By using an in vivo hydroponic rice seedling culture system, we investigated the physiological and biochemical responses of a model rice japonica cultivar Nipponbare to salt stress using proteomics and classical biochemical methods. Yoshida's nutrient solution (YS) was used to grow rice seedlings. YS-grown 18-day-old seedlings manifested highly stable and reproducible symptoms, prominently the wilting and browning of the 3rd leaf, reduced photosynthetic activity, inhibition in overall seedling growth, and failure to develop new (5th) leaf, when subjected to salt stress by transferring them to YS containing 130 mM NaCl for 4 days. As leaf response to salt stress is least investigated in rice by proteomics, we used the 3rd leaf as source material. A comparison of 2-DE protein profiles between the untreated control and salt-stressed 3rd leaves revealed 55 differentially expressed CBB-stained spots, where 47 spots were increased over the control. Of these changed spots, the identity of 33 protein spots (27 increased and 5 decreased) was determined by nESI-LC-MS/MS. Most of these identified proteins belonged to major metabolic processes like photosynthetic carbon dioxide assimilation and photorespiration, suggesting a good correlation between salt stress-responsive proteins and leaf morphology. Moreover, 2-DE immunoblot and enzymatic activity analyses of 3rd leaves revealed remarkable changes in the key marker enzymes associated with oxidative damage to salt stress: ascorbate peroxidase and lipid peroxidation were induced, and catalase was suppressed. These results demonstrate that hydroponic culture system is best suited for proteomics of salt stress in rice seedling.

Nitrogen fertilization is essential for increasing rice production to meet the food demands of increasing world's population. We established an in vivo hydroponic rice seedling culture system to investigate physio-biochemical/molecular responses of various rice japonica and indica cultivars to low nitrogen (N). Three-week-old seedlings grown in Yoshida's nutrient solution manifested stable and reproducible symptoms, such as reduced shoot growth and length under low N. Out of 12 genetically selected cultivars, 11 cultivars showed varied degrees of growth reduction response to applied N (4 and 40 ppm N for treatment and control, respectively), whereas one cultivar (no. 12) showed similar growth as the control though its leaf width was smaller than control. Leaves of a representative low N-responsive cultivar (BG90-2) were sampled for revealing protein profiles between low and normal (control) N application by two-dimensional gel electrophoresis (2-DGE). Forty-one proteins were identified with MALDI-TOF-MS and nESI-LC-MS/MS. Assignment of proteins into major (energy metabolism, photosynthesis and oxidative stress) and minor functional categories, revealed many novel low N-responsive proteins, including those having energy/photosynthesis-and defense/stress-and iron homeostasis-related functions. Results suggest the usefulness of proteomics in identifying novel N-responsive proteins and may provide potential markers for rice response to low N.

Ethanol for fuel uses is normally produced from sugar‐ and starch‐containing crops, which are expensive raw materials and require the consumption of traditional food sources. Rice hull, an abundant agricultural waste, consists of 32.3 ± 0.5% glucan and 13.0 ± 1.3% xylan (dry weight) and has the potential for use as a low‐cost feedstock in ethanol production. Processing challenges need to be addressed in order to achieve commercial rice hull conversion to fermentable sugars. In the present study, the effect of various acid‐to‐biomass ratios and impregnation and hydrolysis durations was evaluated from glucan and xylan conversion yields of rice hull to monomeric sugars and the production of fermentation inhibitors. Additionally, the effect of the inhibitors, 5‐hydroxymethylfurfural (HMF), furfural, and acetic acid, on fermentability by Saccharomyces cerevisiae was evaluated by the comparison between fermentation parameters of rice‐hull hydrolysate and a reference medium. Under optimal conditions of acid pretreatment (2:1 of acid‐to‐biomass ratio, 5‐h impregnation, and 2‐h hydrolysis), high glucan and xylan conversion yields of 92 and 95% were achieved, respectively. Extreme acidity and increased retention time under high temperature were the most important factors influencing the degradation of glucose and xylose, increasing HMF and furfural concentrations, respectively. Although rice‐hull hydrolysate was sufficient for ethanol production, the presence of inhibitors in rice‐hull hydrolysate decreased the specific growth rate and cell growth of S. cerevisiae, and the ethanol yield (0.47 g‐ethanol/g‐glucose) of rice‐hull hydrolysate was slightly lower than that of the reference medium (0.49 g/g). These results confirm that rice hull is a promising and alternative biomass for ethanol production by means of the acid impregnation and hydrolysis technique. The data obtained can be used for the design of an acid hydrolysis reactor for lignocellulosic biomass. Copyright © 2010 Curtin University of Technology and John Wiley & Sons, Ltd.

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