The radiation of nodulated Chamaecrista species from the rainforest into more diverse habitats has been accompanied by a reduction in growth form and a shift from fixation threads to symbiosomes

• Published in: Journal of experimental botany Vol. 75; no. 11; pp. 3643 - 3662
• Main Authors: Casaes, Patricia Alves, Ferreira dos Santos, José Miguel, Silva, Verônica Cordeiro, Rhem, Mariana Ferreira Kruschewsky, Teixeira Cota, Matheus Martins, de Faria, Sergio Miana, Rando, Juliana Gastaldello, James, Euan K, Gross, Eduardo
• Format: Journal Article
• Language: English
• Published: UK Oxford University Press 26-03-2024
• Subjects: nodulation; fixation thread (FT); cellulose; homogalacturonan (HG); Paraburkholderia; Caesalpinioideae; nodC; Chamaecrista; nifH; Bradyrhizobium
• ISSN: 0022-0957; 1460-2431
• DOI: 10.1093/jxb/erae134

Abstract
All non-Mimosoid nodulated genera in the legume subfamily Caesalpinioideae confine their rhizobial symbionts within cell wall-bound ‘fixation threads’ (FTs). The exception is the large genus Chamaecrista in which shrubs and subshrubs house their rhizobial bacteroids more intimately within symbiosomes, whereas large trees have FTs. This study aimed to unravel the evolutionary relationships between Chamaecrista growth habit, habitat, nodule bacteroid type, and rhizobial genotype. The growth habit, bacteroid anatomy, and rhizobial symbionts of 30 nodulated Chamaecrista species native to different biomes in the Brazilian state of Bahia, a major centre of diversity for the genus, was plotted onto an ITS-trnL-F-derived phylogeny of Chamaecrista. The bacteroids from most of the Chamaecrista species examined were enclosed in symbiosomes (SYM-type nodules), but those in arborescent species in the section Apoucouita, at the base of the genus, were enclosed in cell wall material containing homogalacturonan (HG) and cellulose (FT-type nodules). Most symbionts were Bradyrhizobium genotypes grouped according to the growth habits of their hosts, but the tree, C. eitenorum, was nodulated by Paraburkholderia. Chamaecrista has a range of growth habits that allow it to occupy several different biomes and to co-evolve with a wide range of (mainly) bradyrhizobial symbionts. FTs represent a less intimate symbiosis linked with nodulation losses, so the evolution of SYM-type nodules by most Chamaecrista species may have (i) aided the genus-wide retention of nodulation, and (ii) assisted in its rapid speciation and radiation out of the rainforest into more diverse and challenging habitats.
Chamaecrista has shrunk from trees to shrubs as it has radiated from the rainforest into drier habitats; this has been accompanied by the adoption of more intimately efficient nodulating symbioses.