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Although the biochemical pathways for fatty acid synthesis are more or less similar in plants and animals (Harwood, 1988), there is a major cell biological difference between these two groups of eukaryotes. In plants, the major site of fatty acid synthesis is the plastid, an organelle absent from the animal cell. Many aspects of plastid biology, including fatty acid synthesis, reflect the organelle's origins as a prokaryotic symbiont. The synthesis of fatty acids, such as palmitic acid, the prototype 16-carbon fatty acid, requires one molecule of acetyl-COA and seven molecules of malonyl-COA, which are added sequentially with the addition of two carbons to the growing fatty acid chain and the release of CO, at each step. These reactions are catalyzed by fatty acid synthase, an enzyme complex known to exist in a prokaryotic and a eukaryotic form (Wakil et al., 1983;Harwood, 1988). The prokaryotic form (type 11) of fatty acid synthase is found in plants. The synthase is composed of severa1 dissociable proteins, whereas the eukaryotic form (type I) found in animals and yeasts is composed of one or two large multifunctional, nondissociable proteins. For either form, the synthesis requires malonyl-COA, which is supplied by ACCase in the following reaction:In plant cells, large amounts of malonyl-COA are needed in the plastids to sustain fatty acid synthesis, but malonyl-COA is also needed in the cytosol for the elongation of fatty acids exported from the plastids and for the synthesis of flavonoids and phytoalexins. As with fatty acid synthase, ACCase also occurs in prokaryotic and eukaryotic forms in nature. The prokaryotic form is composed of dissociable polypeptides, whereas the eukaryotic form is a homodimer of a multifunctional protein. But which form(s) of this enzyme occur(s) in plants? Both or only one? This biochemical mystery, which has been around since 1972, has finally been solved and the answer is intriguing, both from a ' Present address:

Plants induce immune responses against fungal pathogens by recognition of chitin, which is a component of the fungal cell wall. Recent studies have revealed that LysM receptor-like kinase 1/chitin elicitor receptor kinase 1 (LysM RLK1/CERK1) is a critical component for the immune responses to chitin in Arabidopsis thaliana. However, the molecular mechanism of the chitin recognition by LysM RLK1 still remains unknown. Here, we present the first evidence for direct binding of LysM RLK1 to chitin. We expressed LysM RLK1 fused with yeast-enhanced green fluorescent protein (LysM RLK1-yEGFP) in yeast cells. Binding studies using the solubilized LysM RLK1-yEGFP and several insoluble polysaccharides having similar structures showed that LysM RLK1-yEGFP specifically binds to chitin. Subsequently, the fluorescence microscopic observation of the solubilized LysM RLK1-yEGFP binding to chitin beads revealed that the binding was saturable and had a high affinity, with a K d of ϳ82 nM. This binding was competed by the addition of soluble glycol chitin or high concentration of chitin oligosaccharides having 4 -8 residues of N-acetyl glucosamine. However, the competition of these chitin oligosaccharides is weaker than that of glycol chitin. These data suggest that LysM RLK1 has a higher affinity for chitin having a longer residue of N-acetyl glucosamine. We also found that LysM RLK1-yEGFP was autophosphorylated in vitro and that chitin does not affect the phosphorylation of LysM RLK1-yEGFP. Our results provide a new dimension to chitin elicitor perception in plants.When a plant is attacked by pathogens, it rapidly induces immune responses to stop the infection. A key step of the rapid induction of immune responses is a prompt and efficient detection of microbial invaders. In plants, this is achieved by pattern recognition receptors that recognize the conserved structures of the microbial pathogens. These conserved microbial structures are called pathogen-associated molecular patterns.The pathogen-associated molecular patterns recognized by plants include lipopolysaccharides, peptidoglycan (PGN), 3 flagellin, and bacterial elongation factor-Tu, which are derived from the bacteria (1, 2). Plants also recognize fungal pathogen-associated molecular patterns, such as chitin and ergosterol, which are components of the fungal cell wall and plasma membrane, respectively (1, 3). Recent studies have discovered the plant pattern recognition receptors participating in the perception of chitin elicitor (4 -6).A LysM motif-containing plasma membrane protein, CEBiP (chitin oligosaccharide elicitor-binding protein), participates in the perception of chitin oligosaccharides in rice (4). The CEBiP has two LysM motifs in the extracellular domain and lacks the intracellular kinase domain that is required for signal transduction. It has been demonstrated that CEBiP directly binds to chitin oligosaccharides and plays an essential role in the perception of chitin and the induction of immunity in rice (4).On the other hand, more recently, two groups ha...

Acetyl-CoA carboxylase (ACCase) in plastids is a key enzyme regulating the rate of de novo fatty acid biosynthesis in plants. Plastidic ACCase is composed of three nuclear-encoded subunits and one plastid-encoded accD subunit. To boost ACCase levels, we examined whether overexpression of accD elevates ACCase production. Using homologous recombination, we replaced the promoter of the accD operon in the tobacco plastid genome with a plastid rRNA-operon (rrn) promoter that directs enhanced expression in photosynthetic and non-photosynthetic organs, and successfully raised the total ACCase levels in plastids. This result suggests that the level of the accD subunit is a determinant of ACCase levels, and that enzyme levels are in part controlled post-transcriptionally at the level of subunit assembly. The resultant transformants grew normally and the fatty acid content was significantly increased in leaves, but not significantly in seeds. However, the transformants displayed extended leaf longevity and a twofold increase of seed yield over the control value, which eventually almost doubled the fatty acid production per plant of the transformants relative to control and wild-type plants. These findings offer a potential method for raising plant productivity and oil production.

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The genus Citrus includes mandarin, orange, lemon, grapefruit and lime, which have high economic and nutritional value. The family Rutaceae can be divided into 7 subfamilies, including Aurantioideae. The genus Citrus belongs to the subfamily Aurantioideae. In this study, we sequenced the chloroplast matK genes of 135 accessions from 22 genera of Aurantioideae and analyzed them phylogenetically. Our study includes many accessions that have not been examined in other studies. The subfamily Aurantioideae has been classified into 2 tribes, Clauseneae and Citreae, and our current molecular analysis clearly discriminate Citreae from Clauseneae by using only 1 chloroplast DNA sequence. Our study confirms previous observations on the molecular phylogeny of Aurantioideae in many aspects. However, we have provided novel information on these genetic relationships. For example, inconsistent with the previous observation, and consistent with our preliminary study using the chloroplast rbcL genes, our analysis showed that Feroniella oblata is not nested in Citrus species and is closely related with Feronia limonia. Furthermore, we have shown that Murraya paniculata is similar to Merrillia caloxylon and is dissimilar to Murraya koenigii. We found that “true citrus fruit trees” could be divided into 2 subclusters. One subcluster included Citrus, Fortunella, and Poncirus, while the other cluster included Microcitrus and Eremocitrus. Compared to previous studies, our current study is the most extensive phylogenetic study of Citrus species since it includes 93 accessions. The results indicate that Citrus species can be classified into 3 clusters: a citron cluster, a pummelo cluster, and a mandarin cluster. Although most mandarin accessions belonged to the mandarin cluster, we found some exceptions. We also obtained the information on the genetic background of various species of acid citrus grown in Japan. Because the genus Citrus contains many important accessions, we have comprehensively discussed the classification of this genus.

RNA editing is an important post-transcriptional process in chloroplasts and is thought to be functionally significant. Here we show a requirement of RNA editing for a functional enzyme. In peas, acetyl-CoA carboxylase (ACCase), a key enzyme of fatty acid synthesis, is composed of biotin carboxylase with the biotin carboxyl carrier protein and carboxyltransferase (CT). CT is composed of the nuclear-encoded ␣ polypeptide and the chloroplast-encoded ␤ polypeptide in peas. One nucleotide of the ␤ polypeptide mRNA, which is edited in pea chloroplasts, converts the serine codon to the leucine codon. We show that this RNA editing is required for functional CT by comparing the unedited and edited recombinant enzymes. In plants not having a leucine codon at the same position, editing was shown to take place so as to create the leucine codon, indicating that editing is needed for in vivo CT activity and therefore for ACCase. To our knowledge, ACCase is an essential enzyme, suggesting that the chloroplast RNA editing is necessary for these plants.

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