Evaluation of the roles of two compatible solutes, glycine betaine and trehalose, for the Acacia senegal–Sinorhizobium symbiosis exposed to drought stress

Abstract

Acacia senegal (Mimosoideae) is a leguminous, nitrogen-fixing tree that grows in arid areas of Africa and the Near East. In this work, we studied the effects of drought stress on the development of symbiosis between A. senegal seedlings and Sinorhizobium arboris. We also evaluated if two exogenous compatible solutes, glycine betaine and trehalose, are advantageous for the A. senegal–Sinorhizobium symbiosis and if these solutes are capable of protecting two Sinorhizobium strains from salt stress (NaCl) and osmotic stress induced by polyethylene glycol (PEG 6000). A. senegal seedlings exposed to severe drought stress developed more root hairs than plants grown under moderate stress. After inoculation with the GUS marked S. arboris strain HAMBI 2180 the hairs were deformed but infection threads occurred only occasionally. Non-typically deformed hairs were dwarfed and swollen. Severely stressed roots contained less nodules but more nodule initials than moderately stressed ones. Nodules formed showed a lowered glucuronidase activity and signs of premature senescence. The numbers of culturable rhizobia in soil mix were reduced from 107 to 106 CFU g−1. Thirty days after inoculation severely drought-stressed A. senegal seedlings were wilted. Regarding endogenous glycine betaine, A. senegal appeared to be a non-accumulator plant but was able to translocate foliar-applied glycine betaine into roots. Both glycine betaine (0.01 M) and trehalose (0.01, 0.05 and 0.09 M) protected cell cultures of the wild type S. arboris strain HAMBI 1552 and S. saheli strain HAMBI 1496 from osmotic stress (9 and 17% PEG). In the case of salt stress only trehalose had a favourable effect. Application of 0.0003 M glycine betaine or trehalose into A. senegal soil mix exposed to severe drought stress maintained the numbers of culturable rhizobia at the same level as in moderately stressed soils. The presence of glycine betaine in the soil mix also helped A. senegal seedlings survive under severe drought.