The operations between Nasu-Shiobara and Morioka stations are currently suspended in the wake of the March 11 earthquake, but JR East will reopen the route step by step after completing repair work and fully resume services around late April.
http://e.nikkei.com/e/fr/tnks/Nni20110328D28JF246.htm
As for the Yamagata Shinkansen Line, operations between Fukushima and Shinjo stations will be resumed on Thursday...
So that will bring an end to natto shortages.