Histochemical evidence for the presence of a suberinlike compound in Rhizobium-induced nodules of the nonlegume Parasponia rigida

Abstract

Parasponia rigida (Ulmaceae) nodules, like nodules of pea and alfalfa, are elongate and cylindrical and of indeterminate morphology. However, unlike other indeterminate nodules, the Rhizobium (Rp501) endosymbiont is not released into the host cell cytoplasm and remains confined to threads. Two types of threads are found: invasive infection threads that traverse the nodule tissue intracellularly and intercellularly; and threads confined to the nodule cell, proliferating intracellularly, which are the presumed sites of nitrogen fixation. The thread walls contain pectic and cellulosic components as evidenced by periodic acid – Schiff’s staining and digestion with pectinase and cellulase. Using histochemical analyses coupled with enzymatic and chemical extraction, we determined that the walls of the intracellularly confined threads also contain a suberinlike compound. The thread walls stain positively with Sudan black B and other lipid stains. In contrast, the invasive infection threads appear to be lignified as well as suberized.