Genome Sequence of Striga asiatica Provides Insight into the Evolution of Plant Parasitism

Parasitic plants in the genus Striga, commonly known as witchweeds, cause major crop losses in sub-Saharan Africa and pose a threat to agriculture worldwide. An understanding of Striga parasite biology, which could lead to agricultural solutions, has been hampered by the lack of genome information. Here, we report the draft genome sequence of Striga asiatica with 34,577 predicted protein-coding genes, which reflects gene family contractions and expansions that are consistent with a three-phase model of parasitic plant genome evolution. Striga seeds germinate in response to host-derived strigolactones (SLs) and then develop a specialized penetration structure, the haustorium, to invade the host root. A family of SL receptors has undergone a striking expansion, suggesting a molecular basis for the evolution of broad host range among Striga spp. We found that genes involved in lateral root development in non-parasitic model species are coordinately induced during haustorium development in Striga, suggesting a pathway that was partly co-opted during the evolution of the haustorium. In addition, we found evidence for horizontal transfer of host genes as well as retrotransposons, indicating gene flow to S. asiatica from hosts. Our results provide valuable insights into the evolution of parasitism and a key resource for the future development of Striga control strategies.Peer reviewe

Gene Expression Profiling of the Tetrapyrrole Metabolic Pathway in Arabidopsis with a Mini-Array System

Tetrapyrrole compounds, such as chlorophylls, hemes, and phycobilins, are synthesized in many enzymatic steps. For regulation of the tetrapyrrole metabolic pathway, it is generally considered that several specific isoforms catalyzing particular enzymatic steps control the flow of tetrapyrrole intermediates by differential regulation of gene expression depending on environmental and developmental factors. However, the coordination of such regulatory steps and orchestration of the overall tetrapyrrole metabolic pathway are still poorly understood. In this study, we developed an original mini-array system, which enables the expression profiling of each gene involved in tetrapyrrole biosynthesis simultaneously with high sensitivity. With this system, we performed a transcriptome analysis of Arabidopsis seedlings in terms of the onset of greening, endogenous rhythm, and developmental control. Data presented here clearly showed that based on their expression profiles at the onset of greening, genes involved in tetrapyrrole biosynthesis can be classified into four categories, in which genes are coordinately regulated to control the biosynthesis. Moreover, genes in the same group were similarly controlled in an endogenous rhythmic manner but also by a developmental program. The physiological significance of these gene clusters is discussed

Induction of Isoforms of Tetrapyrrole Biosynthetic Enzymes, AtHEMA2 and AtFC1, under Stress Conditions and Their Physiological Functions in Arabidopsis12[W][OA]

In the tetrapyrrole biosynthetic pathway, isoforms of glutamyl-tRNA reductase (HEMA2) and ferrochelatase1 (FC1) are mainly expressed in nonphotosynthetic tissues. Here, using promoter-β-glucuronidase constructs, we showed that the expressions of Arabidopsis (Arabidopsis thaliana) HEMA2 (AtHEMA2) and FC1 (AtFC1) were induced in photosynthetic tissues by oxidative stresses such as wounding. Transcript levels and β-glucronidase activity were rapidly induced within 30 min, specifically in the wound area in a jasmonate-independent manner. Transcriptome analysis of wound-specific early inducible genes showed that AtHEMA2 and AtFC1 were coinduced with hemoproteins outside plastids, which are related to defense responses. Ozone fumigation or reagents generating reactive oxygen species induced the expression of both genes in photosynthetic tissues, suggesting that reactive oxygen species is involved in the induction. Since cycloheximide or puromycin induced the expression of both genes, inhibition of cytosolic protein synthesis is involved in the induction of these genes in photosynthetic tissues. The physiological functions of AtHEMA2 and AtFC1 were investigated using insertional knockout mutants of each gene. Heme contents of the roots of both mutants were about half of that of the respective wild types. In wild-type plants, heme contents were increased by ozone exposure. In both mutants, reduction of the ozone-induced increase in heme content was observed. These results suggest the existence of the tetrapyrrole biosynthetic pathway controlled by AtHEMA2 and AtFC1, which normally functions for heme biosynthesis in nonphotosynthetic tissues, but is induced in photosynthetic tissues under oxidative conditions to supply heme for defensive hemoproteins outside plastids