Weed seeds as nutritional resources for soil Ascomycota and characterization of specific associations between plant and fungal species

摘要

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 2 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 to be associated with seeds of woolly cupgrass and Pennsylvania smartweed, respectively. More diverse fungi were 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 colonized resulted in seed decay. The results showed that 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.