Java8 Method Reference and generics, type cast needed in some cases

I am trying to deal with Java8 method references and encountered very strange compile error.

I had a map with object values and I want to provide for user to apply some functions on values.

So I used generic method which takes map key and functional interface as parameter

And I am wondering why I don't need to cast Method reference to precise real type (in case of trim)

import java.util.HashMap;
import java.util.Map;
import java.util.function.UnaryOperator;

public class ReferenceDiscovering {

    Map<String, Object> values = new HashMap<>();

    public static void main(String[] args) {
        ReferenceDiscovering main = new ReferenceDiscovering();
        main.values.put("key1", " some text with space in start");
        main.values.put("key2", "AAAAAAAAAAAAAA");

        System.out.println(main.values);

        main.applyFunctionByKey("key1", String::trim);
        //        cannot resolve method toLowerCase
        //        COMPILE ERROR HERE. WHY???? WHY NOT FOR trim?
        //        main.applyFunctionByKey("key2", String::toLowerCase);
        main.applyFunctionByKey("key2", (UnaryOperator<String>)String::toLowerCase);


        System.out.println(main.values);
    }

    private <T> void applyFunctionByKey(String key, UnaryOperator<T> binaryOperator) {
        if (values.containsKey(key)) {
            values.put(key, binaryOperator.apply((T)values.get(key)));
        }
    }
}

Result:

{key1= some text with space in start, key2=AAAAAAAAAAAAAA} {key1=some text with space in start, key2=aaaaaaaaaaaaaa}