title: Apoplasmic loading in the rice phloem supported by the presence of sucrose synthase and plasma membrane-localized proton pyrophosphatase.
abstract: OBJECTIVE
  Although Oryza sativa (rice) is one of the most important cereal crops, the mechanism by which sucrose, the major photosynthate, is loaded into its phloem is still a matter of debate. Current opinion holds that the phloem loading pathway in rice could involve either a symplasmic or an apoplasmic route. It was hypothesized, on the basis of a complementary body of evidence from arabidopsis, which is an apoplasmic loader, that the membrane specificity of proton pyrophosphatases (H(+)-PPases; OVPs) in the sieve element-companion cell (SE-CC) complexes of rice source leaves would support the existence of either of the aforementioned phloem loading mechanisms. Additionally, it was contended that the presence of sucrose synthase in the SE-CC complexes would be consistent with an apoplasmic sucrose loading route in rice.
  
  
  METHODS
  Conventional chemical fixation methods were used for immunohistochemical localization of H(+)-PPases and sucrose synthase in rice and arabidopsis at the light microscopy level, while ultrastructural immunogold labelling of H(+)-PPases and sucrose synthase was performed on high-pressure frozen source leaves of rice.
  
  
  RESULTS
  Using immunogold labelling, it was found that OVPs predominantly localize at the plasma membrane (PM) of the SE-CC complexes in rice source leaf minor veins, while in the root meristematic cells, OVPs preferentially localize at the vacuoles. The PM specificity of OPVs in the SE-CC complexes was deemed to support apoplasmic loading in the rice phloem. Further backing for this interpretation came from the sucrose synthase-specific immunogold labelling at the SE-CC complexes of rice source leaves.
  
  
  CONCLUSIONS
  These findings are consistent with the idea that, in the same way as in arabidopsis and a majority of grasses, sucrose is actively loaded into the SE-CC complexes of rice leaves using an apoplasmic step.
authors.affiliation: School of Life Sciences, Arizona State University, 427 E. Tyler Mall, Tempe, AZ 85287, USA.
authors.forename: Kamesh C
authors.initials: KC
authors.lastname: Regmi
authors.affiliation: School of Life Sciences, Arizona State University, 427 E. Tyler Mall, Tempe, AZ 85287, USA.
authors.forename: Shangji
authors.initials: S
authors.lastname: Zhang
authors.affiliation: School of Life Sciences, Arizona State University, 427 E. Tyler Mall, Tempe, AZ 85287, USA roberto.gaxiola@asu.edu.
authors.forename: Roberto A
authors.initials: RA
authors.lastname: Gaxiola
mesh_terms: D017360:Arabidopsis / Q000378:metabolism; D029681:Arabidopsis Proteins / Q000378:metabolism; D019068:Bryopsida / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism; D005964:Glucosyltransferases / Q000378:metabolism*; D007150:Immunohistochemistry; D043564:Inorganic Pyrophosphatase / Q000378:metabolism*; D018519:Meristem / Q000166:cytology / Q000378:metabolism; D012275:Oryza / Q000378:metabolism*; D052585:Phloem / Q000378:metabolism*; D018515:Plant Leaves / Q000378:metabolism; D010940:Plant Proteins / Q000378:metabolism*; D032107:Populus / Q000378:metabolism; D014617:Vacuoles / Q000378:metabolism; D003313:Zea mays / Q000378:metabolism
chemical_list: D029681:Arabidopsis Proteins; D010940:Plant Proteins; D005964:Glucosyltransferases; C022699:sucrose synthase; C505048:AVP1 protein, Arabidopsis; D043564:Inorganic Pyrophosphatase
keywords: Rice; apoplasmic loading; phloem loading; proton pyrophosphatase; sieve element–companion cell complex; sucrose synthase; symplasmic loading