If you are intrested in genetic information I would recommend phytozome!

You can also go to http://www.boldsystems.org for species identification and taxonomy

I suggest KOME (http://cdna01.dna.affrc.go.jp/cDNA/) and Phytozome(http://www.phytozome.net/). It will be helpful for you.

ENSEMBL plants: http://plants.ensembl.org/index.html

and the BioMART part allows you to perform searches of genes filtered by various criteria, or SNPs etc. 

http://plants.ensembl.org/biomart/martview/a43270fb15a93c2aa4f51c73a1fd5748

I agree that it depends on which plant you're interested in. Both ENSEMBL and Phytozome provide extensive resources for a number of plant species. 

I think you need to know the genes associated with biotic/abiotic stress, right? I guess you could try https://www.arabidopsis.org/portals/expression/microarray/ATGenExpress.jsp as well as STIFDB2

I suggest you to use www.arabidopsis.org and www.phytozome.net. But again don't limit yourself.