Tropical rainforest bromeliads (Bromeliaceae) are largely epiphytic and as such lack functional root or stem water uptake mechanisms. Tank bromeliads maintain a modified overlapping leaf structure which forms a reservoir from which water and nutrients can be drawn. Water movement from the tank into the plant tissue occurs via trichomes on the leaf surface. Aquaporin proteins may be important in facilitating rapid water movement across leaf cell membranes. Changes in aquaporin gene expression have been linked to changes in water permeability in many plant species. Thus, we hypothesized that gene expression of aquaporins in the plasma membrane intrinsic protein (PIP) subclass (previously shown to be important in water uptake in roots of non-epiphytic plants) may be important in the regulation of water and small solute uptake in two congeneric tank bromeliads in Guzmania, a wide-ranging genus in the Neotropics. In addition, because tank water acidity varies widely in the field, and intracellular pH has been shown to regulate aquaporins in many species, we hypothesized that aquaporin expression would vary with tank water pH.
Results/Conclusions
Using oligonucleotides previously identified for the aquaporin TiPIP1 in the bromeliad Tillandsia ionantha (Ohrui et al., 2007), we identified a putative PIP1 in Guzmania lingulata and G. monostachia with 98% sequence similarity between the congeners and 80% sequence similarity with PIP4-2 in banana (M. acuminata). PIP1 transcript accumulation was monitored by qRT-PCR in leaf segments collected from the tank to the tip. Preliminary data suggest that PIP1 transcript accumulation was greatest in tissue from the region directly above the submerged tank region, further suggesting that intracellular water uptake by PIP aquaporins is most highly regulated in the region of the leaf where water availability is most frequently in flux. We also collected tissue from the tank region of Guzmania congeners in a Costa Rican rainforest with tank water pH ranging from 3.21 to 6.65. Preliminary data suggest that PIP1 mRNA accumulation was higher in tissue from tanks at a higher pH compared to plants at lower pH. Overall, studies of PIP1gene expression in tank bromeliads will inform larger questions of water and nutrient uptake in epiphytic tropical rainforest species in a more variable global climate.