Effect of dopamine and progesterone on the physiological and molecular responses of tomato seedlings to drought and salt stress

Drought and salt stress are the most common abiotic stresses resulting in yield reduction or complete loss of agricultural production in recent years. Stress mitigation by external biostimulator molecules has been an active research topic recently. Neurotransmitters (NTs) dopamine and progesterone, found in both animal and plant kingdom were shown to take role in plant abiotic and biotic stress defense in limited number of studies. This study investigated the effects of exogenous dopamine and progesterone application in tomato seedlings under drought and salt stress by examining various morphological and physiological parameters (tissue length and weights, relative water content, ion leakage, malondialdehyde and proline levels), as well as expressions of various genes encoding enzymes; superoxide dismutase (FeSOD), catalase (CAT2), glutathione reductase (GR1), ascorbate peroxidase (APX1), 1-aminocyclopropane-1-carboxylic acid synthase (ACS2) and delta 1-pyrroline-5-carboxylate synthase (P5CS), that play a direct role in the antioxidative defense system or measured as stress indicators. The results showed that dopamine and progesterone alleviated drought stress mainly by increasing superoxide dismutase and catalase antioxidative enzyme gene expressions and decreasing ethylene production in tomato seedlings, thereby improving cell membrane integrity and increasing root dry weight. Although morphological and physiological responses of the seedlings were mostly similar under drought and salinity stresses, antioxidative defense enzyme gene expressions were not upregulated under salinity stress, except for the GR1 expression under progesterone treatment.