However if you use the correct URL in your R query you currently encounter a different error. I get this same error when using the Ensembl BioMart web interface, suggesting this is a problem at Ensembl's end, rather than something I can address in biomaRt.
There is a more recent archive at oct2017-plants.ensembl.org although I can't find any documentation about this, so it may not be permanent. You can query this via:
Bear in mind with this that all of the information you retrieve from here will be potentially out of date, and it might be worth finding out why the tigr_locus attribute was removed from the current Ensembl release. That's something that you will have to contact Ensembl about.