Yasuo Yamauchi

Comparison of adaptability to flash flood between rice cultivars differing in flash flood tolerance

Abstract The mechanism of flash flood tolerance was investigated by using a flash flood-intolerant rice cultivar (Oryza sativa), IR42 and a tolerant one, FR13A. The photosynthetic rate of the rice seedlings from both types of cultivars decreased during submergence, whereas the photosynthetic rate of FR13A remained higher than that of IR42. The decrease in photosynthesis in IR42 was partially due to a higher inhibition of the activities of ribulose-1,5-bisphosphate carboxylase, fructose-1,6-bisphosphatase, and glyceraldehyde-3-phosphate dehydrogenase, than in FR13A. After submergence receded (hereafter referred as “desubmergence”), both types of submerged rice cultivars experienced oxidative damage. However the oxidation of the lipids and proteins in FR13A was maintained at a lower level than that in IR42 after desubmergence. On the other hand, the superoxide dismutase activity in FR13A increased more than that in IR42 after desubmergence. Moreover, the content of total ascorbate (reduced and oxidized ascorbates), a main antioxidant increased in FR13A, while in IR42 the content remained low after desubmergence. It is assumed that the photosynthetic activity during submergence may be related to flash flood tolerance. Moreover, flash flood tolerance requires a rapid stimulation of antioxidant systems after desubmergence.

Appearance of endopeptidases during the senescence of cucumber leaves

Abstract In cucumber ( Cucumis sativus L.) leaves at different ontogenic stages, a differential appearance of three major endopeptidases was observed by employing activity staining using gelatin as a substrate. On the basis of this observation, we discussed their physiological roles in senescing leaves. The most active endopeptidase in young mature leaves was a glutamyl endopeptidase with a pI of 4.5. It might be involved in active protein catabolism in young leaves because its activity became maximal just after the leaf had fully expanded and when protein and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) contents rapidly decreased. An endopeptidase with a pI of 4.3 was not observed in young leaves, however, it was highly active in senescing leaves. Interestingly, its activity in cotyledons was eliminated when the upper metabolically active leaves were removed. This implies that the appearance of this enzyme is regulated by the presence of sink tissues, and it is involved in the degradation of protein in senescing leaves facilitating N transfer to upper developing leaves. A trypsin-like endopeptidase with a pI of 5.0 showed relatively constant activity during the whole period. This endopeptidase has been shown to be inhibited by arginine, guanidino compounds and Mg 2+ , therefore, it might exist constitutively and its activity might be regulated mainly at a post-translational level responding to nutrient and environmental conditions.

A serine endopeptidase from cucumber leaves is inhibited by l-arginine, guanidino compounds and divalent cations

An endopeptidase was purified and characterized from green leaves of cucumber (Cucumis sativus L. suyo). The purified enzyme, a basic amino acid-specific endopeptidase with a pI of 5.0, was a monomeric protein of 80 kDa whose pH optimum was 9.5. Inhibitor analysis suggested that it was a serine endopeptidase and contained sulfhydryl groups essential for catalytic activity. Analysis of internal amino acid sequences of the endopeptidase showed no significant similarity to other proteins. Its activity was inhibited by L-Arg and guanidino compounds having high hydrophobicity, as well as divalent cations such as Mg2+ and Ca2+. The K(i) values of L-Arg and Mg2+, which are also likely in vivo inhibitors, were 3.5 and 10 mM, respectively. Inhibition by L-Arg and Mg2+ was additive, and more than 70% of the activity was reversibly inhibited under their physiologically significant concentrations. These results suggest that the enzyme is possibly regulated by L-Arg and/or guanidino compounds, and by divalent cations in vivo.