Effect of HvPIP2;1 Expression on Hydraulic Conductance and Formation of Apoplast Barriers in Transgenic Tobacco Plants Grown under Normal and Salt Stress Conditions

Hydraulic conductance of plant tissues depend on the presence of water channels aquaporins in the membranes. Previous experiments showed that over-expression of aquaporin genes in transgenic plants improved plant vigor under favorable growth conditions. However, when growing such plants under stressful conditions, contradictory results were obtained. We hypothesized that, to explain this discrepancy, attention should be paid to the deposition of lignin and suberin, which influence conductivity of the alternative pathway for water passage through the apoplast. Transgenic tobacco (Nicotiana tabacum L.) plants that carry the gene encoding barley plasma membrane aquaporin (HvPIP2;1) and control transgenic plants transformed with an empty vector were grown under normal and salt stress conditions. Under favorable conditions, hydraulic conductance was higher in plants expressing HvPIP2;1 gene accompanied by increased transpiration and growth rate. Salinity decreased hydraulic conductance in the plants of both genotypes, which helped plants to economize water under the salt caused water shortage. In the transgenic plants transformed with the empty vector, this decline in hydraulic conductance was associated with the down-regulation of NtPIP2;1 gene. Although the mRNA abundancy of the gene encoding NtPIP2;1 was also reduced by salinity in plants carrying HvPIP2;1, transgene expression remained high in plants experiencing salinity stress, while the decrease in hydraulic conductance was likely due to strengthened apoplastic barriers. Since apoplastic barriers reduce the entry of toxic sodium ions into plants, their enhancement may explain the increased salt tolerance found in transgenic plants expressing HvPIP2;1, as evidenced by increased Fv/Fm and growth rate under salinity conditions.