Physiology and proteomics analyses reveal the response mechanisms of Rhizophora mucronata seedlings to prolonged complete submergence.

Mangrove seedlings are subjected to natural tidal inundation, while occasional flooding may lead to complete submergence. Complete submergence reduces light availability and limits gas exchange, affecting several plant metabolic processes. The present study focuses on Rhizophora mucronata, a common mangrove species found along the coasts of Thailand and the Malay Peninsula region. To reveal the response mechanisms of R. mucronata seedlings to submergence, a physiological investigation coupled with proteomic analyses of the leaf and root tissues was carried out in the plants subjected to 20 days of control (drained) and submerged conditions. Submerged seedlings showed decreased photosynthetic activity, lower stomatal conductance, higher total antioxidant capacity in the leaves and higher lipid peroxidation in the roots than control plants. At the same time, tissue nutrient ions displayed organ-specific responses. Proteome analysis revealed a significant change in 240 proteins in the leaves and 212 proteins in the roots. In leaves, most differentially-accumulated proteins (DAPs) are associated with nucleic acid, stress response, protein transport, signal transduction, development and photosynthesis. In roots, most DAPs are associated with protein metabolic process, response to abiotic stimulus, nucleic acid metabolism and transport. Our study provides a comprehensive understanding of submergence responses in R. mucronata seedlings. The results suggest that submergence induced multifaceted stresses related to light limitation, oxidative stress, and osmotic stress, but the responses are organ specific. The results uncovered many candidate proteins, which may be essential for the survival of R. mucronata under prolonged submergence.