Expression and role of plasma membrane aquaporins in maize leaves

(eng) Leaves play key roles in plant growth and development. Since they constitute the major site of evapotranspiration and photosynthesis, they need to be continuously supplied with water and carbon dioxide. On its way through the leaf from the xylem to the stomata, water can either move through cell walls (apoplastic path) or pass from cell to cell via the symplastic and transcellular paths. Transcellular water flow is facilitated and regulated by the presence of water channels, named aquaporins (AQPs), in the membranes. Besides their important role in plant water relations, an increasing number of AQPs were shown to transport small uncharged substrates involved in other physiological phenomena such as carbon fixation, cell signaling, nutrition and stress responses. The present work aimed at studying the expression and transport activity of plasma membrane AQPs (PIPs) in Zea mays leaves. RT-qPCR and immunodetection approaches were performed to quantify and localize ZmPIP expression, and transport assays in heterologous expression systems were used to investigate their potential substrates. We found that all ZmPIP genes except ZmPIP2;7 were expressed in leaves, although to different extents. Their expression was generally dependent on the developmental stage of the leaf tissue and changed diurnally. ZmPIPs are located in tissues in which high transmembrane water permeability might be required. Several ZmPIPs are expressed in stomatal complexes (guard cells and/or subsidiary cells) suggesting their potential involvement in stomatal movements. The study of the transport selectivity of two of them, ZmPIP1;5 and ZmPIP1;6, suggested that both facilitate transmembrane diffusion of water. Interestingly, acidification kinetics of ZmPIP1;6 expressing yeast cells indicated also a possible role of ZmPIP1;6 in transmembrane CO2 transport. In situ immunolocalization using anti-ZmPIP1;6 antibodies indicated that ZmPIP1;6 is present in the plasma membrane of epidermal cells, including subsidiary and guard cells, but also in mesophyll and guard cell chloroplasts. Altogether, these data strengthen the assumption that PIPs play important roles in many different physiological processes in leaves.