Trehalose Biosynthesis Gene Protects against Stress in the Initial Infection Stage of -Bean Bug Symbiosis.

Trehalose, a nonreducing disaccharide, functions as a stress protectant in many organisms, including bacteria. In symbioses involving bacteria, the bacteria have to overcome various stressors to associate with their hosts; thus, trehalose biosynthesis may be important for symbiotic bacteria. Here, we investigated the role of trehalose biosynthesis in the -bean bug symbiosis. Expression levels of two trehalose biosynthesis genes, and , were elevated in symbiotic cells, and hence mutant Δ and Δ strains were generated to examine the functions of these genes in symbiosis. An competition assay with the wild-type strain revealed that fewer Δ cells, but not Δ cells, colonized the host symbiotic organ, the M4 midgut, than wild-type cells. The Δ strain was susceptible to osmotic pressure generated by high salt or high sucrose concentrations, suggesting that the reduced symbiotic competitiveness of the Δ strain was due to the loss of stress resistance. We further demonstrated that fewer Δ cells infected the M4 midgut initially but that fifth-instar nymphs exhibited similar symbiont population size as the wild-type strain. Together, these results demonstrated that the stress resistance role of is important for to overcome the stresses it encounters during passage through the midgut regions to M4 in the initial infection stage but plays no role in resistance to stresses inside the M4 midgut in the persistent stage. Symbiotic bacteria have to overcome stressful conditions present in association with the host. In the -bean bug symbiosis, we speculated that a stress-resistant function of is important and that trehalose, known as a stress protectant, plays a role in the symbiotic association. Using , the trehalose biosynthesis gene, and a mutant strain, we demonstrated that confers with competitiveness when establishing a symbiotic association with bean bugs, especially playing a role in initial infection stage. assays revealed that provides the resistance against osmotic stresses. Hemipteran insects, including bean bugs, feed on plant phloem sap, which may lead to high osmotic pressures in the midguts of hemipterans. Our results indicated that the stress-resistant role of is important for to overcome the osmotic stresses present during the passage through midgut regions to reach the symbiotic organ.