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Bypassing Both Surface Attachment and Surface Recognition Requirements for Appressorium Formation by Overactive Ras Signaling in Magnaporthe oryzae

September 2014 , Volume 27 , Number  9
Pages  996 - 1,004

Xiaoying Zhou,1 Xinhua Zhao,1 Chaoyang Xue,1 Yafeng Dai,2 and Jin-Rong Xu1

1Department of Botany and Plant Pathology, Purdue University, West Lafayette, IN 47907, U.S.A.; 2State Key Laboratory of Crop Stress Biology for Arid Areas, Northwestern A&F University, Yangling, Shaanxi 712100, China

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Accepted 7 May 2014.

Magnaporthe oryzae forms a highly specialized infection structure called an appressorium for plant penetration. In M. oryzae and many other plant-pathogenic fungi, surface attachment and surface recognition are two essential requirements for appressorium formation. Development of appressoria in the air has not been reported. In this study, we found that expression of a dominant active MoRAS2G18V allele in M. oryzae resulted in the formation of morphologically abnormal appressoria on nonconducive surfaces, in liquid suspensions, and on aerial hyphae without attachment to hard surfaces. Both the Pmk1 mitogen-activated protein kinase cascade and cAMP signaling pathways that regulate surface recognition and appressorium morphogenesis in M. oryzae were overactivated in the MoRAS2G18V transformant. In mutants deleted of PMK1 or CPKA, expression of MoRAS2G18V had no significant effects on appressorium morphogenesis. Furthermore, expression of dominant MoRAS2 in Colletotrichum graminicola and C. gloeosporioides also caused the formation of appressorium-like structures in aerial hyphae. Overall, our data indicate that MoRas2 functions upstream from both the cAMP-PKA and Pmk1 pathways and overactive Ras signaling leads to improper activation of these two pathways and appressorium formation without surface attachment in appressorium-forming pathogens.

© 2014 The American Phytopathological Society