Submitted to: Symbiosis
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/13/2004
Publication Date: 9/1/2005
Citation: Kim, S.J., Kremer, R.J. 2005. Scanning and transmission electron microscopy of root colonization of morningglory (Ipomoea spp.) seedlings by rhizobacteria. Symbiosis. 39:117-124.
Interpretive Summary: Morningglory vigorously competes with soybean for nutrients and water and is very difficult to control even with broad-spectrum, non-selective herbicides (i.e., Roundup) currently used in herbicide-resistant soybean production. Management of morningglory in herbicide-resistant soybean might be improved by supplementing herbicide control with specific microorganisms selected from soil around soybean roots for biological control of morningglory. Plant roots provide unique environments for microorganisms whose various activities can have either beneficial or detrimental effects on plant growth. We identified bacteria inhabiting roots (rhizobacteria) of soybean and morningglory that suppressed the growth of morningglory, a weed that infests many row crops. We then evaluated the rhizobacteria in controlled hydroponic studies to characterize growth-suppressive effects on morningglory. This information will be valuable as we develop ways to better control the weed in current soybean production systems. We found that two of the rhizobacteria strains suppressed morningglory root growth by up to 80% due partly to over-production of indoleacetic acid, a plant-growth-promoting hormone. Microscopic observation at very high magnification (5,000 to 10,000 times) revealed that cells of the growth-suppressive rhizobacteria attached tightly to morningglory root surfaces, which indicated that colonization of roots by high rhizobacteria cell numbers also contributed to reduction in root growth. Results are important to scientists and producers because a framework is provided for the development of microorganisms, like the rhizobacteria described here, as potential biological control agents that could be used to help control troublesome weeds like morningglory that infest soybean fields. The use of such non-chemical methods to supplement weed management in herbicide-based crop production may become a more practical option as the number of weeds becoming resistant to herbicides increases.
Technical Abstract: Hydroponically grown ivyleaf morningglory (Ipomoea hederacea) seedlings inoculated with rhizobacteria were studied to observe colonization of roots using scanning and transmission electron microscopy. The deleterious rhizobacteria (DRB), Bradyrhizobium japonicum isolate GD3 and Pseudomonas putida isolate GD4, were compared with a plant growth promoting rhizobacterium (PGPR), Bacillus megaterium isolate GP4. Scanning electron microscopy revealed that the colonization patterns of isolates GP4 and GD4 consisted of cells distributed over the surface of roots; in contrast, cells of isolate GD3 were deeply localized in surface furrows of roots. Transmission electron microscopy showed considerable alterations of root cells by isolates GD3 and GD4, including vesiculation, partial cell wall degradation, and cytoplasmic disorganization. The average population density of isolate GD4 on root surfaces was about 10 and 100 times greater than GP4 and GD3, respectively. Isolates GD3 and GD4 significantly inhibited seedling root elongation by 80% and 30%, respectively. This study showed that inhibition of morningglory root growth by isolate GD3 was related to production of very high concentrations of indoleacetic acid. Rhizobacteria able to suppress morningglory growth may be effective as biological control agents for supplementing herbicide weed management in crops where morningglory is difficult to control.