Accumulation of the amino acid proline is a commonly observed phenomenon in plant roots under drought stress. The proline accumulated is believed to serve as a compatible osmolyte to balance the cytoplasm against higher exterior and vacuolar ionic concentrations. The root nodules of soybean, like other root tissues, accumulate proline under drought conditions. Unlike enteric bacteria, who are also proline accumulators under osmotic stress, the symbiotic nitrogen fixing bacteria in soybean nodules display high activity of the proline catabolic enzyme proline dehydrogenase. Previous research has shown that gene directed knock-out mutations of the proline dehydrogenase gene in the symbiotic nitrogen fixing bacteria had an effect on seed yield and number when the soybean plants nodulated with these strains were subjected to moderate water stress. The basis of the yield decrease is not known but could be due to either loss of a catabolic substrate that provides some energy during drought or due to accumulation of fixed nitrogen (proline) down-regulating the nitrogen fixation machinery. The previously described proline dehydrogenase knock-out mutations and construction of a strain containing a proline dehydrogenase promoter-reporter gene fusion (GUS) are being used to test transcriptional regulation of the proline dehydrogenase promoter with or without the presence of the working enzyme in the bacteria.

Key words: B. japonicum, Proline