p-Coumaric acid increases lignin content and reduces hyperhydricity in in vitro-grown Arabidopsis seedlings

Hyperhydricity (HH) occurs when plants are faced with accumulation of water and reduction of air in the apoplast of the leaves. One of the characteristics of hyperhydric plants is the reduction of cell wall lignification (hypolignification), but how this is related to the abnormalities is still unclear. Lignin is hydrophobic and based on this it can be speculated that a reduction in lignin levels leads to more capillary action of the cell wall and, consequently, to more water in the intercellular spaces. p-coumaric acid is a hydroxy derivative of cinnamic acid and a precursor for lignin and flavonoids in higher plants. In the present work, the relative amount of apoplast water and air to the total apoplast volume of Arabidopsis thaliana wild-type (Col-0) leaves was evaluated. Exogenously applied p-coumaric acid can be channelled into the phenylpropanoid pathway through action of the enzyme 4-hydroxycinnamoyl-CoA ligase (4CL), ultimately resulting in an increase in the total lignin content. Exogenously applied p-coumaric acid led to increases in apoplastic air and lowering of apoplastic water in seedlings grown on medium solidified with gelrite. The symptoms of HH are also greatly diminished. These findings corroborate our hypothesis that lignin plays a role in the development of the HH and that an increase of lignin production by exogenously applying p-coumaric acid can lead to a decrease of water in the apoplast and thus to a reduction in the occurrence of HH.