The effect of polymyxin on plant roots.

Antibiotics are recognized and selected because of their inhibitory effects on the proliferation or activity of microorganisms. Only relatively recently have their effects on plant growth been studied, mostly in connection with the search for compounds which might be employed to suppress invading plant pathogens (l-5). In such cases the antibiotic should control the pathogen without adversely affecting the host plant. Various antibiotics, however, have been found to be inhibitory or inimical to plant growth (2, &lo). The experiments reported herein arose from a desire to suppress microbial development on root surfaces in cation-uptake studies. Ordinarily in such work no attempt is made to establish or maintain sterile conditions. Bacterial counts, which will be reported elsewhere, indicated that in young root cultures organisms are present both at large in the medium and on the root surfaces in great numbers. This fact is ordinarily ignored in the reporting or interpretation of uptake studies. Of a number of antibiotics tested to control microbial development in root cultures, none were found to be entirely satisfactory, but several were observed to affect adversely root growth and function at levels which indicated substantial physiological potency. Polymyxin is in this class. Its effect on roots was examined further in the hope that thereby some light might be thrown on its mode of action. The antibiotic, polymyxin, produced by the organism Bacillus polymyxa, is active in the suppression of gram-negative bacteria (11, 12). It is believed to be a basic cyclic polypeptide with mol. wt. about 1200.