104th General Meeting of the American Society for Microbiology
May 23-27, 2004, New Orleans, Louisiana
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EMBARGOED UNTIL: Monday, May 24, 1:00 p.m. CDT
(Session 78, Paper N-013)
Joanne Chee-Sanford
Urbana, IL, United States
Phone: 217-265-0959

Weed control is essential for successful crop production. Currently, herbicide use is the primary method for weed control, however, increasing environmental and health concerns due to contamination has stimulated interest in research and public sectors to discover new and effective non-chemical methods to manage problem weeds. A big challenge in weed control is the abundant amount of seeds that can persist in soil, creating a reservoir, or seed bank, from which future weeds can emerge every year. Control of weeds by managing seed banks has potential for practical application in agricultural as well as in natural area systems. In this research, we identified natural soil bacteria and fungi that were found associated with seeds of several common species of weeds, including velvetleaf, giant ragweed, Pennsylvania smartweed, jimsonweed, and wooley cupgrass after exposure of the seeds to native soil microbial populations. In these experiments, the seeds provided a main source of nutrition for the soil microorganisms that were present. Velvetleaf seeds were especially susceptible to microbial decay and this activity was widely observed with seeds exposed to several different types of soils. Other weed seeds were not as susceptible to microbial decay, however many microorganisms were found colonizing the surfaces of these seeds. While many of these associations did not result in obvious seed decay, loss of seed viability may have occurred. Using DNA-based techniques that do not require the cultivation of live cells to identify microbial species, many of the bacteria and fungi associated with decaying velvetleaf seeds were found to be normal soil microorganisms. These results suggest that for certain species of weeds, the potential for seed decay by native soil bacteria or fungi is widely distributed. Individual weed species differ in their susceptibility to microbial attack, and the microbial associations with seeds do not necessarily result in seed decay or noticeable loss of weed viability. However, in natural seed banks, seeds are known to persist for years prior to germination, indicating the presence of unknown factors that limit the extent of seed decay or reductions in seed viability. By identifying the important factors involved in relationships between soil microorganisms and weed seeds, we can develop strategies to enhance important soil bacteria or fungi and their seed decay or seed inactivation activities.

Scientists in the Agricultural Research Service of the U.S. Department of Agriculture are carrying out this research, which is funded by the U.S.D.A. The research team is part of the Invasive Weed Management Unit located in Urbana, IL, which conducts studies in Illinois and the surrounding Midwest states where the major crops are primarily corn and soybean. Results are being presented at the 104th General Meeting of the American Society for Microbiology held in New Orleans, LA from May 23 through May 27, 2004.

In the U.S. Corn Belt, over 95% of crop land is treated each year with herbicides to control weeds. These chemicals can enter into the environment, potentially affecting environmental quality and pose risks to human and animal health. Weeds can become resistant or tolerant to herbicides, making chemical controls less effective or unreliable. Heightened interest in alternative, non-chemical methods have led to research that focuses on utilizing more biological methods for effective weed control. In contrast to other biological weed control methods investigated in the past that relied on development of microbial cultures that target specific weed pests, this research looks at how microbes that normally reside in soil can be exploited for their natural abilities to attack weed seeds in soil seed banks. Weeds are resilient and highly adaptive, and the abundance of seeds present in soil offers potential for many interactions with native soil microorganisms, many of which are unidentified and perform unknown functions. Seeds may provide microorganisms with surfaces for attachment, nutrition, protection, and even means of dispersal. This research will help to identify factors or conditions in soil that might be manipulated or modified in agricultural practices to increase important natural microbial activities that enhance weed death. In practical application, incorporating such biologically-based methods into current agriculture could lead to less reliance on herbicide control and more environmentally-sound and sustainable management of weeds for the future.


Technical Abstract

Buried seed reserves, commonly referred to as the soil seed bank, are widely distributed and can persist in the environment. Weed seeds, in particular, are resilient and highly adaptive, and their abundance in soil offers potential for many interactions with native soil microorganisms. As a specialized niche, seeds may provide microorganisms with surfaces for attachment, nutrition, protection, and even means of dispersal. In this study, we used microscopy, terminal restriction fragment (TRF) analysis and ribosomal gene sequences to characterize microbial populations associated, either soil- or seed-borne, with a variety of common weed seeds. The species include velvetleaf, giant ragweed, Pennsylvania smartweed, jimsonweed, and wooley cupgrass. Few fungi and bacteria were loosely associated with some seeds collected directly from plants, but abundant growth and diversity of both fungi and bacteria were found on seeds incubated directly with a variety of soil inocula. PCR-amplified and cloned ribosomal DNA sequences included common soil isolates associated with four major bacterial phyla and a dominance of representatives in the main fungal phylum Ascomycota. Matches between 93-99% sequence identities to bacterial 16S rDNA include members of the genera Flexibacter, Flavobacterium, and Sphingobacterium, and to fungal 18S rDNA primarily associated with the genera Chaetomium, Cephaliophora, and Cordyceps. Analysis of cloned sequences and TRF profiles showed that development of seed-associated populations differed according to individual soils, but within a given soil, a number of common fragments and closely related microbial species were present on replicate seeds, suggesting that there may be some specific relationships. Most seeds, with the exception of velvetleaf, were not decayed over a three-month incubation period in the presence of soil microbial populations. This study investigates the fundamental relationships that may be common between microorganisms and seeds, with broader implications for soil seed bank ecology in many natural systems.