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Title: Biphasic gene expression changes elicited by Phakopsora pachyrhizi in soybean correlates with fungal penetration and haustoria formation

item SCHNEIDER, KATHERINE - Us Deparment Of Homeland Security
item VAN DE MORTEL, MARTIJN - Iowa State University
item BANCROFT, TIMOTHY - Iowa State University
item Nelson, Rex
item NETTLETON, DON - Iowa State University
item BRAUN, EDWARD - Iowa State University
item Frederick, Reid
item BAUM, THOMAS - Iowa State University
item Graham, Michelle
item WHITHAM, STEVEN - Iowa State University

Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/22/2011
Publication Date: 7/22/2011
Citation: Schneider, K.T., Van De Mortel, M., Bancroft, T.J., Nelson, R., Nettleton, D., Braun, E., Frederick, R.D., Baum, T.J., Graham, M.A., Whitham, S.A. 2011. Biphasic gene expression changes elicited by Phakopsora pachyrhizi in soybean correlates with fungal penetration and haustoria formation. Plant Physiology. 157:355-371.

Interpretive Summary: The fungus Phakopsora pachyrhizi causes a serious foliar disease of soybeans, referred to as Asian Soybean Rust (ASR), in most growing regions of the world including the U.S. Five genes have been described that provide resistance to some isolates of P. pachyrhizi, and understanding the molecular basis of resistance might allow for development of new methods to control ASR. The expression levels of ~37,000 genes were monitored in a soybean line in both resistant and susceptible ASR reactions overtime. An initial burst of increased soybean gene expression occurred at 12 hours followed by a second burst of increased gene expression after 48 hours in the resistant ASR reaction. Both phases of increased soybean gene expression correlated with the observation of specialized infection structures by P. pachyrhizi using light microscopy. The genes identified in this study will provide targets for future studies designed to assess the specific role of individual genes in ASR resistance.

Technical Abstract: Inoculation of soybean plants with Phakopsora pachyrhizi, the causal organism of Asian soybean rust, elicits a biphasic response characterized by a burst of differential gene expression in the first 12 h. A quiescent period occurs from 24 to 48 h after inoculation in which P. pachyrhizi continues to develop but does not elicit strong host responses, followed by a second phase of intense gene expression. To correlate soybean responses with P. pachyrhizi growth and development, we inoculated the soybean cultivar Ankur (accession PI462312), which carries the Rpp3 resistance gene with avirulent and virulent isolates of P. pachyrhizi. The avirulent isolate Hawaii 94-1 (HW94-1) elicits hypersensitive cell death that limits fungal growth on Ankur and results in an incompatible response while the virulent isolate Taiwan 80-2 (TW80-2) grows extensively, sporulates profusely, and produces a compatible reaction. Inoculated leaves were collected over a 288-h time course for microarray analysis of soybean gene expression and microscopic analysis of P. pachyrhizi growth and development. The first burst in gene expression correlated well with appressorium formation and penetration of epidermal cells, while the second burst of gene expression changes followed the onset of haustoria formation in both compatible and incompatible interactions. Furthermore, in the incompatible interaction, the development of haustoria correlated well with elicitation of Rpp3-mediated resistance suggesting that an AvrRpp3 effector was expressed as HW94-1 haustoria matured. The temporal relationships between P. pachyrhizi growth and host responses provide an important context in which to view the interacting gene networks that mediate the outcomes of their interactions.