Abstract
Aquaporins in plants show more abundant and greater diversity than aquaporins in bacteria and animals. This unique characteristic provided versatile tool boxes for plants, dealing with environmental changes, which overcome the disadvantage of immobility. Aquaporins were first known for their function as water channel proteins. Later on, more and more studies showed that other small solutes, i.e., ammonia, glycerol, urea, hydrogen peroxide and metalloids, can also pass through the channel of various aquaporins. Moreover, the function of aquaporins as CO2 gas channels was studied by several groups (Nakhoul et al. Am J Physiol Cell Physiol 43(2): C543-C548, 1998; Yang et al. J Biol Chem 275(4): 26862692, 2000; Tholen and Zhu Plant Physiol 156(1): 90-105, 2011). In parallel, studies on model reconstituted membranes claim that no such type of channel would be needed due to the high permeability of those model membranes (Missner et al. Proc Natl Acad Sci USA 105(52): E123, 2008a; J Biol Chem 283(37): 25340-25347, 2008b). However, experimental data showed the physiological significance of CO2-conducting channels, particularly in plants. It is generally accepted that plant science presented the first evidence for the physiological relevance and importance of aquaporins as CO2 transport facilitators (Boron Exp Physiol 95(12): 1107-1130, 2010; Terashima and Ono Plant Cell Physiol 43(1): 70-78, 2002; Uehlein et al. Nature 425 (6959): 734-737, 2003; Heckwolf et al. Plant J 67 (5): 795-804, 2011; Uehlein et al. Plant Cell 20(3): 648-657, 2008). In this chapter, we discuss the CO2 diffusion across membranes and the role of plant aquaporins during this process.
