Infection process and the interaction of rice roots with rhizobia

Most rhizobial strains inhibit rice root growth in the presence of calcium or potassium nitrates, but not ammonium nitrate. Certain rhizobial strains, however, such as strain R4, do not inhibit rice growth and can enter rice roots and multiply in the intercellular spaces. By using the green fluorescent protein (GFP) as a visual marker, it was found that Rhizobium became intimately associated with rice seedling roots within 24–48 h. During this initial period it was observed that strain R4 could cause structural changes resembling infection threads within the rice root hairs. Generally, the sites of the emerging lateral roots provide a temporary entry point for rhizobia, either by root hair entry or crack entry. All tested GFPlabelled Rhizobium strains infected the root hairs near the base of growing lateral roots. This study suggests that some strains may have the ability to infect rice root tissues via root hairs located at the emerging lateral roots and to spread extensively throughout the rice root.