Acknowledgment
Yao Shuluan, Shi Ao and Li Jun contributed equally to this work and should be considered joint first authors.
Declaration of interest
The authors declare no conflict of interest. The authors alone are responsible for the content and writing of the article.
Funding
The authors are supported by grants from the National Natural Science Foundation of China (NSFC; Grant Number 81560502), the National Natural Science Foundation of Yunnan Province (Grant Numbers 2013FB044 and 2014FB008), the Education Department Fund of Yunnan Province (Grant Numbers 2014Y165, 2015Z082); Doctoral Graduate Academic Newcomer Award of Yunnan Province (2014) and Doctoral Innovation Fund (Grant Numbers 2016D02).