Maize (Zea mays L.) leaves damaged by lepidopteran herbivores emit a complex volatile blend that can attract natural enemies of the herbivores and may also have roles in direct defense and inter- or intra-plant signaling. The volatile blend is dominated by sesquiterpenes of which the majority is produced by two herbivore-induced terpene synthases, TPS10 and TPS23. However, little is known about the pattern of volatile emission within maize leaves.

Translocator proteins (TSPO) are the products of a family of genes that is evolutionarily conserved from bacteria to humans and expressed in most mammalian tissues and cells. Human TSPO (18 kDa) is expressed at high levels in steroid synthesizing endocrine tissues where it localizes to mitochondria and functions in the first step of steroid formation, the transport of cholesterol into the mitochondria. TSPO expression is elevated in cancerous tissues and during tissue injury, which has lead to the hypothesis that TSPO has roles in apoptosis and the maintenance of mitochondrial integrity. We recently identified a new paralog of Tspo in both the human and mouse. This paralog arose from an ancient gene duplication event before the divergence of the classes aves and mammals, and appears to have specialized tissue-, cell-, and organelle-specific functions. Evidence from the study of TSPO homologs in mammals, bacteria, and plants supports the conclusion that the TSPO family of proteins regulates specialized functions related to oxygenmediated metabolism. In this review, we provide a comprehensive overview of the divergent function and evolutionary origin of Tspo genes in Bacteria, Archaea, and Eukarya domains.

A key element in the regulation of mammalian steroid biosynthesis is the 18 kDa peripheral-type benzodiazepine receptor (PBR), which mediates mitochondrial cholesterol import. PBR also possess an affinity to the tetrapyrrole metabolite protoporphyrin. The bacterial homolog to the mammalian PBR, the Rhodobacter TspO (CrtK) protein, was shown to be involved in the bacterial tetrapyrrole metabolism. Looking for a similar mitochondrial import mechanism in plants, protein sequences from Arabidopsis and several other plants were found with significant similarities to the mammalian PBR and to the Rhodobacter TspO protein. A PBR-homologous Arabidopsis sequence was cloned and expressed in E. coli. The recombinant gene product showed specific high affinity benzodiazepine ligand binding. Moreover, the protein applied to E. coli protoplasts caused an equal benzodiazepine-stimulated uptake of cholesterol and protoporphyrin IX. These results suggest that the PBR like protein is involved in steroid import and is directing protoporphyrinogen IX to the mitochondrial site of protoheme formation.