Regulation of Plant Growth & Development

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Plants of the genera Striga and Orobanche are noxious root parasitic weeds on many food crops. Their life cycle is strongly cued to that of their hosts. To germinate the parasites need a moist pre-treatment (conditioning) at specified temperatures followed by exposure to an exogenous stimulant. On perception of a second signal the parasites produce haustoria that are the bridge for transfer of water, minerals and organic compounds from the hosts. I have been studying the interactions between the parasites and their hosts. Based on proposed structure of sorgolactone, the main germination stimulant isolated from sorghum root exudates, all eight stereoisomers were synthesized. Only those isomers possessing the same stereochemistry as the natural sorgolactone at two adjacent chiral centers exhibited significant germination stimulatory activity. The involvement of endogenous ethylene in germination of S. hermonthica was confirmed. Expression of ACC synthase and ACC oxidase genes was found to be responsive to germination stimulants and to conditioning, respectively. Lotus japonicus, a model legume for nodulation and mycorrhizal colonization, was compatible with O. aegyptiaca but not with O. minor, S. hermonthica and S. gesnerioides. O. aegyptiaca and S. hermonthica parasitism specifically induced the expression of genes involved in nodulation and phytoalexin biosynthesis, respectively. Reactions of the model legume and its mutants to O. aegyptiaca suggest that nodulation, mycorrhizal colonization and plant parasitism are modulated by similar mechanisms.

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Cucumber seedlings develop a protuberance called peg on the transition zone between the hypocotyl and the root. The lateral placement of a peg occurs because peg formation on upper side of the horizontally placed transition zone is suppressed in response to gravity. Reduction of auxin level on the upper side of the transition zone is responsible for the suppression of peg formation. We isolated six cDNAs encoding auxin efflux facilitators (CsPINs) in cucumber, and showed that CsPIN1 and CsPIN6 could control auxin dynamics for the gravimorphogenesis. Of auxin response factors (CsARFs) we isolated, CsARF2 and CsARF5 appeared to function on the auxin-mediated transcription as positive and negative regulators, respectively. CsIAA1 and CsIAA2 belonging to Aux/IAA family appeared to be negative regulators for the transcription. Furthermore, an auxin-inducible gene, CS-ACS1, was differentially expressed in the gravistimulated transition zone, which could cause an increase in ethylene production to promote peg development on the lower side. In addition, we found that mRNA of glycine-rich protein (CsGRP1) accumulated more abundantly on the upper side than the lower side of the transition zone. CsGRP1 gene was auxin-repressive, and the transition zone responded to gravistimulation in a specific manner by an asymmetric expression of CsGRP1 gene. Based on the results of these studies, we hypothesized the mechanism underlying the negative control of peg formation by gravity.

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Isoprenoids comprise a broad range of natural products that are synthesized by the condensation of isopentenyl diphosphate and dimethylallyl diphosphate. Plants have two distinct routes for the synthesis of these common isoprenoid precursors, the methylerythritol phosphate (MEP) pathway in plastids and the mevalonate (MVA) pathway in the cytosol. We have extensively studied the role of MEP and MVA pathways in the biosynthesis of gibberellins (GAs) and cytokinins (CKs) in plants. To determine the contribution of MEP and MVA pathways to hormone biosynthesis, we performed tracer experiments in which each metabolite from the MEP or MVA pathways are selectively and efficiently ^<13>C-labeled in Arabidopsis. We found that GAs are mainly produced from the MEP pathway in plastids, but also partially from the MVA pathway in the cytosol. We also showed that a large fraction of prenyl side chains of isopentenyladenine-type and trans-zeatin-type CKs are mainly produced from the MEP pathway, while those of cis-zeatin type CKs are predominantly derived from the MVA pathway. Moreover, we investigated the biosynthesis of CKs in crown gall, which is induced by infection of Agrobacterium tumefaciens to host plants. Our study showed that Agrobacterium efficiently produces trans-zeatin-type CKs by creating a non-plant-type CK biosynthetic pathway in plastids. This result provided new insight into the function of Agrobacterium CK biosynthetic enzyme that can move into plastids without typical signal peptides.

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Purine alkaloids, including caffeine (1,3,7-trimethylxanthine) and theobromine (3,7-dimethylxanthine) are found in varying quantities in the leaves, seeds and flowers of nearly 100 plants. It is interesting that only some species contain purine alkaloids, despite the distribution of these alkaloids in many genera. Caffeine synthase and the related methyltransferases belong to motif B' methyltransferase family. Most members of this family catalyze the formation of small molecule methyl esters, using s-adenosyl-L-methionine as a methyl donor and substrate with a carboxyl group as methyl acceptors. It is interesting to note that caffeine biosynthetic pathways seem to have evolved independently several times in plants.

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