Why do you need to use 2,4-D as a growth regulator to inhibit the growth of test-tube seedlings and promote the generation and growth of callus?

This is related to the concentration of 2,4d. 2,4d is an auxin-like substance. High concentration of 2,4d inhibited growth, while low concentration promoted growth. This is a law similar to "low promotion and high inhibition" of plant hormones, depending on who is more sensitive to 2,4-D test-tube seedlings or calli.