The Plant Ontology (PO: http://www.plantontology.org) is a structured vocabulary and database resource for all plant scientists that links plant anatomy, morphology and development to the rapidly expanding field of plant genomics. The primary purpose of the PO is to facilitate cross-database querying and to foster consistent use of vocabularies in annotation. Originally designed to span the monocot-dicot divide in flowering plants, the PO has been redesigned to encompass a wider variety of angiosperm species, as well as gymnosperms, pteridophytes (ferns), lycophytes (lycopods) and bryophytes (liverworts, mosses and hornworts). Recent changes in the PO include the addition of more than 80 new terms to accommodate non-seed plants, with an emphasis on those needed to annotate gene expression from the Physcomitrella patens genome. An essential feature of the PO is the set of freely accessible web links from terms to associated annotations, which are structure- or development-specific genes, proteins and phenotypes sourced from numerous plant genomics datasets. Currently, the PO includes over 2 million such annotations associated with over 1,300 terms. Outreach activities include workshops, conference presentations and outreach booths. Also, image libraries are being created through PlantSystematics.org and linked to PO terms to provide reference images for plant structure terms. The PO is a valuable resource for both research and teaching that can be used as a guide to plant structures and growth and developmental landmarks in life cycles of plants across many taxa.