Abnormal root and nodule vasculature in R50 (sym16), a pea nodulation mutant which accumulates cytokinins

R50 (sym16) is a pea nodulation mutant with fewer and shorter lateral roots (LR), fewer nodules and high levels of cytokinins (CK). Because a link exists between CK imbalance and abnormal vasculature, the vasculature of the primary root (PR) and LR of R50 was studied and it was compared with that of the wild-type 'Sparkle'. Also nodule vasculature was investigated to correlate R50 low nodulation phenotype with CK accumulation.

PR and first-order LR were hand-sectioned transversely in different locations and at different ages. Vascular poles were counted and root and stele diameters measured. To evaluate LR primordia number, roots were cleared. Nodules obtained from inoculated plants were either fixed and sectioned or cleared; numbers of vascular strands and of tracheary elements in the strands were counted.

'Sparkle' PR is triarch, whereas that of R50 can be triarch, tetrarch or pentarch. Furthermore, as the R50 roots developed, supernumerary vascular strands appeared but, as they aged, the new growth of more roots displayed the triarch pattern. LR vasculature differed from that of PR: whereas 'Sparkle' LR had three or four poles, those of R50 had two or three. No differences in PR or PR stele diameters existed between the two lines. Whereas 'Sparkle' nodules had two vascular strands, most R50 nodules possessed three; however, because R50 nodules were variable in size, their vasculature was highly diverse in terms of strand length. A strong correlation was found between nodule length and number of tracheary elements in strands.

R50 displays an additional number of vascular poles in its PR, a smaller number of vascular poles in its first-order LR and an altered vasculature in its nodules. It appears that these three characteristics are linked to the high levels of CKs that the mutant accumulates over its development.