Root architecture of provenances, seedlings and cuttings of Melia volkensii: implications for crop yield in dryland agroforestry

Melia volkensii (Gürke) is being increasingly promoted as an on-farm tree in Kenya. Researchers’ and farmers’ views on its competitiveness with crops differ; research station studies have found it to be highly competitive whereas farmers do not consider it to be so. Because of difficulties in seed germination, it is probable that dissemination programmes will rely upon plants produced from root and stem cuttings, rather than on seedlings. This study evaluates differences in root system architecture of plants raised from seed (of four provenances), stem or root cuttings and the relationships between the competitivity index (CI) and crop yield. Cuttings were more shallowly rooting than seedlings, and had higher competitivity indices, and there was a negative relationship between CI and crop yield. No differences in root architecture between provenances were found. Therefore, to reduce tree-crop competition, the use of seedlings rather than cuttings should be recommended when promoting the use of this species on dryland farms. If cuttings are used to circumvent the problems of seed germination, alternative methods of controlling competition, such as root pruning, need to be considered

Biogeography of nodulated legumes and their nitrogen-fixing symbionts

In the last decade, analyses of both molecular and morphological characters, including nodulation, have led to major changes in our understanding of legume taxonomy. In parallel there has been an explosion in the number of genera and species of rhizobia known to nodulate legumes. No attempt has been made to link these two sets of data or to consider them in a biogeographical context. This review aims to do this by relating the data to the evolution of the two partners: it highlights both longitudinal and latitudinal trends and considers these in relation to the location of major land masses over geological time. Australia is identified as being a special case and latitudes north of the equator as being pivotal in the evolution of highly specialized systems in which the differentiated rhizobia effectively become ammonia factories. However, there are still many gaps to be filled before legume nodulation is sufficiently understood to be managed for the benefit of a world in which climate change is rife

Burkholderia sabiae sp nov., isolated from root nodules of Mimosa caesalpiniifolia

Two rhizobial strains, Br3407(T) and Br3405, were isolated from nitrogen-fixing nodules on the roots of Mimosa caesalpiniifolia, a legume tree native to Brazil. On the basis of 16S rRNA gene sequence similarities, both strains were shown previously to belong to the genus Burkholderia. A polyphasic approach, including DNA-DNA hybridizations, pulsed-field gel electrophoresis of whole-genome DNA profiles, whole-cell protein analyses, fatty acid methyl ester analysis and extensive biochemical characterization, was used to clarify the taxonomic position of these strains further; the strains are here classified within a novel species, for which the name Burkholderia sabiae sp. nov. is proposed. The type strain is strain Br3407(T) (=LMG 24235(T) =BCRC 17587(T))

Burkholderia nodosa sp nov., isolated from root nodules of the woody Brazilian legumes Mimosa bimucronata and Mimosa scabrella

Three strains, Br3437(T), Br3461 and Br3470, were isolated from nitrogen-fixing nodules on the roots of Mimosa scabrella (Br3437(T)) and Mimosa bimucronata (Br3461, Br3470), both of which are woody legumes native to Brazil. On the basis of 16S rRNA gene sequence similarities, all the strains were shown previously to belong to the genus Burkholderia. A polyphasic approach, including DNA-DNA hybridizations, PFGE of whole-genome DNA profiles, whole-cell protein analyses, fatty acid methyl ester analysis and extensive biochemical characterization, was used to clarify the taxonomic position of these strains further; the strains are here classified within a novel species, for which the name Burkholderia nodosa sp. nov. is proposed. The type strain, Br3437(T) (=LMG 23741(T) =BCRC 17575(T)), was isolated from nodules of M. scabrella

The innovation of the symbiosome has enhanced the evolutionary stability of nitrogen fixation in legumes

Nitrogen-fixing symbiosis is globally important in ecosystem functioning and agriculture, yet the evolutionary history of nodulation remains the focus of considerable debate. Recent evidence suggesting a single origin of nodulation followed by massive parallel evolutionary losses raises questions about why a few lineages in the N2-fixing clade retained nodulation and diversified as stable nodulators, while most did not. Within legumes, nodulation is restricted to the two most diverse subfamilies, Papilionoideae and Caesalpinioideae, which show stable retention of nodulation across their core clades. We characterize two nodule anatomy types across 128 species in 56 of the 152 genera of the legume subfamily Caesalpinioideae: fixation thread nodules (FTs), where nitrogen-fixing bacteroids are retained within the apoplast in modified infection threads, and symbiosomes, where rhizobia are symplastically internalized in the host cell cytoplasm within membrane-bound symbiosomes (SYMs). Using a robust phylogenomic tree based on 997 genes from 147 Caesalpinioideae genera, we show that losses of nodulation are more prevalent in lineages with FTs than those with SYMs. We propose that evolution of the symbiosome allows for a more intimate and enduring symbiosis through tighter compartmentalization of their rhizobial microsymbionts, resulting in greater evolutionary stability of nodulation across this species-rich pantropical legume clade

A Model for the Development of the Rhizobial and Arbuscular Mycorrhizal Symbioses in Legumes and Its Use to Understand the Roles of Ethylene in the Establishment of these two Symbioses

We propose a model depicting the development of nodulation and arbuscular mycorrhizae. Both processes are dissected into many steps, using Pisum sativum L. nodulation mutants as a guideline. For nodulation, we distinguish two main developmental programs, one epidermal and one cortical. Whereas Nod factors alone affect the cortical program, bacteria are required to trigger the epidermal events. We propose that the two programs of the rhizobial symbiosis evolved separately and that, over time, they came to function together. The distinction between these two programs does not exist for arbuscular mycorrhizae development despite events occurring in both root tissues. Mutations that affect both symbioses are restricted to the epidermal program. We propose here sites of action and potential roles for ethylene during the formation of the two symbioses with a specific hypothesis for nodule organogenesis. Assuming the epidermis does not make ethylene, the microsymbionts probably first encounter a regulatory level of ethylene at the epidermis–outermost cortical cell layer interface. Depending on the hormone concentrations there, infection will either progress or be blocked. In the former case, ethylene affects the cortex cytoskeleton, allowing reorganization that facilitates infection; in the latter case, ethylene acts on several enzymes that interfere with infection thread growth, causing it to abort. Throughout this review, the difficulty of generalizing the roles of ethylene is emphasized and numerous examples are given to demonstrate the diversity that exists in plants