The latest Plant Ontology Paper is available online from Plant and Cell Physiology
We just heard the news that the recent Plant Ontology Paper is available online from Plant and Cell Physiology: http://pcp.oxfordjournals.org/content/early/2012/12/05/pcp.pcs163.abstract?keytype=ref&ijkey=y8ieoOz5Ec5iIaT
Right now it is just the abstract with a link to the full text accepted proof. But it has been assigned a DOI, in case you would like to cite it.