posing major challenges on agricultural supply chains. 

1.  有时候，介词的搭配可能与动词有关，例如网友所说的pose ...to...
If something poses a problem or a danger, it is the cause of that problem or danger. 造成 (问题或危险)
pose...to...意为“对......造成/构成......”
This could pose a threat to jobs in the coal industry.
这可能对煤炭行业的就业造成威胁。

2.  有时候，介词搭配可能与前面的名词有关，即与challenges 有关。challenge on 对......施加的挑战；压在......的挑战。

The speed of China's population aging has been fast, which has posed a severe challenge on the mode of providing for the rural old people.
我国人口老龄化速度逐步加快，对农村养老方式提出了严峻的挑战。