TRANSLOCATABLE LEAF SIGNAL AUTOREGULATES SOYBEAN NODULATION

In soybean (Glvcine max (L.) Merr.), expression of the hypernodulation phenotype appears to be controlled by the shoot. The current study attempted to localize the source of the nodulation signal in soybean cv. Williams 82 and its hypernodulating mutant NOD1-3, Wedge grafts (replacement of entire shoot) and approach grafts (addition of a shoot) were utilized and the shoot (scion) treatments ranged front complete shoots to various combinations of leaf, cotyledon, and apex removal from the grafted scion. Leaf-bearing wedge-grafted or approach-grafted scion types dictated the nodulation phenotype of the host plant. Scions with more leaves were clearly more effective in altering nodulation. Scions with leaves and without apices were as effective in altering nodulation as were scions with both leaves and apices. This confirmed the report by Delves et al. (Plant Cell Environ., 15 (1992) 249-254), but in neither case are results definitive as new buds continued to form which may provide stimulus for nodulation control. Leaf and shoot cuttings which were stimulated to form roots were also examined for expression of nodulation. Leaf cuttings, which were devoid of any meristematic apices, exhibited nodulation phenotypes similar to that of the shoot cuttings, i.e. leaf and shoot cuttings of Williams 82 were normally nodulated while those of NOD1-3 were hypernodulated. The leafcutting approach definitively showed that the leaf, and not the apex, was the synthesis site of a translocatable signal controlling autoregulation of nodule number.