Expression of PIP aquaporins is enhanced in olive dwarf genotypes and is related to root and leaf hydraulic conductance

The expression of OePIP1.1 and OePIP2.1 aquaporins in root, stem and leaf was higher in Olea europaea L. (olive) plants carrying a dwarfing (D) clone as rootstock, than in plants carrying a vigorous (M) clone as rootstock. The highest difference of expression between plants with D and M rootstocks was detected in the root and for the PIP2 gene, the transcripts of which show a high water channel activity in oocytes. Compared with plants with M rootstock, plants with D rootstock showed reduced root and canopy biomass and reduced hydraulic conductance of the bulk root system (K-root). Hydraulic conductance of D roots was higher than that of M roots when K-root was scaled by root DW, in agreement with a higher expression of PIP genes. Leaf hydraulic conductance scaled by unit leaf surface area was almost twice as high in leaves from D plants compared with that in leaves from M plants. Modification of root growth imposed by grafting M onto D and D onto M slightly changed expression levels of root OePIP1.1 gene, the transcripts of which show almost null water channel activity in oocytes, suggesting a role less evident for OePIP1.1 in root hydraulic reequilibration. The role of PIP aquaporin gene expression in partially compensating reductions of plant hydraulic conductance is discussed.