Theme 15: Advances in Rhizobial Research
Description
Symbiotic N fixation from legumes is one of the most important biological processes on the planet. It currently provides the majority of the N requirement in agriculture, yet will have to double if cereal crop production is to meet world demand by 2020 (Kennedy and Cocking 1997). To effectively harness the value of biological N fixation from legumes we need to more fully understand G2 x E; where G refers to the genotypes of both the legume (Gl) and its microsymbiont (rhizobia; Gr), and E refers to the edaphic environment in which the symbiosis is to function.
In the Mediterranean basin, indigenous legumes are nodulated by specific rhizobial genotypes. Despite co-evolution of the symbionts, their relationship is not always optimal for N fixation. It has been proposed that rhizobial genotypes are differentially adapted to soil conditions and that it is this adaptation, rather than the relationship with their host, that most strongly governs outcomes relating to competition and persistence (Sprent 1994, Howieson 2000). Expressed in terms of the above formula, symbiotic effectiveness (G2) is unimportant to the persistence and success of rhizobia relative to adaptation of the rhizobia to soil and climate (Gr x E).
In this paper we investigate whether rhizobia which have co-evolved with tropical legumes show similar sub-optimal patterns of N fixation. Tropical legumes differ from Mediterranean legumes in that many nodulate promiscuously, and often effectively, with a broad range of rhizobial genotypes (both rhizobia and bradyrhizobia). This introduces the possibility that impacts of soil and climate on the persistence and success of some tropical rhizobia (Gr x E) are less important for optimal N fixation (G2) than they are in the Mediterranean region. However, observations on N fixation in promiscuous tropical legumes are mostly based on agricultural species associated with rhizobia from outside the centre of origin of the host legume. We examine N fixation by tropical rhizobia which have co-evolved with their host.
We show that sub-optimal N fixation can be improved using several research options, the choice of which depends on the nature of the limitation to the legume or the performance of the rhizobia. We describe three scenarios that might compromise N fixation: a) where the legume is sown into soil containing a high population of variably effective rhizobia, b) where the rhizobial population is low, and c) where there is no background population of rhizobia capable of nodulation with the legume. Each scenario has presented with it several research strategies for improving symbiotic N fixation. These strategies have application for both Mediterranean and tropical environments.
Included in
The Legume – Rhizobia Symbiosis. Does It Vary for the Tropics Relative to the Mediterranean Basin?
Symbiotic N fixation from legumes is one of the most important biological processes on the planet. It currently provides the majority of the N requirement in agriculture, yet will have to double if cereal crop production is to meet world demand by 2020 (Kennedy and Cocking 1997). To effectively harness the value of biological N fixation from legumes we need to more fully understand G2 x E; where G refers to the genotypes of both the legume (Gl) and its microsymbiont (rhizobia; Gr), and E refers to the edaphic environment in which the symbiosis is to function.
In the Mediterranean basin, indigenous legumes are nodulated by specific rhizobial genotypes. Despite co-evolution of the symbionts, their relationship is not always optimal for N fixation. It has been proposed that rhizobial genotypes are differentially adapted to soil conditions and that it is this adaptation, rather than the relationship with their host, that most strongly governs outcomes relating to competition and persistence (Sprent 1994, Howieson 2000). Expressed in terms of the above formula, symbiotic effectiveness (G2) is unimportant to the persistence and success of rhizobia relative to adaptation of the rhizobia to soil and climate (Gr x E).
In this paper we investigate whether rhizobia which have co-evolved with tropical legumes show similar sub-optimal patterns of N fixation. Tropical legumes differ from Mediterranean legumes in that many nodulate promiscuously, and often effectively, with a broad range of rhizobial genotypes (both rhizobia and bradyrhizobia). This introduces the possibility that impacts of soil and climate on the persistence and success of some tropical rhizobia (Gr x E) are less important for optimal N fixation (G2) than they are in the Mediterranean region. However, observations on N fixation in promiscuous tropical legumes are mostly based on agricultural species associated with rhizobia from outside the centre of origin of the host legume. We examine N fixation by tropical rhizobia which have co-evolved with their host.
We show that sub-optimal N fixation can be improved using several research options, the choice of which depends on the nature of the limitation to the legume or the performance of the rhizobia. We describe three scenarios that might compromise N fixation: a) where the legume is sown into soil containing a high population of variably effective rhizobia, b) where the rhizobial population is low, and c) where there is no background population of rhizobia capable of nodulation with the legume. Each scenario has presented with it several research strategies for improving symbiotic N fixation. These strategies have application for both Mediterranean and tropical environments.
