title: Hydrogel control of water uptake by pectins during in vitro pollen hydration of Eucalyptus globulus.
abstract: OBJECTIVE
  Upon pollination, dehydrated pollen grains take water out of the stigma surface, an event that constitutes the first functional checkpoint of sexual reproduction in higher plants. Little is known about possible functional connections between rehydration speed and further steps of fertilization. Here we addressed the mechanisms of water uptake control by dehydrated pollen grains. Because dehydrated cells have no energy-driven active mechanism such as membrane-based osmoregulation for controlling water movement, we tested the hypothesis that another mechanism might exist, namely, the use of hydrogel-behaving molecules.
  
  
  METHODS
  We developed an imaging protocol to visualize and quantify the rate of water entry into pollen grains of Eucalyptus globulus and tested the influence of different treatments linked to hydrogel-behaving molecules. We complemented these analyses by immunostaining pectins in the pollen grain with monoclonal antibodies JIM5 and JIM7.
  
  
  RESULTS
  Water entry seemed to proceed exclusively through the germination apertures of the pollen grain, and the changes observed in different hydration media are compatible with hydrogel behavior. When JIM5 and JIM7 were used to characterize pectins on the germination apertures during hydration, pectin localization and esterification changed during hydration and were affected by the hydration solutions. These results suggest that chemical modification of the pectins may change their hydrogel behavior, thus modifying the hydration speed.
  
  
  CONCLUSIONS
  The hydrogel behavior of pectins and pectin localization on apertures strongly suggest that pectins act like "valves" for water entry, enabling a regulated process of water uptake into the dehydrated pollen grain. We propose that this regulation evolved in terms of achieving the correct self-organization of molecules and cellular components to resume metabolism and pollen tube growth, especially in species that are subject to demanding environmental water stress.
authors.affiliation: Instituto Gulbenkian de Ciência; Oeiras, 2780-156, Portugal.
authors.forename: Ana Maria
authors.initials: AM
authors.lastname: Vieira
authors.affiliation: Instituto Gulbenkian de Ciência; Oeiras, 2780-156, Portugal University of Maryland, Department of Cell Biology and Molecular Genetics, 0118 BioScience Research Building, College Park, Maryland 20742-5815 USA jfeijo@umd.edu.
authors.forename: José A
authors.initials: JA
authors.lastname: Feijó
mesh_terms: D005052:Eucalyptus / Q000187:drug effects / Q000502:physiology*; D018525:Germination / Q000187:drug effects; D020136:Hydrogel, Polyethylene Glycol Dimethacrylate / Q000494:pharmacology*; D006863:Hydrogen-Ion Concentration; D008954:Models, Biological; D010368:Pectins / Q000378:metabolism*; D011058:Pollen / Q000187:drug effects / Q000502:physiology*; D012997:Solvents; D014867:Water / Q000378:metabolism*
chemical_list: D012997:Solvents; D014867:Water; D020136:Hydrogel, Polyethylene Glycol Dimethacrylate; D010368:Pectins
keywords: Eucalyptus globulus; Myrtaceae; hydration; hydrogel; pectin; pollen