7.3. Ca2+ and pH
Cytosolic Ca2+ and pH, which function as potent regulators of plant
aquaporins, are crucial signaling intermediates in plant responses to
stresses and hormones. For instance, soil flooding results in an oxygen
deprivation in roots, which traps cells in a deep cytosolic acidosis, and
thereby inhibits Lpr by proton-dependent gating of PIPs [55]. The effects
of Ca2+ seem to be more diverse and contribute to multiple signaling
cascades. For instance, an exogenous supply of Ca2+ enhanced the stimulation
of sunflower Lpr by ABA [129], and counteracted its inhibition by
salt stress in maize and melon [130,131]. Whereas structure–function
analyses have elucidated themode of Ca2+ and proton-dependent gating
of PIPs [33,55,70], the numerous Ca2+-dependent protein kinases
that exist in plant certainly provide additional regulation mechanisms
that need to be elucidated.