Welcome to the forum, maly.
The structure is not particularly complex or uncommon, though the second and fourth words will probably be new to most native speakers, except biologists. By the way, plant should be plants.
Being ....., = As.. I/we/it/they/etc.. am/is/are/was/were...,
So, your sentence could be re-worded as: As plants are sessile and photoautotrophic, they are particularly sensitive to this crucial external signal.