# Enzymatic Antioxidant Defense System of Scots Pine Seedlings Under Conditions of Progressive Manganese Deficiency

## TL;DR

Scots pine seedlings respond to manganese deficiency by reducing a key antioxidant enzyme, unlike most plants, which increase antioxidant activity to combat stress.

## Contribution

The study reveals a paradoxical response in pine seedlings to manganese deficiency, with reduced superoxide dismutase activity despite expected antioxidant defense activation.

## Key findings

- Manganese-deficient pine seedlings showed a twofold decrease in Cu/Zn-containing superoxide dismutase activity in needles.
- Other antioxidant enzymes like peroxidase showed no increase or even decreased activity under manganese deficiency.
- Reduced reactive oxygen species production in Photosystem II may explain the lack of antioxidant enzyme activation.

## Abstract

Manganese (Mn) is a vital micronutrient for plants. In particular, Mn plays an important role in the plant life cycle, participating in processes such as photosynthesis, respiration, reactive oxygen species (ROS) scavenging, pathogen defense, and hormonal signaling. Currently, approximately 400 enzymes, including the enzymes of Photosystem II, are known to contain Mn in the metal-binding site. Impaired photosynthesis leads to the formation of ROS, which can damage DNA, inactivate proteins, and impair cell membranes. It is generally believed that Mn deficiency leads to the generation of ROS. Plants typically respond to increased ROS levels by increasing the activity of antioxidant enzymes to neutralize excess oxygen radicals. Unlike most plants, pine seedlings, as shown in our study, respond to progressive Mn deficiency in a completely different way. Specifically, a twofold decrease in the activity of one of the key antioxidant enzymes, superoxide dismutase (SOD), which is present in Cu/Zn-containing forms, was observed in the needles of Mn-deficient plants. The activities of other components of the antioxidant defense system remained unchanged or, as in the case of peroxidase, even decreased. This paper discusses possible causes for the paradoxical response of pine seedlings to Mn deficiency.

Manganese (Mn) is a crucial micronutrient for plants. The impaired function of the oxygen-evolving complex in Photosystem II (PSII) due to Mn deficiency is believed to result in the overproduction of reactive oxygen species and the induction of an enzymatic antioxidant system. In our study, we investigated the effects of progressive Mn deficiency (the difference in Mn content between the needles of control and Mn-deficient plants increased from 17-fold at the beginning of the experiment to 59-fold at the end) on the activities of superoxide dismutase (SOD), catalase, ascorbate peroxidase, and guaiacol peroxidase in the roots and needles of Scots pine seedlings. We found that the soluble protein content in plant organs under Mn deficiency was maintained at a level comparable to that of the control. Regardless of the severity of Mn deficiency, the needles of the Mn-deficient plants presented twofold lower SOD activity than the needles of the control plants. These differences were observed even when Mn deficiency did not negatively affect plant growth. Additionally, the total SOD activity in the needles of both plant groups was determined solely by the activity of the Cu/Zn-containing SOD isozymes. Compared with the control plants, Mn deficiency did not result in an increase in any of the studied H2O2-degrading enzymes in the needles of the seedlings. In contrast, the needles of the Mn-deficient plants presented a lower level of guaiacol peroxidase activity. Despite the inhibition of root growth, Mn deficiency led to changes in the balance of the enzymatic antioxidant system in plant roots. The data obtained suggest that the lack of activation of SOD and other antioxidant enzymes in Scots pine seedlings against the background of progressive Mn deficiency is due to the reduced ability of PSII to generate ROS under these conditions.