Molecular Characterization of LjABCG1, an ATP-Binding Cassette Protein in Lotus japonicus.

LjABCG1, a full-size ABCG subfamily of ATP-binding cassette proteins of a model legume, Lotus japonicus, was reported as a gene highly expressed during the early stages of nodulation, but have not been characterized in detail. In this study we showed that the induction of LjABCG1 expression was remarkable by methyl jasmonate treatment, and reporter gene experiments indicated that LjABCG1 was strongly expressed in the nodule parenchyma and cell layers adjacent to the root vascular tissue toward the nodule. LjABCG1 was suggested to be localized at the plasma membrane based on the fractionation of microsomal membranes as well as separation via aqueous two-phase partitioning. The physiological functions of LjABCG1 in symbiosis and pathogenesis were analyzed in homologous and heterologous systems. LjABCG1 knock-down L. japonicus plants did not show clear phenotypic differences in nodule formation, and not in defense against Pseudomonas syringae, either. In contrast, when LjABCG1 was expressed in the Arabidopsis pdr8-1 mutant, the penetration frequency of Phytophthora infestans, a potato late blight pathogen, was significantly reduced in LjABCG1/pdr8-1 than in pdr8-1 plants. This finding indicated that LjABCG1, at least partially, complemented the phenotype of pdr8 in Arabidopsis, suggesting the multiple roles of this protein in plant-microbe interactions

Expression profile of <i>LjABCG1</i> in <i>L</i>. <i>japonicus</i>.

<p>(A) Expression of <i>LjABCG</i>1 in various tissues. Leaves, stems and inoculated roots were obtained from plants inoculated with <i>M</i>. <i>loti</i>. (B) Response of <i>LjABCG1</i> expression to various treatments. Two-week-old seedlings were treated for 24 h with 10 μM indole-3-acetic acid (IAA), 10 μM 1-naphthaleneacetic acid (NAA), 10 μM abscisic acid (ABA), 10 μM gibberellic acid (GA₃), 100 μM salicylic acid (SA), 100 μM methyl jasmonate (JA), or 100 μM sclareol (Scl). There chemicals were applied into the medium at the final concentration described above. Significant difference was based on one-way ANOVA with Tukey's HSD test. Asterisks indicate significant differences from control (p<0.05). (C) Time course of expression of <i>LjABCG1</i> after treatment with 100 μM MeJA. (D) Response of <i>LjABCG1</i> expression to MeJA applied to shoots. A cotton ball containing MeJA was placed onto the wall of an air-tight plant box. After 24 hours, shoots and roots were frozen in liquid nitrogen until RNA preparation. Data represent the means ± S.D. of three replicates.</p