title: The Role of Aquaporins in pH-Dependent Germination of Rhizopus delemar Spores.
abstract: Rhizopus delemar and associated species attack a wide range of fruit and vegetables after harvest. Host nutrients and acidic pH are required for optimal germination of R. delemar, and we studied how this process is triggered. Glucose induced spore swelling in an acidic environment, expressed by an up to 3-fold increase in spore diameter, whereas spore diameter was smaller in a neutral environment. When suspended in an acidic environment, the spores started to float, indicating a change in their density. Treatment of the spores with HgCl2, an aquaporin blocker, prevented floating and inhibited spore swelling and germ-tube emergence, indicating the importance of water uptake at the early stages of germination. Two putative candidate aquaporin-encoding genes-RdAQP1 and RdAQP2-were identified in the R. delemar genome. Both presented the conserved NPA motif and six-transmembrane domain topology. Expressing RdAQP1 and RdAQP2 in Arabidopsis protoplasts increased the cells' osmotic water permeability coefficient (Pf) compared to controls, indicating their role as water channels. A decrease in R. delemar aquaporin activity with increasing external pH suggested pH regulation of these proteins. Substitution of two histidine (His) residues, positioned on two loops facing the outer side of the cell, with alanine eliminated the pH sensing resulting in similar Pf values under acidic and basic conditions. Since hydration is critical for spore switching from the resting to activate state, we suggest that pH regulation of the aquaporins can regulate the initial phase of R. delemar spore germination, followed by germ-tube elongation and host-tissue infection.
authors.affiliation: Department of Postharvest Sciences of Fresh Produce, Agricultural Research Organization (ARO), The Volcani Center, Bet-Dagan, Israel.|Department of Plant Sciences and Genetics in Agriculture, The Robert H. Smith Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem, Rehovot, Israel.
authors.forename: Tidhar
authors.initials: T
authors.lastname: Turgeman
authors.affiliation: Department of Plant Sciences and Genetics in Agriculture, The Robert H. Smith Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem, Rehovot, Israel.
authors.forename: Arava
authors.initials: A
authors.lastname: Shatil-Cohen
authors.affiliation: Department of Plant Sciences and Genetics in Agriculture, The Robert H. Smith Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem, Rehovot, Israel.
authors.forename: Menachem
authors.initials: M
authors.lastname: Moshelion
authors.affiliation: Department of Postharvest Sciences of Fresh Produce, Agricultural Research Organization (ARO), The Volcani Center, Bet-Dagan, Israel.
authors.forename: Paula
authors.initials: P
authors.lastname: Teper-Bamnolker
authors.affiliation: Renewable Product Technology Research Unit, NTL Center for Agricultural Utilization Research, Peoria, Illinois, United States of America.
authors.forename: Christopher D
authors.initials: CD
authors.lastname: Skory
authors.affiliation: Department of Postharvest Sciences of Fresh Produce, Agricultural Research Organization (ARO), The Volcani Center, Bet-Dagan, Israel.
authors.forename: Amnon
authors.initials: A
authors.lastname: Lichter
authors.affiliation: Department of Postharvest Sciences of Fresh Produce, Agricultural Research Organization (ARO), The Volcani Center, Bet-Dagan, Israel.
authors.forename: Dani
authors.initials: D
authors.lastname: Eshel
mesh_terms: D020346:Aquaporins / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D012233:Rhizopus / Q000378:metabolism* / Q000502:physiology*; D013172:Spores, Fungal / Q000378:metabolism* / Q000502:physiology*
chemical_list: D020346:Aquaporins