Strategies used by rhizobia to lower plant ethylene levels and increase nodulation

Agriculture depends heavily on biologically fixed nitrogen from the symbiotic association between rhizobia and plants. Molecular nitrogen is fixed by differentiated forms of rhizobia in nodules located on plant roots. The phytohormone, ethylene, acts as a negative factor in the nodulation process. Recent discoveries suggest several strategies used by rhizobia to reduce the amount of ethylene synthesized by their legume symbionts, decreasing the negative effect of ethylene on nodulation. At least one strain of rhizobia produces rhizobitoxine, an inhibitor of ethylene synthesis. Active 1-aminocyclopropane-1-carboxylate (ACC) deaminase has been detected in a number of other rhizobial strains. This enzyme catalyzes the cleavage of ACC to α-ketobutyrate and ammonia. It has been shown that the inhibitory effect of ethylene on plant root elongation can be reduced by the activity of ACC deaminase.Key words: rhizobia, nodulation, ethylene, ACC deaminase, rhizobitoxine.

Histological and Cytological Effects of Haloxyfop on Sorghum (Sorghum bicolor) and Unicorn-Plant (Proboscidea louisianica) Root Meristems

The effects of haloxyfop on elongation, mitotic index, and morphology of sorghum and unicorn-plant primary roots were examined. Elongation of sorghum roots was completely inhibited by haloxyfop concentrations of 10–6M or greater 24 h after treatment, whereas unicorn-plant root elongation was unaffected by the same concentrations 72 h after treatment. Mitotic indices of sorghum roots were reduced by both 10–6and 10–8M haloxyfop, with the higher concentration reducing the index to near zero by 24 h of exposure. The mitotic indices of unicorn-plant roots were unaffected by the same levels of the herbicide after both 24- and 48-h treatment. Histological analyses showed that after 24 h exposure to 10–6M haloxyfop, large vacuoles were present in cells at the root apex that normally did not exhibit these organelles. After 48 h of exposure many cells in the apical region appeared to lack visible cytoplasm and/or nuclei, and by 72 h only cell walls remained visibly evident, and many cells had collapsed. These changes are similar to those that occur in tissues undergoing senescence. Treatment of sorghum roots with 10–8M haloxyfop did not cause discernible changes after 72 h. Unicorn-plant roots treated with 10–6M haloxyfop appeared unaffected after 72 h.