Submitted to: Biology and Fertility of Soils
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 1, 2007
Publication Date: May 1, 2008
Citation: Chee Sanford, J.C. 2008. Weed seeds as nutritional resources for soil Ascomycota and characterization of specific associations between plant and fungal species. Biology and Fertility of Soils. 44(5):763-771. Interpretive Summary: Soil seed banks contain highly abundant seeds of many annual weed species. Natural soil microbial populations are likely to interact with seeds as part of their environment, and the mechanism of interaction may include nutritional benefits to microorganisms provided by the seeds. In this study, four common annual weed species, velvetleaf (Abutilon theophrasti), woolly cupgrass (Eriochloa villosa), Pennsylvania smartweed (Polygonum pensylvanicum), and giant ragweed (Ambrosia trifida) were examined for fungal populations found in association with seeds following exposure to natural soil microbial populations. Velvetleaf, which were highly susceptible to complete decay, clearly provided a nutritional benefit to microorganisms at the detriment of the seeds. Seeds of the other weed species were not as susceptible to decay processes, but neverthesless sustained distinct microbial populations directly associated with the seed surfaces. Even when the seeds were not decayed, the seeds were thought to provide a nutritional benefit to the attached microbes, either through seed exudates or enhanced access to available nutrients in the soil. The identity of specific soil fungi within the Ascomycota group, suggested specific populations were associated with certain weed species. The impact of these findings suggest that different weed species may be found associated with certain species of soil microorganisms, and the interactions between seeds and microorganisms may provide certain nutritional benefit to the microbes. These types of interactions may suggest the future ability to manipulate conditions that might enhance seed decay, leading to seed bank depletion and contribute to weed management strategies.
Technical Abstract: Current interest in biological-based management of weed seed banks in agriculture furthers the need to understand how microorganisms affect seed fate in soil. Many annual weeds produce seeds in high abundance; their dispersal presenting ready opportunity for interactions with soil-borne microorganisms. In this study, we investigated seeds of four common broadleaf weeds, velvetleaf (Abutilon theophrasti), woolly cupgrass (Eriochloa villosa), Pennsylvania smartweed (Polygonum pensylvanicum), and giant ragweed (Ambrosia trifida), for potential as sources of carbon nutrition for soil fungi. Seeds, as the major source of carbon in an agar matrix, were exposed to microbial populations derived from four different soils for two months. Most seeds were heavily colonized and the predominant 18S rRNA gene sequences cloned from these assemblages were primarily affiliated with Ascomycota. Further, certain fungi corresponded to weed species, regardless of soil population. Relatives of Chaetomium globosum (98-99% sequence identity) and Cordyceps sinensis (99%) were found associated with seeds of woolly cupgrass and Pennsylvania smartweed, respectively. More diverse fungi associated with velvetleaf seeds, which were highly susceptible to decay. The velvetleaf seed associations were dominated by relatives of Cephaliophora tropica (98-99%). In contrast to the other species, only few giant ragweed seeds were heavily colonized, but those that were, resulted in seed decay. The results showed seeds could provide significant nutritional resources for saprophytic microbes, given the extant populations can overcome intrinsic seed defenses against microbial antagonism. Further, weed species-specific associations may occur with certain fungi, with nutritional benefits conferred to microorganisms that may not always result in seed biodeterioration.