Pathways of assimilation and transfer of fixed nitrogen in coralloid roots of cycad-Nostoc symbioses

Abstract

Freshly detached coralloid roots of several cycad species were found to bleed spontaneously from xylem, permitting identification of products of nitrogen transfer from symbiotic organ to host. Structural features relevant to the export of fixed N were described for Macrozamia riedlei (Fisch. ex Gaud.) Gardn. the principal species studied. Citrulline (Cit), glutamine (Gln) and glutamic acid (Glu), the latter usually in a lesser amount, were the principal translocated solutes in Macrozamia (5 spp.), Encephalartos (4 spp.) and Lepidozamia (1 sp.), while Gln and a smaller amount of Glu, but no Cit were present in xylem sap of Bowenia (1 sp.),and Cycas (2 spp.). Time-course studies of ¹⁵N enrichment of the different tissue zones and the xylem sap of ¹⁵N₂-pulse-fed coralloid roots of M. riedlei showed earlier ¹⁵N incorporation into Gln than into Cit, and a subsequent net decline in the ¹⁵N of Gln of the coralloid-root tissues, whereas Cit labeling continued to increase in inner cortex and stele and in the xylem sap. Hydrolysis of the ¹⁵N-labeled Cit and Gln consistently demonstrated much more intense labeling of the respective carbamyl and amide groups than of the other N-atoms. Coralloid roots of M. riedlei pulse-fed ¹⁴CO₂ in darkness showed ¹⁴C labeling of aspartic acid (Asp) and Cit in all tissue zones and of Cit of xylem bleeding sap. Lateral roots and uninfected apogeotropic roots of M. riedlei and M. moorei also incorporated ¹⁴CO₂ into Cit. The ¹⁴C of Cit was restricted to the carbamyl-C. Comparable ¹⁵N₂ and CO₂-feeding studies on corallid roots of Cycas revoluta showed Gln to be the dominant product of N₂ fixation, with Asp and alanine as other major ¹⁴C-labeled amino compounds, but a total absence of Cit in labeled or unlabeled form.