Synergistic induction of wheat stem rust appressoria by chemical and topographical signals

The leaf alcohols cis-3-hexen-1-ol and trans-2-hexen-1-ol have been previou sly shown to induce the wheat stem rust (Puccinia graminis f. sp. tritici) to differentiate appressoria in a complex axenic culture medium. In this pa per. the possible role of these chemicals in appressorium induction during infection has been analysed further. The inductive potential of ethylene an d three cutin monomers were also tested because they have been found to ind uce appressoria in other fungi. Both hexenols were found to be inductive in aqueous solution in the absence of media constituents. 0.5 1 mM trans-2-he xen-1-ol Nas the most inductive resulting in a maximum of 51% appressorium differentiation. trans-2-hexen-1-ol was also shown to be inductive in vapou r form. In aqueous solution, trans-2-hexen-1-ol acted synergistically with topographical signals by increasing the rate of appressorium induction. Com bining the two signals also increased the total number of appresoria differ entiated (88%). The other chemicals tested were noninductive although a bio chemical domain rich in exposed cutin was localized around stomatal apertur es of heat leaves. The characteristic bipolar morphology of appressoria for med over stomata was found to be determined lj topographical signals. Overa ll. Our data Support a role for both chemical signals (hexenols or their an alogues) and topographical signals being involved in appressorium induction by wheat stem rust. (C) 2001 Academic Press.