It depends on what the Russian means. I English, we have a SEED OF DOUBT, which refers to suggestion something to someone that will make them start doubting, make them skeptical. Then we have a GRAIN OF DOUBT, which refers to a very small amount of doubt — what we feel when we’re not 100% convinced of something.

Обычно №1 - "заронить/посеять зерно сомнения"

to plant a seed of doubt 👍🏼

a seed of doubt

думаю, тогда и оригинал лучше подредактировать

So, we can't plant a grain of doubt?