Investigation of morphological differences leading to rust resistance in Sorghastrum nutans

The rust fungus Puccinia virgata is an obligate parasite of Sorghastrum nutans (indiangrass). Uredospores of P. virgata initiate infection via the formation of germ tubes that terminate in penetration structures, appressoria, over host stomatal openings. Previous research has shown that different varieties of indiangrass have varying levels of resistance to P. virgata that, at least in part, may be due to leaf morphological features. Scanning electron microscopy was used to characterize the morphological features of three indiangrass varieties with varying degrees of rust resistance. Stomatal density, trichome density, prickle density, subsidiary cell gap distance, and intercostal cell length were compared using the abaxial leaf surfaces from replicate plants in a growth chamber study. The susceptible variety Rumsey was characterized as having significantly shorter intercostal cells, higher prickle density and wider subsidiary cell gaps. Intercostal cells were significantly longer and subsidiary cell gaps were significantly smaller in the resistant variety Osage. Osage's more topographically uniform surface provides decreased tactile signaling that can be correlated with a reduction in the number of uredospore germ tubes that successfully form appressoria over host stomata. Osage's features would be the most favorable for selection in breeding programs to improve rust resistance.