title: Is expression of aquaporins (plasma membrane intrinsic protein 2s, PIP2s) associated with thermonasty (leaf-curling) in Rhododendron?
abstract: It is postulated that leaf thermonasty (leaf curling) in rhododendrons under sub-freezing temperatures is caused by water redistribution due to extracellular freezing. We hypothesize that aquaporins (AQPs), the transmembrane water-channels, may be involved in regulating water redistribution and thus leaf curling. Our experimental system includes two Rhododendron species with contrasting leaf curling behavior whereby it was observed in R. catawbiense but not in R. ponticum. We compared leaf movements and the expression of two AQPs, i.e. R. catawbiense/ponticum plasma-membrane intrinsic protein 2 (Rc/RpPIP2;1 and Rc/RpPIP2;2), in the two species under freezing-rewarming and dehydration-rehydration cycles. To determine the relationship between extracellular freezing and leaf-curling, we monitored leaf-curling in R. catawbiense with or without controlled ice-nucleation. Our data indicate that extracellular freezing may be required for leaf curling. Moreover, in both species, PIP2s were up-regulated at temperatures that fell in ice-nucleation temperature range. Such up-regulation could be associated with the bulk-water efflux caused by extracellular freezing. When leaves were frozen beyond the ice-nucleation temperature range, PIP2s were continuously down-regulated in R. catawbiense along with the progressive leaf curling, as also observed for RcPIP2;2 in dehydrated leaves; as leaves uncurled during re-warming/rehydration, RcPIP2 expression was restored. On the other hand, R. ponticum, a non-curling species, exhibited substantial up-regulation of RpPIP2s during freezing/dehydration. Taken together, our data suggest that RcPIP2 down-regulation was associated with leaf curling. Moreover, the contrasting PIP2 expression patterns combined with leaf behavior of R. catawbiense and R. ponticum under these two cycles may reflect different strategies employed by these two species to tolerate/resist cellular dehydration.
authors.affiliation: Department of Horticulture, Iowa State University, Ames, IA 50011, USA.
authors.forename: Keting
authors.initials: K
authors.lastname: Chen
authors.forename: Xiang
authors.initials: X
authors.lastname: Wang
authors.forename: Anania
authors.initials: A
authors.lastname: Fessehaie
authors.forename: Yanhai
authors.initials: Y
authors.lastname: Yin
authors.forename: Xiaolei
authors.initials: X
authors.lastname: Wang
authors.forename: Rajeev
authors.initials: R
authors.lastname: Arora
mesh_terms: D020346:Aquaporins / Q000235:genetics* / Q000378:metabolism; D001483:Base Sequence; D003080:Cold Temperature; D018076:DNA, Complementary / Q000235:genetics / Q000378:metabolism; D005615:Freezing; D018506:Gene Expression Regulation, Plant; D018515:Plant Leaves / Q000235:genetics / Q000502:physiology*; D010940:Plant Proteins / Q000235:genetics* / Q000378:metabolism; D016133:Polymerase Chain Reaction; D029793:Rhododendron / Q000235:genetics / Q000502:physiology*; D013045:Species Specificity; D014867:Water / Q000378:metabolism
chemical_list: D020346:Aquaporins; D018076:DNA, Complementary; D010940:Plant Proteins; C110065:major intrinsic protein, plant; D014867:Water
keywords: AQP; Dehydration; Extracellular freezing; Ice-nucleation; PIP; R. catawbiense; R. ponticum; aquaporin; plasma membrane intrinsic protein