Mitogen activated protein kinase 4 (MPK4) influences growth in Populus tremula L. × tremuloides

• MPK4 is involved in the cell wall thickening and lignification of xylem vessels.
• MPK4 may control growth and photosynthesis by modulation of ROS/hormonal homeostasis.
• MPK4 seem to regulate transpiration, ensuring water supply in plant tissues.

Mitogen activated protein kinases (MAPKs or MPKs) are important molecular regulators involved in plant development and stress responses. Arabidopsis MPK4 was proved to play a crucial role in the regulation of cytokinesis, hormonal homeostasis and defence responses against bacteria and oomycetes. The aim of this work was to distinguish the function of MPK4 in poplar. Therefore, we generated transgenic lines of Populus tremula × tremuloides with the reduction of poplar MPK4 expression. Two independent lines mpk4-5 and mpk4-7 were selected and cultivated in the field for four vegetative seasons. Transgenic lines with reduced expression of MPK4 displayed higher foliar content of free salicylic acid (SA), elevated cellular reactive oxygen species (ROS) level and antioxidant enzymes activities. Moreover, they proved increased photosynthetic efficiency as well as chlorophyll and carotenoid contents. Despite effective photosynthetic processes, MPK4-silenced trees demonstrated reduced growth, which could be caused by significantly elevated stomatal conductance that lead to higher transpiration and lower water use efficiency. Changes in the lignification process and secondary growth of stems could be the other reason for retarded growth in these lines. Altogether, our results indicate that MPK4 is responsible for controlling PSII efficiency, chloroplast retrograde signalling, cellular ROS/SA homeostasis and water management, leading to changes in growth and development in woody plant species grown in the field.