Acid tolerance in the Rhizobium meliloti - Medicago symbiosis

Abstract

Several strains of Rhizobium meliloti that originated from acid soils in Sardinia, Italy, were markedly superior in colonizing a moderately acid loamy sand (pH 5.0 in 1:5 0.01 M CaCl2) than two Australian commercial inoculant strains (U45 and CC169), and a group of strains that originated from alkaline soils in Syria and Iraq. Six Medicago hosts also varied greatly in their ability to achieve nodulation in this soil. M. polymorpha and M. murex were far superior in this respect to M. littoralis, M. truncatula and M. tornata. The most acid-tolerant strains of R. meliloti, WSM419 and WSM413, were able to nodulate a high proportion of plants of M. polymorpha and M. murex sown in the second year between 11 and 20 cm from the point of introduction of the rhizobia into the soil the previous year. It is suggested that these more saprophytically competent isolates of R. meliloti, combined with the species of Medicago more able to nodulate readily in acid soil, will extend the range of soils suitable for successful regenerative growth of these species.

Enhanced nodulation and symbiotic effectiveness of Medicago truncatula when co-inoculated with Pseudomonas fluorescens WSM3457 and Ensifer (Sinorhizobium) medicae WSM419

“…E. medicae WSM419 was isolated from nodules of Medicago murex in Sardinia (Howieson and Ewing 1986). Both strains were sourced from the Western Soil Microbiology (WSM) collection held at the Centre for Rhizobium Studies at Murdoch University.…”

An Essential Role for actA in Acid Tolerance of Rhizobium Melilotix

“…The establishment of medick pastures on mildly acidic soils in Western Australia has, until recently, been prevented by the acid sensitivity of R. meliloti (Robson & Loneragan, 1970). This problem was largely overcome by the isolation from the southern Mediterranean region of acid-tolerant strains of R. meliloti like WSM419 and WSM540 which perform better in acidic soils than traditional inoculant strains (Howieson & Ewing, 1986;Howieson et al, 1988). This raises an obvious question about the underlying physiological mechanisms which assist such tolerant rhizobial inoculant strains to survive and colonize acidic soil environments.…”

Selecting improved Lotus nodulating rhizobia to expedite the development of new forage species

“…Generally speaking, the persistence of the best strains was outstanding, with significant levels of nodulation occurring even in the 22-32 cm sampling region in South Australia. Where strains persist poorly, such as is the case with Sinorhizobium meliloti in very acid soils, nodules are rarely found past the 12 cm sampling region (Howieson and Ewing 1986;Ballard et al 2004). While the persistence of the Lotus rhizobia in the sandy neutral soils (pH Ca 6.3 and 7.2) is encouraging, their evaluation in the more acidic soil was important as Lotus spp.…”