Function of type–2 A rabidopsis hemoglobin in the auxin‐mediated formation of embryogenic cells during morphogenesis

Summary Suppression of A rabidopsis GLB 2 , a type–2 nonsymbiotic hemoglobin, enhances somatic embryogenesis by increasing auxin production. In the glb2 knock‐out line ( GLB 2 − / − ), polarization of PIN 1 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 upregulation of WUSCHEL ( WUS ) and SOMATIC EMBRYOGENESIS RECEPTOR KINASE ( SERK 1 ), which are markers of embryogenic competence. The increased auxin levels in the GLB 2 − / − line were ascribed to the induction of several key enzymes of the tryptophan and IAA biosynthetic pathways, including ANTHRANILATE SYNTHASE (α subunit; ASA 1 ), CYTOCHROME P 79 B 2 ( CYP 79 B 2 ) and AMIDASE 1 ( AMI 1 ). The effects of GLB 2 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 MYC 2, a well‐characterized repressor of the auxin biosynthetic pathway. A model is proposed in which the suppression of GLB 2 reduces the degree of NO scavenging by oxyhemoglobin, thereby increasing the cellular NO concentration. The increased levels of NO repress the expression of MYC 2 , 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.