Trimethylamine-N-oxide enhances drought tolerance in Eucalyptus by increasing photosynthesis.

Drought stress significantly reduces agricultural productivity, threatening global food security and timber production. Although trimethylamine-N-oxide (TMAO) has been shown to enhance drought tolerance in plants such as Arabidopsis thaliana and tomato, the physiological and molecular mechanisms by which it regulates drought tolerance in plants remain unclear. In this study, we investigated the physiological and transcriptomic changes in Eucalyptus under drought stress following exogenous TMAO treatment. Physiological analyses showed that TMAO treatment improved the drought resistance of Eucalyptus, and the optimal application concentration was 10 mM. Under drought stress, exogenous TMAO reduced the malondialdehyde content and electrolyte leakage in Eucalyptus leaves, and maintained the stability of the cell membrane. At the same time, TMAO maintained the stability of the photosynthetic electron transport chain and regulates stomatal aperture, which results in a 59% increase in the net photosynthetic efficiency of Eucalyptus under drought. Transcriptomic analysis revealed that TMAO activated pathways for phenylpropanoid biosynthesis, photosynthesis, and carbon metabolism, and influenced the drought resistance of Eucalyptus by regulating the expression of genes such as Phenylalanine ammonia-lyase (PAL), photosystem II reaction center PSB28 protein (Psb28), and FTSH protease 1 (FTSH1), thereby mediating the growth and development of Eucalyptus and its adaptation to adverse conditions. The findings of this study provide an important theoretical basis for using exogenous substances to alleviate plant stress under drought conditions and lay the foundation for exploring the use of exogenous substances in forestry and agriculture.