and hyperhydricity, prominent oxidative damage was found. This result suggests that oxidative stress could be associated with the inability of partially organized teratogenous TM to regenerate plant-lets with normal morphology.