Preformed aerenchyma determines the differential tolerance response under partial submergence imposed by fresh and saline water flooding in rice

Plant's response to fresh- and saline-water flooding and the resulting partial submergence, seems different due to the added complexities of element toxicity of salinity. We identified a few rice genotypes which can tolerate combined stresses of partial submergence and salinity during saline water flooding. To gain mechanistic insights, we compared two rice genotypes: Varshadhan (freshwater-flooding tolerant) and Rashpanjor (both fresh- and saline-water flooding tolerant). We found greater ethylene production and increased "respiratory burst oxidase homolog" (RBOH)-mediated reactive oxygen species (ROS) production led to well-developed constitutive aerenchyma formation in Rashpanjor, which makes it preadapted to withstand fresh- and saline-water flooding. On the contrary, an induced aerenchyma formation-dependent tolerance mechanism of Varshadhan worked well for freshwater flooding but failed to provide tolerance to saline-water flooding. Additional salt stress was found to significantly inhibit the induced aerenchyma formation process due to the dampening of ROS signaling by the action of metallothionein in Varshadhan. Besides, inconspicuous changes in ionic regulation processes in these two genotypes under saline-water flooding suggest preadapted constitutive aerenchyma formation plays a more significant role than elemental toxicity per se in tolerating combined stresses encountered during saline water flooding in rice. Overall, our study indicated that well-developed constitutive aerenchyma provide an adaptive advantage during partial submergence due to saline water flooding in rice as the key process of induced aerenchyma formation is hampered in the presence of salinity stress coupled with partial submergence.