The system variables for default transaction isolation and
read only transaction are 'tx_read_only' and 'tx_isolation'.
This differs from the start up options which are:
'--transaction_read_only' and '--transaction_isolation'
This naming convention can be quite confusing for users.

There are separate patches for 5.7 and trunk:

In 5.7, the variables 'tx_read_only' and 'tx_isolation' are
deprecated and usage of these variables would result in a
deprecation warning. Two aliases for the deprecated variables
 have been introduced: 'transaction_read_only' and
'transaction_isolation'.

In trunk, the variables 'tx_read_only' and 'tx_isolation' are
removed. 'transaction_read_only' and 'transaction_isolation'
introduced in 5.7 serves as the alternative. This ensures that
the system variables are in sync with the start up option names.