African Species Belonging to the Tribe Psoraleeae Produce Ureides, a Symbiotic Trait Characteristic of the Phaseoleae
Modern studies in plant physiology have revealed the importance of nodulation and symbiotic traits in the taxonomy of legumes (Sprent, 2001). A recent phylogenetic tree showed a close relationship between the tribes Phaseoleae and Psoraleeae. This study aimed to assess the symbiotic traits of eight Psoralea species, which occur uniquely as endemics in the Cape region of South Africa and to compare them to Phaseoleae. Eight Psoralea species were collected from six sites, namely, P. pinnata, P. repens, P. aphylla, P. asarina, P. monophylla, P. aculeata, P. restioides and P. laxa. All nodules had a spherical shape resembling that of cowpea nodules (Phaseoleae). Nodule anatomy and ultrastructure of P. pinnata revealed an internal arrangement similar to that of Phaseoleae (Dakora and Atkins, 1989; Dakora 2000). Ureides were present in all plant organs of all eight species (assayed according to Dakora et al., 1992). The concentration of ureide was found to be significantly different (p > 0.5) at the species and organ levels. P. pinnata had the highest ureides concentration. This ureide production by all eight species supports their relatedness to the tribe Phaseoleae. Interestingly, ureide biosynthesis and the formation of round determinate nodules with an internal anatomy similar to cowpea and soybean nodules are unique symbiotic features of the tribe Phaseoleae (Dakora 2000). Based on these shared symbiotic traits of nodule morphology, nodule anatomy and ureide biogenesis, we propose that Psoralea species be re-classified in the tribe Phaseoleae.