Abscisic acid regulates root hydraulic conductance via aquaporin expression modulation in Nicotiana tabacum

Abscisic acid (ABA) modifies the hydraulic properties of roots by increasing root water flux. The effects of ABA on aquaporin content and root hydraulic conductance are controversial. We addressed these effects via a combination of experiments. Tobacco (Nicotiana tabacum) plants were grown hydroponically, and ABA (1 μM) was exogenously applied to the roots. Then, the water transport properties of tobacco roots and expression of PIP-type aquaporins were examined. ABA increased the sap flow rate (Jv) and also the osmotic root hydraulic conductance (Lpr-o) of excised tobacco roots after 24 h. The expression of three aquaporin PIP-type genes and PIP1s proteins abundance in tobacco roots were analyzed by real-time PCR and protein gel blot analysis, respectively. Interestingly, the accumulation of NtAQP1, NtPIP1;1 and NtPIP2;1 transcripts and NtPIP1;1 and NtAQP1 proteins abundance was significantly increased. Although the antibody used recognize NtPIP1;1 and NtAQP1, most probably it also recognizes other PIP1 proteins present in tobacco. Thus, the increase in the expression of the three PIP-type genes and other PIP1s proteins abundance caused by ABA were correlated with an increase in Lpr-o and Jv. ABA therefore facilitated the cell-to-cell component of water transport across the root cylinder. The subcellular localization of NtPIP1;1- and NtPIP2;1-GFP was investigated by protoplast transformation with chimeric gene, showing NtPIP2;1 localization in plasma membrane and NtPIP1;1 retention in the endoplasmic reticulum (ER). However, ABA did not change subcellular localization of NtPIP1;1 from ER to plasma membrane.