Rice (Oryza sativa L.) seedlings, when kept at 42 degrees C for 24 h before being kept at 5 degrees C for 7 d, did not develop chilling injury. Chilling resistance was enhanced in parallel with the period of heat-treatment. The level of APX activity was higher in heated seedlings whereas CAT activity was decreased by heat stress. There was no significant difference in SOD activity between heated and unheated seedlings. The elevated activity of APX was sustained after 7 d of chilling. The cytosolic APX gene expression in response to high and low temperature was analysed with an APXa gene probe. APXa mRNA levels increased within 1 h after seedlings were exposed to 42 degrees C. Elevated APXa mRNA levels could also be detected after 24 h of heating. The APXa mRNA level in preheated seedlings was still higher than unheated seedlings under cold stress. The promoter of the APXa gene was cloned from rice genomic DNA by TAIL-PCR, and characterized by DNA sequencing. The promoter had a minimal heat shock factor-binding motif, 5'-nGAAnnTTCn-3', located in the 81 bp upstream of the TATA box. Heat shock induction of the APXa gene could be a possible cause of reduced chilling injury in rice seedlings.

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The pyridoxal 5-phosphate (PLP)-dependent enzyme 1-aminocyclopropane-1-carboxylate deaminase (ACCD) catalyzes a reaction that involves a ring opening of cyclopropanoid amino acid, yielding ␣-ketobutyrate and ammonia. Unlike other PLP-dependent enzymes, this enzyme has no ␣-hydrogen atom in the substrate. Thus, a unique mechanism for the bond cleavage is expected. The crystal structure of ACCD from Hansenula saturnus has been determined at 2.0 Å resolution by the multiple wavelength anomalous diffraction method using mercury atoms as anomalous scatterers. The model was built on the electron density map, which was obtained by the density averaging of multiple crystal forms. The final model was refined to an R-factor of 22.5% and an R free -factor of 26.8%. The ACCD folds into two domains, each of which has an open twisted ␣/␤ structure similar to the ␤-subunit of tryptophan synthase. However, in ACCD, unlike in other members of the ␤ family of PLPdependent enzymes, PLP is buried deep in the molecule. The structure provides the first view of the catalytic center of the cyclopropane ring opening.

Natural abundance of tSN (6 tSN) of leaves harvested from tropical plants in Brazil and Thailand was analyzed. The 8 ~SN values of non-Nz-fixing trees in Brazil were +4.5 -+ 1.9%o, which is lower than those of soil nitrogen (+8.0-+2.2%0). In contrast, mimosa and kudzu had very low 6 ~SN values ( -1 . 4 -+ 0 . 5 % @ The t5 1-~N values of Panicum maximum and leguminous trees, except Leucaena leucocephala, were similar to those of non-N2-fixing trees, suggesting that the contribution of fixed N in these plants is negligible. The 6 15N values of non-N2-fixing trees in Thailand were +4.9 +_ 2.0%0. Leucaena leucocephala, Sesbania grandiflora, Casuarina spp. and Cycas spp. had low 6 tSN values, close to the value of atmospheric N 2 (0%e), pointing to a major contribution of N 2 fixation in these plants. Cassia spp. and Tamarindus indica had high 6 15N values, which confirms that these species are non-nodulating legumes. The 6 ~SN values of Acacia spp. and Gliricidia sepium and other potentially nodulating tree legumes were, on average, slightly lower than those of non-N2-fixing trees, indicating a small contribution of N 2 fixation in these legumes.

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