Aquaporins in poplars: Characterization, transcript profiling and drought stress

This dissertation describes a series of studies that examined: (1) physiological responses of Populus balsamifera and Populus simonii x Populus balsamifera to drought (2) characterization of poplar aquaporins (PtAQPs) (3) abundance of AQP transcripts between poplar tissues (4) effects of drought on transcript abundance of poplar leaf AQPs. In the first study, effects of drought on gas exchange and growth were examined. Contrary to P. balsamifera, P. simonii x balsamifera used drought avoidance as main drought resistance mechanism by reducing stomatal conductance and growth rates. In the second study, 53 aquaporin sequences were identified in the genome sequence of Populus trichocarpa that represented PtPIP, PtTIP, PtNIP, PtSIP, and PtXIP subfamilies. One, two or four PtAQPs for each Arabidopsis thaliana AQP were identified, supporting the notion of genome expansion during poplar genome evolution. PtPIPs and PtTIPs were predicted to be water channels by in silico characterization. The osmotic water permeability coefficient was determined for five PIPs, in which PIP2s were functional water transporters, and PIP1s did not show this functionality. In the third study, the transcript abundance for eleven AQP genes was examined in different tissues of Populus trichocarpa x Populus deltoides . Four AQPs were highly abundant in stems, two in secondary xylem and roots, three in mature leaves, one in aboveground tissues, and one in roots. In the fourth study, expression of these same genes was compared in leaves from P. balsamifera and P. simonii x balsamifera exposed to drought. Contrary to P. balsamifera, P. simonii x balsamifera showed four different expression patterns among the examined AQPs. The first group was characterized by high abundance during mild and severe drought. The second group showed high abundance during severe drought. The third group showed low abundance during drought. Finally, the fourth group was not altered by stress. The abovementioned results allowed me to conclude that AQPs expressed in leaves might be involved in the different drought strategies observed between P. balsamifera and its descendant hybrid poplar, and that the different patterns of gene expression for these AQPs among tissues suggests that there has been functional divergence of these genes in poplars.