Are single-celled organisms capable of learning?
I'd like to know what is the reference for amoebic learning. I cannot comment directly on this, but there is some evidence for  "adaptive anticipation" in both prokaryotes and single-celled Eukaryotes which do not have a nervous system. 

In the case of E. coli, it has been shown that the bacteria can anticipate the environment it is about to enter.  E. coli in the digestive tracts of mammals will typically be exposed to initially a lactose, and then later to a maltose environment as the bacteria pass down through the animal tract. This suggests that upon encountering a lactose environment, maltose operons are induced. I.e., upon encountering lactose, maltose is anticipated. This suggests a "genetic memory" of the sequence of sugar types where lactose is always encountered before maltose.

Further cultures (500 generations) of E. coli in the absence of maltose but in the presence of lactose reduced the maltose operon activity to negligible levels, suggesting that this is an adaptive prediction of environmental changes.

Mitchell, A et al., Adaptive Prediction of environmental changes by microorganisms, 2009, 460, 1038