I am working with potato and I found one of my interested genes in Arabidopsis, what I need now is to find this gene in potato. I have tried NCBI and all of these websites but I could not find it.
At, Potato and Tomato databases are represented as whole genomes, and annotated features like Transcripts etc. There are both cultivated S. tuberosum and relative S. phureja, that I'm unfamiliar with, in addition there are S. lycopersicum (tomato) and S. pimpinellifolium (wild relative) but you may have additional success searching CIP website for more information on the tubers.
http://cipotato.org/region-quito
and more about tomato at the Sol Genome Net.
http://solgenomics.net/
Note that Arabidopsis is represented by the TAIR databse the link above. If its an annotated At gene, there are TAIR resources that links to knock-out mutants (SALK lines). The Nature paper of the tomato-potato comparative genomics is available at Nature.