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Monogalactosyldiacylgycerol (MGDG) is a membrane lipid unique to photosynthetic organisms, and a major structural component of plastid membrane. Its formation is catalyzed in plastid envelope membrane by MGDG synthase. To date, three types of MGDG synthase gene have been identified in Arabidopsis and designated as atMGD1, 2 and 3. Our previous analyses revealed that expression of each gene was detected in various organs and developmental stages. In this study, we present the histochemical analysis of these genes in major organs and several conditions using β-glucuronidase (GUS) assay in Arabidopsis. GUS staining of whole transformant revealed that the expression of atMGD1-GUS was detected highly in green tissues, such as cotyledons, stems and sepals, while the expression of atMGD2-GUS and atMGD3-GUS was observed only a few parts, such as tips of leaves. In floral tissues, GUS stainings of all atMGD-GUS constructs was detected in pollen grains. Moreover, only the pollen tube of atMGD2-GUS transformant was stained by cross-pollination of the transformant pollen and wild type stigma. Thus, atMGD2 might have special roles on the event of pollen germination. Under phosphate-deprived condition, the expression of atMGD2-GUS and atMGD3-GUS was detected intensely, especially in root, while atMGD1-GUS did not show significant difference. Considering that DGDG is accumulated under phosphate-deprived condition, particularly in root, type B MGDG synthase might supply MGDG to DGDG synthesis as a substrate.

MGDG synthase (MGD, UDP-galactose: 1,2-diacylglycerol 3-β-D-galactosyltransferase; EC 2.4.1.46) catalyzes the final step of MGDG synthesis. This enzyme transfers a galactose from UDP-galactose to 1,2-diacylglycerol in chloroplast envelope (Douce, 1974). We have revealed that irradiation of white light on etiolated seedlings induced a transient increase in the level of MGD transcripts, followed by activation of the enzyme and a light-dependent accumulation of MGDG and DGDG (Yamaryo et al. 2000). Light-dependent expression of photosynthesis-related genes is generally mediated by the photoreceptor phytochrome. However, it is still unknown which photoreceptor is responsible for light-dependent changes in MGDG synthesis.

Jasmonates are known as growth regulators, which have cyclopentanone or cyclopentenone ring, and synthesized through lipoxygenase pathway. Jasmonates are widely distributed in the plant kingdom and modulates wounding responses, disease responses, and anther development. Some kinds of jasmonates were shown to have specific effects on plants. 12-oxo-phytodienoic acid (OPDA), a precursor for jasmonic acid (JA) biosynthesis, promotes tendril coiling, and this effect is stronger than MeJA (Falkenstein E., et al., 1991). However, except for the fact, little is known about the OPDA specific function in various physiological events.

Formation of the photosynthetic membranes of purple non-sulfur bacteria is regulated mainly by molecular oxygen and light[1-3]. Recent studies revealed that several regulatory genes such as the two-component regulatory genes(RegBCAIPrrBCA) are involved in the regulation [4,5].

All known photosynthetic organisms possess magnesium-containing tetrapyrroles, namely chlorophylls and bacteriochlorophylls (Bchls), for light harvest and energy-generating charge separation. Very recently, one exception was found in the purple bacterium, Acidiphilium rubrum, which possesses zinc-containing Bchl a (Zn-Bchl a) as its major photosynthetic pigments [1]. This finding modifies our common understanding that the naturally occurring chlorophylls and BchIs ubiquitously have a chelated magnesium atom at the center of macrocyclic ring. Zinc porphyrin derivatives are usually more stable than Mg-derivatives, and have been widely used in the studies of artificial photosynthesis [2]. All known species of the genus Acidiphilium are chemoheterotrophic aerobes and extreme acidophilic bacteria showing optimum pH for growth at around 3.0, and belong to the α-subclass of Proteobacteria that includes many of typical Bchl-utilizing photosynthetic bacteria [3]. Therefore, it is most likely that Acidiphilium species utilize Zn-Bchl instead of acid-labile Bchl in order to adapt their acidic environment, rather than they are evolutional ancestor of magnesium-containing Bchl-utilizing photosynthesis. Thus, an elucidation of Zn-Bchl a biosynthetic pathway in Acidiphilium species may provide insights as to how these bacteria adapt their photosynthesis by modulating a central metal of tetrapyrrole ring.

Monogalactosyldiacylglycerol, MGDG, is the most abundant lipids in photosynthetic membrane in higher plants and eukaryotic algae. MGDG reached to almost 40 to 50% in thylakoid lipids of the chloroplasts, and therefore, this lipid is considered as the most abundant lipid in the earth. The last step of the MGDG synthesis was catalyzed by MGDG synthesis (UDP-galactose:diacylglycerol galactosyltransferase EC2.4.I.46). Digalactosyldiacylglycerol (DGDG), another major lipid in thylakoid membranes, is synthesized by dismutation between two molecules of MGDG. And thus, UDGT is a key enzyme in the formation of thylakoid membranes.

UDP-galactose:diacylglycerol galactosyltransferase (UDGT) catalyzes the final step of the synthesis of monogalactosyl diacylglycerol (MGDG), a major glycohpid in plastid membrane. We found that the enzyme activity was strongly activated coincided with accumulation of galactolipids during germination of cucumber seedlings, suggesting that activation step of the UDGT was important in the development of chloroplast membrane [1]. However, mechanism of regulation of the UDGT activity during chloroplast formation remains unclear.