THE SYMBIOTIC PHENOTYPES OF EXOPOLYSACCHARIDE-DEFECTIVE MUTANTS OF RHIZOBIUM SP STRAIN-TAL1145 DO NOT DIFFER ON DETERMINATE-NODULATING AND INDETERMINATE-NODULATING TREE LEGUMES

Three classes of exopolysaccharide (EPS) defective mutants were isolat ed by Tn3Hogus-insertion mutagenesis of Rhizobium sp. strain TAL1145, which nodulates tree legumes. The class I and class III mutants produc ed 10-22% of the EPS produced by TAL1145 and appeared partially mucoid while the class II mutants formed small, opaque and non-mucoid coloni es. Size-fractionation of the soluble EPSs made by these mutants in th e culture supernatant indicated that the class I and the class III mut ants produced reduced levels of both high and low-molecular-mass EPSs while the class II mutants lacked both these EPSs but produced a small amount of a medium-molecular-mass anthrone-reactive EPS. The succinyl and acetyl substituents observed in the TAL1145 EPS were absent in th e EPS of the class II mutants. When examined under UV, the class I and class III mutants grown on Calcofluor-containing YEM agar showed dim blue fluorescence, compared to the bright blue fluorescence of the wil d-type strain, whereas the class II mutants did not fluoresce. While t he dim blue fluorescence of the class III mutants changed to yellow-gr een after 10 d, the fluorescence of the class I mutants did not change after prolonged incubation. Unlike the EPS-defective mutants of other rhizobia, these mutants did not show different symbiotic phenotypes o n determinate- and indeterminate-nodulating tree legumes. The class I and the class III mutants formed small ineffective nodules on both typ es of legumes whereas the class II mutants formed normal nitrogen-fixi ng nodules on both types. The genes disrupted in the class I and class III mutants form a single complementation group while those disrupted in the class II mutants constitute another. All the three classes of EPS-defective mutants were located within a 10.8 kb region and complem ented by two overlapping cosmids.