Reactive oxygen species ( ROS ) are ubiquitous signaling molecules involved in diverse physiological processes, including stomatal closure. Photosynthetic electron transport ( PET ) is the main source of ROS generation in plants, but whether it functions in guard cell signaling remains unclear. Here, we assessed whether PET functions in abscisic acid ( ABA ) signaling in guard cells. ABA ‐elicited ROS were localized to guard cell chloroplasts in Arabidopsis thaliana , Commelina benghalensis , and Vicia faba in the light and abolished by the PET inhibitors 3‐(3, 4‐dichlorophenyl)‐1, 1‐dimethylurea and 2, 5‐dibromo‐3‐methyl‐6‐isopropyl‐ p ‐benzoquinone. These inhibitors reduced ABA ‐induced stomatal closure in all three species, as well as in the NADPH oxidase‐lacking mutant atrboh D/F . However, an NADPH oxidase inhibitor did not fully eliminate ABA ‐induced ROS in the chloroplasts, and ABA ‐induced ROS were still observed in the guard cell chloroplasts of atrboh D/F . This study demonstrates that ROS generated through PET act as signaling molecules in ABA ‐induced stomatal closure and that this occurs in concert with ROS derived through NADPH oxidase.

Steroidal saponins are natural surfactants with various biological activities, and the tubers of Dioscorea, known as yam, contain a furostanol glycoside protodioscin and a spirostanol glycoside dioscin, which are valuable saponins required for semi-synthetic production of pharmaceutical steroidal drugs. Steroidal saponins are biosynthesized from cholesterol via several steps of oxygenation and transglycosylation, and a β-glucosidase is involved in the hydrolytic conversion from furostanol glycosides to spirostanol glycosides. To investigate steroidal saponin biosynthesis in Dioscorea spps, comparative transcriptome analysis of high saponin producers, D. esculenta and D. cayenensis, and a low producer, D. alata, was performed using 454 pyrosequencing. In this study, we isolated and characterized a β-glucosidase (DeF26G1) from D. esculenta. The DeF26G1 cDNA encodes a family 1 glucosidase, and the DeF26G1 transcript was present at high levels in D. esculenta but not detected in D. alata. The recombinant DeF26G1 protein hydrolyzed the 26-O-glycosidic bond of protodioscin to form dioscin, indicating that the DeF26G1 gene encodes furostanol glycoside 26-O-β-glucosidase. These results suggested that DeF26G1 is involved in the conversion of furostanol saponins to spirostanol saponins, which seems to be related to biological defense response in the leaves of Dioscorea plants.

Cucumisin [EC 3.4.21.25], a subtilisin-like serine endopeptidase, was isolated from melon fruit, Cucumis melo L. Mature cucumisin (67 kDa, 621 residues) is produced by removal of the propeptide (10 kDa, 88 residues) from the cucumisin precursor by subsequence processing. It is reported that cucumisin is inhibited by its own propeptide. The crystal structure of mature cucumisin is reported to be composed of three domains: the subtilisin-like catalytic domain, the protease-associated domain and the C-terminal fibronectin-III-like domain. In this study, the crystal structure of the mature cucumisin•propeptide complex was determined by the molecular replacement method and refined at 1.95 Å resolution. In this complex, the propeptide had a domain of the α-β sandwich motif with four-stranded antiparallel β-sheets, two helices and a strand of the C-terminal region. The β-sheets of the propeptide bind to two parallel surface helices of cucumisin through hydrophobic interaction and 27 hydrogen bonds. The C-terminus of the propeptide binds to the cleft of the active site as peptide substrates. The inhibitory assay suggested that the C-terminal seven residues of the propeptide do not inhibit the cucumisin activity. The crystal structure of the cucumisin•propeptide complex revealed the regulation mechanism of cucumisin activity.

A reliable means to induce sprouting in dormant seed tubers of yams (Dioscorea species) is required to enhance flexibility in planting date and rate of propagation of the crop. Experiments were conducted to assess the potential of two gibberellin inhibitors, Uniconazole-P and Prohexadione-calcium, to induce sprouting in tubers from three varieties of D. rotundata and four of D. alata. Uniconazole-P and Prohexadionecalcium shortened the period of dormancy in tubers of some varieties. In others, they either had no effect or extended dormancy. The varietal responses were also influenced by whether the tubers were treated at harvest (before shoot senescence) or four weeks after harvest by which time shoots had senesced fully. Tubers stored in darkness at a constant temperature of 30 • C sprouted earlier but lost weight faster than did those stored under natural daylight and ambient temperature. The apparent slow action of Uniconazole-P and Prohexadione-calcium, and their variable effectiveness in relation to variety, would limit their usefulness in seed or breeding programmes, or in determining treatment timing and the best storage environment for the tubers after treatment. I N T RO D U C T I O NTuber dormancy is an important mechanism for adaptation of yams (Dioscorea spp.) to their natural environments (Craufurd et al., 2001) and, because yams are purchased predominantly as fresh tubers for food preparation, long dormancy is very important for shelf life. Hence, long dormancy of tubers is a desirable attribute in yam breeding and selection programmes. It complicates the use of the tuber for propagation, however. Irrespective of when a yam seed tuber is planted, the critical starting point of the growing season is when dormancy ends and sprouts are produced. There is a period of about four months after harvest during which tuber losses are incurred in storage but propagation of the planting material would not be successful. In some environments, early planting could largely obviate losses incurred during seed-yam storage if there was more flexibility in the control of sprouting date through effective means of artificially terminating dormancy. Successful protocols for this would have a tremendous impact on yam cultivation. They could lead to more uniformity in sprouting, off-season propagation of planting materials, multiple cropping cycles of early maturing varieties per year and better control over planting date. Developing such protocols has been rather challenging (Barker et al., 1999a;b;Craufurd et al., 2001).

The yield of 'Daikichi', eddo cultivar (Colocasia esculenta Schott (L). var. antiquorum Hubbard & Rehder) cultivar generally grown in upland fields is increased by pot culture under flooded conditions. Here, we studied the effect of flooding on the photosynthesis in eddo under upland field conditions and flooded conditions in which flooding treatment started when two to three leaves emerged. In comparison with upland field conditions, stomatal conductance, transpiration rate, amount of chlorophyll and photosynthetic rate were increased with the plant growth under flooded conditions. The increase in transpiration under flooded conditions prevented the rise of daytime leaf temperature thus preventing the decrease in photosynthetic rate that occurs under high temperature conditions.