Michael Dilworth

I. Introduction The problems for agriculture on acidic soils are especially significant in relation to legume nodulation on such soils. The low rainfall Mediterranean climate of the eastern wheat-belt of Western Australia represents one particular type of problem. Here the nitrogen input for wheat crops needs to come from N2 fixation from pasture legumes, but the growing season is inadequate for normal annual clovers and the acidity of the soils is too great for normal strains of Sinorhizohium meliloi to persist and nodulate the annual species of Medicago with suitable short growing season requirements. Introduction of strains of S. melilola isolated from annual Medicago spp. growing on acid soils in Mediterranean Europe (Howieson, Ewing 1986) have, however, helped establish wide areas (Ca. 400 000 ha) of medic pastures in this environment. Our laboratory is trying to understand why these introduced strains of S. meliloti arc able to colonize, persist and nodulate in such soils when so many others cannot. The initial question was whether strains were acid-tolerant because they carried genes, which others did not, or whether they simply had more effective versions of genes which were of general occurrence. Other questions concern how many genes arc specifically required for acid tolerance, how the acid environment is perceived, how the acid-tolerant cells respond to It, and what physiological mechanisms arc brought into play. Some of these questions arc addressed in this brief review.

The Sinorhizobium-legume interaction is sensitive to a number of environmental factors: one of the important stresses is soil acidity. Low pH imposes severe constraints on legume productivity. In the Sinorhizobium-Medicago partnership, prokaryotic partner is the most acid-sensitive. The isolation of acid-tolerant Sinorhizobium strains from the eastern and southern Mediterranean rim have allowed the successful development of medic pastures on over I million ha of land in Western Australia, posing two important questions: • How do acid-tolerant strains differ from acid-sensitive counterparts? • How do agriculturally important root nodule bacteria respond to low pH? Knowledge of the genetic circuits required and activated by the cell in acidic conditions is vital to understand the response and survival of bacteria in these conditions. The molecular basis of acid tolerance has been investigated with several approaches (Glenn et al. 1999).