Please note that some important tweaks were made to makeTxDbFromBiomart() last week to improve its support for EnsemblGenomes (see here A: Errors with makeTxDbFromBiomart for the details) so make sure you use the latest version of GenomicFeatures (1.28.5) before trying the above.
Ensemblgenomes provides gene models for many plants. Check http://plants.ensembl.org/index.html . You could either download a gtf or gff3 file for rice from there and build a TxDb using makeTxDbFromGff (GenomicFeatures package) or, since the data is in Ensembl format, an EnsDb using ensDbFromGtf (ensembldb package - EnsDb and TxDb packages/databases provide the same functionality/annotations).
For EnsDb, creating an EnsDb from a GTF you might lack some annotations since they are not provided in the file. If you tell me what release and species (which of the many oryza forms e.g. oryza_sativa, oryza_meridionalis etc) you'd need, I could build the EnsDb database/package for you directly from the ensemblgenomes MySQL databases - just let me know.