Sesbania rostrata is a flooding-tolerant tropical legume with nodulation at lateral root bases and at the position of stem-located adventitious rootlets. One of the first visible features of Azorhizobium caulinodans-induced nodule initiation on S. rostrata is the formation of infection pockets. Infection pockets are big intercellular spaces occupied with proliferating bacteria, and are formed during both stem and root nodule formation. Infection pocket formation is accompanied by a phenomenon of local plant cell death. It was demonstrated that Nod factors purified from the wild-type strain complement ORS571-V44, abolished in the synthesis of Nod factors, to form infection pockets in the adventitious rootlets on S. rostrata stems. These observations suggested that Nod factors are required for infection pocket formation, since ORS571-V44 alone is unable to infect the outer cortical tissue (D'Haeze et al. 1998). This peculiar plant cell death-accompanied infection pocket formation and the fact that Nod factors are required for infection pocket formation, and thus may play a role in the induction of a plant cell to die, tempted us to further investigate this topic. Nod factor-induced root hair formation and nodulation were used as biological assays in a pharmacological and morphological approach, including the in situ localization of hydrogen peroxide.

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