这句话用英文怎么翻译呢？

首先我不明白为什么既“互惠互利”又“拮抗平衡”，两个意思自相矛盾。
互惠互利的共生关系在生物学中有专门的词汇symbiosis

可以翻译成The interaction between endophytic fungi and host plant is an instance of symbiosis.
“拮抗平衡”我认为放在句中是错误的。
symbiosys本身的英文解释就是...relationship，所以前文用interaction以避免英文语意重复。

另外，一楼这种人，白痴也会拿翻译软件来翻译，请不要再做白痴才做的事情。

Endophytic fungi and host plants is a mutually beneficial symbiotic relationship balanced antagonism.

The relationship between endophytic fungi and host plant is beneficial and balanced.

内生真菌 endophytic fungi
宿主植物 host plant

between…and…两者之间

The relationship between in-living fungus and paresitic flora is mutual benefit and be blanced while living together``

Endophytic fungi and host plants is a mutually beneficial symbiotic relationship balanced antagonism.