The time and spatial strategy of Blumeria graminis f. sp. hordei for surviving after failure of first infection

Abstract

The powdery mildew fungus of barley, Blumeria graminis f. sp. hordei (Bgh), is an obligate parasite. If the fungus fails to form a haustorium in a barley cell, it cannot absorb nutrients from the host cell and will die. To survive after failure of the first penetration, the fungus attempts a secondary attack from a second lobe of the appressorium (APP). If the secondary attack is successful, the fungus can form haustoria in barley cells and take up nutrients, and then finally propagate, forming new conidia. However, a highly resistant condition is induced in the same host cell as a result of the failure of the first attack by Bgh. This condition makes it difficult for the fungus to successfully penetrate the same cell that the first lobe failed to penetrate. Interestingly, when the fungus fails to penetrate with the first lobe, there is a high tendency for a second lobe to emerge on the side opposite the first lobe. By means of the manner, a second lobe would penetrate a cell adjacent to the cell of the first attack when the appressorial germ tube (AGT) lies parallel to the border between host cells. Moreover, when the conidia land on part of the coleoptile instead of the cell border, most of the apices of the APP reach the cell border and attempt to penetrate adjacent to the cell on which the APPs were formed. This phenomenon means that a second lobe penetrates a cell adjacent to the cell of the first attack whichever the second lobe emerges on the side opposite or same the first lobe. Surprisingly, most of the secondary attack on the cell adjacent to the cell of the first attack ends before the resistance induced by the failure of the first attack is transferred. Hence, the results show that the second lobes escape, in both a timely and spatial manner, the highly inaccessible state in the same cell enhanced by the failure of a prior attack by the first lobe.