How can two strings be concatenated?

How can I concatenate (merge, combine) two values?
For example I have:

tmp = cbind("GAD", "AB")
tmp
#      [,1]  [,2]
# [1,] "GAD" "AB"

My goal is to concatenate the two values in “tmp” to one string:

tmp_new = "GAD,AB"

Which function can do this for me?