title: Developmental changes in guard cell wall structure and pectin composition in the moss Funaria: implications for function and evolution of stomata.
abstract: OBJECTIVE
  In seed plants, the ability of guard cell walls to move is imparted by pectins. Arabinan rhamnogalacturonan I (RG1) pectins confer flexibility while unesterified homogalacturonan (HG) pectins impart rigidity. Recognized as the first extant plants with stomata, mosses are key to understanding guard cell function and evolution. Moss stomata open and close for only a short period during capsule expansion. This study examines the ultrastructure and pectin composition of guard cell walls during development in Funaria hygrometrica and relates these features to the limited movement of stomata.
  
  
  METHODS
  Developing stomata were examined and immunogold-labelled in transmission electron microscopy using monoclonal antibodies to five pectin epitopes: LM19 (unesterified HG), LM20 (esterified HG), LM5 (galactan RG1), LM6 (arabinan RG1) and LM13 (linear arabinan RG1). Labels for pectin type were quantitated and compared across walls and stages on replicated, independent samples.
  
  
  RESULTS
  Walls were four times thinner before pore formation than in mature stomata. When stomata opened and closed, guard cell walls were thin and pectinaceous before the striated internal and thickest layer was deposited. Unesterified HG localized strongly in early layers but weakly in the thick internal layer. Labelling was weak for esterified HG, absent for galactan RG1 and strong for arabinan RG1. Linear arabinan RG1 is the only pectin that exclusively labelled guard cell walls. Pectin content decreased but the proportion of HG to arabinans changed only slightly.
  
  
  CONCLUSIONS
  This is the first study to demonstrate changes in pectin composition during stomatal development in any plant. Movement of Funaria stomata coincides with capsule expansion before layering of guard cell walls is complete. Changes in wall architecture coupled with a decrease in total pectin may be responsible for the inability of mature stomata to move. Specialization of guard cells in mosses involves the addition of linear arabinans.
authors.affiliation: Department of Plant Biology, Southern Illinois University, Carbondale, IL 62901, USA ameliamerced@siu.edu.
authors.forename: Amelia
authors.initials: A
authors.lastname: Merced
authors.affiliation: Department of Plant Biology, Southern Illinois University, Carbondale, IL 62901, USA.
authors.forename: Karen
authors.initials: K
authors.lastname: Renzaglia
mesh_terms: D005075:Biological Evolution; D019068:Bryopsida / Q000254:growth & development / Q000378:metabolism / Q000648:ultrastructure*; D002473:Cell Wall / Q000648:ultrastructure*; D010368:Pectins / Q000378:metabolism*; D054046:Plant Stomata / Q000254:growth & development / Q000378:metabolism / Q000648:ultrastructure*; D011134:Polysaccharides / Q000378:metabolism
chemical_list: D011134:Polysaccharides; C042491:rhamnogalacturonan I; C030080:araban; D010368:Pectins; C003181:polygalacturonic acid
keywords: Arabinan; Funaria hygrometrica; LM13; LM19; LM6; homogalacturonan; immunolabelling; moss; pectin; plant cell wall; rhamnogalacturonan; stomata; ultrastructure