The functions of PPOs in higher plants are not fully known but they are thought to
play a key part in the plant's defence mechanism against disease causing micro-organisms
and insect attack. When microbial infection occurs the cell integrity is broken and the
enzymic reaction takes place. An impervious scab of melanin forms and this acts as a physical
anti-microbial barrier. The quinones formed during the reaction are known to denature
proteins in the invading microorganisms and the polymeric phenols complexes can act as
inhibitors of microbial growth.