You cannot directly interface with an LDAP data source from apex. The only supported way of calling a remote service via apex is either HTTP(S) or SMTP (aka email; not helpful here).
As @pbattisson mentioned there are some middleware applications that allow proxying LDAP to and from HTTP(S) data. Be it a RESTful or SOAP-based stack apex should support them (although excessively complex WSDL can be an issue).
It's also worth noting that webservice callouts are not allowed directly from a trigger - you'd have to use something like an @future method in order to process the callout asynchronously.
At the end of the day it's certainly possible, but it is rather labor intensive to make this happen, and requires 3rd party software.