I am comparing two variants of one gene. The unspliced/nascent RNAs show no difference, but spliced/mature RNA show very big difference. So I guess the mRNA stability of the two variants may be changed. So I used Actinomycin D to treat the seedlings and monitored the RNA abundance in time course, and found both mRNA are too stable, I could not see any reduction of the mRNA after inhibitor treatment for more than 12 hours.

How can I check the stability of very stable mRNA in vivo?