Hemoglobins of Lotus japonicus: differential gene expression in plant organs and in response to hormones in nodules

Abstract

In plants, three distinct types of hemoglobins (Hbs) are known: symbiotic, nonsymbiotic, and truncated. Nonsymbiotic Hbs are further divided into class-1 and class-2 based on primary sequences and oxygen-binding properties. Symbiotic Hbs are well-characterized, whereas the functions of the other two types of Hbs are not fully understood. We have identified five genes encoding two class-1 (GLB1-1, GLB1-2), one class-2 (GLB2), and two truncated (GLB3-1, GLB3-2) Hbs in Lotus japonicus. GLB1-2 and GLB3-2 are expressed in all plant organs, whereas GLB1-1, GLB2, and GLB3-1 are expressed abundantly only in nodules. We have assessed also the response of GLB genes in nodules to hormone treatments. GLB1-1 was up-regulated by abscisic acid (ABA), 1-aminocyclopropane-1-carboxylic acid (ACC), and polyamines (PAs), whereas GLB2 and GLB3-1 were drastically down-regulated by cytokinins (CKs). In situ RNA hybridization showed that the GLB1-1 and GLB1-2 mRNAs are mainly localized in the cortex, vascular bundles, and fixation zone, whereas GLB2 and GLB3 mRNAs are mainly confined to the cortex and slightly to the vascular bundles. GLB3-2 promoter activity was localized by GUS staining in the vascular and endodermis/sclerenchyma cells in the cortex of mature nodules.