I'm not sure what it represents but I made wheat on the pumpkin patch cake in my photos. I rolled a snake of gumpaste and then snipped it with scissors to get the look of wheat. I picked it up from the world championship of baking which dh laughed at me for watching, they were doing it on bread 
hth
Sue
To address your first question, I'm no expert, but if I recall from years of Catholic school, the church doctrine is communion wafers must have unleavened wheat in them (like the bread served at the Last Supper). This is why there is often wheat incorported into first communion celebration decorations.
snowshoe1 is right about the wheat.
I just uploaded a First Communion cake this weekend that had wheat on it out of gumpaste. I made the stalk first then each triangular wheat bud? and attached. when it was dry, I painted with thinned ivory gel color.
http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1199376
Quote by @%username% on %date%
%body%