Elhiti, Mohamed2; Hebelstrup, Kim5; Wang, Aiming3; Li, Chenlong3; Cui, Yuhai3; Hill, Robert D4; Stasolla, Claudio4
1 Department of Molecular Biology and Genetics - Afgrødegenetik og Bioteknologi, Department of Molecular Biology and Genetics, Science and Technology, Aarhus University2 Department of Botany, Faculty of Science, Tanta University3 Southern Crop Protection and Food Research Centre, Agriculture and Agri-Food Canada4 Department of Plant Science, University of Manitoba5 Department of Molecular Biology and Genetics - Afgrødegenetik og Bioteknologi, Department of Molecular Biology and Genetics, Science and Technology, Aarhus University
Suppression of the Arabidopsis GLB2, a type-2 nonsymbiotic hemoglobin, enhances somatic embryogenesis by increasing auxin production. In the glb2 knock-out line (GLB2 -/-) polarization of PIN1 proteins and auxin maxima occurred at the base of the cotyledons of the zygotic explants, which are the sites of embryogenic tissue formation. These changes were also accompanied by a transcriptional up-regulation of WUSCHEL (WUS) and SOMATIC EMBRYOGENESIS RECEPTOR KINASE (SERK1), markers of embryogenic competence. The increased auxin levels in the GLB2 -/- line were ascribed to the induction of several key enzymes of the tryptophan and IAA biosynthetic pathways, including ANTHRANILATE SYNTHASE (α subunit) (ASA1), CYTOCHROME P79B2 (CYP79B2) and AMIDASE1 (AMI1). The effects of GLB2 suppression on somatic embryogenesis and IAA synthesis are mediated by increasing levels of nitric oxide (NO) within the embryogenic cells, which repress the expression of the transcription factor MYC2, a well characterized repressor of the auxin biosynthetic pathway. A model is proposed in which the suppression of GLB2 reduces the degree of NO scavenging by oxyhemoglobin, thereby increasing the cellular NO concentration. The increased levels of NO repress the expression of MYC2, relieving the inhibition of IAA synthesis and increasing cellular IAA, which is the inductive signal promoting embryogenic competence. Besides providing a model for the induction phase of embryogenesis in vitro, these studies propose previously undescribed functions for plant hemoglobins.