Ethylene is involved in the nodulation phenotype of Pisum sativum R50 (sym16), a pleiotropic mutant that nodulates poorly and has pale green leaves

R50 is characterized as a pleiotropic pea mutant; it forms few nodules and has short lateral roots, short stature and pale leaves. Using grafting tech niques, R50 paleness was found to be controlled by the shoot of the mutant whereas the nodulation phenotype was regulated by its root. The paleness of R50 is due to a lower than normal total chlorophyll content in its young l eaves. The defect appears to be overcome with age because, as the plant mat ures, the chlorophyll levels increase in the older leaves. The reduction in stature is attributed to shorter internodes, and the oldest internodes are thicker than those of the parent Sparkle. Upon rhizobial inoculation, R50 forms as many infection threads as Sparkle. However, most of these are arre sted in the inner cortex. The threads appear to have lost their directional growth towards the stele, and they coil around within enlarged cortical ce lls. In addition, very few infection threads are associated with divisions of the inner cortical cells. These aborted nodule primordia are abnormal, f lat and mainly composed of cells which have divided anticlinally only. Nodu lation of R50 was restored by treating the roots with ethylene inhibitors. The R50 mutant further supports the postulated role of ethylene in regulati ng rhizobial infection with a probable role in the control of the primordiu m development.