Comparaison du mode d'infection et du développement des nodosités racinaires et caulinaires chez quelques légumineuses des genres Sesbania et Aeschynomene

Sesbania rostrata and fifteen species of Aeschynomene are tropical legumes which can grow in flooded soils because they have both root and stem nodulation. They are of particular agronomic interest because of their potential for maintaining soil fertility. Three genera of rhizobia are associated symbiotically with these stem nodulating legumes: Rhizobium infects S. rostrata and some Aeschynomene species; Azorhizobium is specific to S. rostrata and can live outside the plant; Bradyrhizobium infects only Aeschynomene species. In S. rostrata root infection occurs by root hair curling, whereas stem nodulation is achieved by entry of bacteria at the site of emerging adventitious rootlets. Many studies have reported that infection by thread formation is observed on both roots and stems of S. rostrata. In S. rostrata , the process of root infection has been shown to follow several stages: recognition and attachment of rhizobia to the root surfaces induction of root hair curling; formation of infection threads that guide bacteria to the adjacent cortical root cells; release of bacteria into the cytoplasm of host cells where they are enveloped by host plasmalemm. Stem infection of S. rostrata has been shown to have four stages: rhizobial penetration in degenerated cortical cells through cracks at the site of emerging rootlets; Rhizobium proliferates in the intercellular cavities and initiates meristematic nodules; infection threads form in the intercellular cavities and in the host cells; rhizobia are intercellularly released by an endocytotic process. Most of the stem nodulated Aeschynomene spp. have displayed the same process of root infection as S. rostrata. A.indica is an exception in that it appears that the path of infection is constituted by the rupture of tissue restricted to the place of rootlet emergence. No infection thread formation is observed in A. indica. The unique characteristic of all stem‐nodulating legumes is the presence of pre‐determined nodulation sites on their stems. In Aeschynomene , the process of stem infection does not involve formation of infection threads but is effected by penetration of Rhizobinum through cracks which surround the root primordia. This penetration is followed by hydrolysis of host epidermic cell walls causing the release of the bacteria into cell cytoplasm. Stem‐nodulating legumes infection depends on both ecological and intrinsic legumes factors. It is important to note that the nitrogen fixation of S. rostrata. A. afraspera , and A. scabra was not inhibited by mineral nitrogen. The role of leaghemoglobin in the regulation and protection of nitrogenase activity is well established. Recently, it was suggested that superoxide dismutase. which is present at high levels in bacteroides isolated from root and stem nodules of a few stem‐nodulating legumes, would be another regulator and protector of the process of nitrogen fixation.