add top-level plant ontology term

[planteome-user]

as I was updating PO in the sol genomics database, I've ran into an issue with participates_in relationship, which creates references between terms in plant_anatomy node and 'sporophyte phase' or 'gametophyte phase' from the growth stages node. (e.g. http://plantontology.org/amigo/go.cgi?xpand=PO:0025025&search_constraint=terms&query=PO:0025025&session_id=9390b1327425114&view=details&show_associations=list)

Since the 2 PO components are now merged into one plant_ontology file, and there are relationships between terms of these 2 components, would it make sense to have one root term called 'plant ontology' ? Currently the ontology has 2 roots, and the PO browser top level term displays as 'all'.

On the technical side, I am not sure how the ontology is loaded into Amigo, but the gmod loader, which I maintain assumes each namespace is separate without relationships between terms (as in GO and its 3 components). This behavior could be changed, but I think one top level root for PO should be considered .

Reported by: *anonymous

Original Ticket: obo/plant-ontology-po-term-requests/436