Submitted to: Biology and Fertility of Soils
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 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: The current interest in more biological-based management of weed seedbanks in agriculture has increased the need to understand how microorganisms might affect seed fate in soil. Many annual weeds produce seeds in high abundance and following their dispersal, opportunity for interactions between seed and native soil microorganisms can readily occur. This study investigates the potential for seeds of four common broadleaf weeds, velvetleaf (Abutilon theophrasti), woolly cupgrass (Eriochloa villosa), Pennsylvania smartweed (Polygonum pensylvanicum), and giant ragweed (Ambrosia trifida), to provide significant sources of nutrition for growth of common soil microbial inhabitants such as fungi. With the exception of giant ragweed, all seeds became heavily colonized by microorganisms following exposure to soil microbial populations. Only velvetleaf seeds were highly susceptible to seed decay as a result of microbial degradation, indicating that the associated microorganisms used seed resources for nutrition at the expense of the seed. In contrast, seeds of woolly cupgrass and Pennsylvania smartweed were highly colonized on the surface by microorganisms, however, these seeds remained largely intact, with little obvious signs of decay. Few giant ragweed seeds became colonized, however, those that did resulted in seed decay. Analysis of the fungi on the seed surfaces revealed preferential associations between species, suggesting that certain native soil fungi may form interactions with specific weeds. These associations may not be detrimental to the seed, but could benefit native soil fungi. The significance of this study demonstrates different species of weed seeds are potentially accessed by specific soil fungal species as nutritional resources, either in the form of seed components leading to decay, or seed chemicals produced by seeds that support microbial growth, but do not lead to decay processes. These results further suggest the possibility of enhancing or disrupting natural soil microorganisms for antagonistic processes that target seeds.
Technical Abstract: Current interest in biological-based management of weed seedbanks 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 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, 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.