Uptake of an endocytic marker by rice cells: variations related to osmotic and saline stress

Saline and osmotic stress are the main abiotic factors limiting the productivity of rice and other crop plants. Although both coincide in generating water deficit and affect many aspects of plant growth and development similarly, some effects of salinity are distinctively related to the ionic component of salt stress. At the cellular level, dessication tolerance is largely dependent on the cell's ability for osmotic adjustment. Here, we have studied the effects of saline and osmotic stress on endocytosis by rice cells, to investigate the common and distinctive effects of saline-generated stress and osmotically generated stress, and the possible involvement of endocytosis in tolerance mechanisms. For this purpose, we have used rice cell lines with different levels of tolerance and biotinylated bovine serum albumin (bBSA) as an endocytic marker, which in our previous experiments has been shown to enter rice cells by a process with the characteristics of receptor-mediated endocytosis. Our results indicate that the pattern of uptake is common to both types of stress. Thus, when rice cells were subjected to saline or osmotic stress there was an initial dose-dependent inhibition of uptake. However, after more extended stress periods, there was an activation of uptake in the stressed cells. This late activation appears mainly related to the inhibition of growth commonly caused by the different stress agents used in this study. When using cell lines with different degrees of tolerance, the level of uptake activation varied as a function of the type of stress. Thus, under osmotic stress, a higher stress tolerance was directly related to a higher bBSA uptake, while the opposite occurred under saline stress. The possible role of endocytosis in the cellular responses to osmotic and saline stress is discussed.