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MiR398 targets two Cu or Zn superoxide dismutases (CSD1 and CSD2) in Arabidopsis thaliana (L.) Heynh. Here we provide evidence that rice (Oryza sativa L.) miR398 mediates responses to abiotic and biotic stresses through regulating the expression of its target genes, Os-CSD1 and Os-CSD2. Rice plants were exposed to various stresses, including high Cu2+, high salinity, high light, methyl viologen, water stress, pathogens and ethylene, and the molecular response was investigated. Rice plants overexpressing Os-miR398 and the miR398-resistant form of Os-CSD2 were also exposed to these stresses. Both abiotic and biotic stresses significantly inhibited Os-miR398 expression and thus stimulated the expression of Os-CSD1 and Os-CSD2. The plant hormone ethylene produced an especially marked response. Transgenic rice lines that overexpressed Os-miR398 had a lower expression of CSD1 and -2 and were more sensitive to environmental stress. Conversely, transgenic rice lines which overexpressed the miR398-resistant form of Os-CSD2 showed more tolerance to high salinity and water stress than non-transgenic rice. We conclude that Os-miR398 regulates the responses of rice to a wide range of environmental stresses and to ethylene, and exerts its role through mediating CSDs expression and cellular ROS levels.

The physiological and biochemical factors contributing to poor grain filling of indica-japonica rice (Oryza sativa L.) hybrids were studied by analyzing the role of grain sink strength in dry matter accumulation of grains of two types of rice cultivars, Yayou 2 (an indica-japonica hybrid) and Yanjing 2 (a japonica cultivar). Carbon dioxide enrichment and plant hormone application were imposed at anthesis and the number of endosperm cells, dry matter accumulation and the activities of some sugar-metabolizing enzymes of grains were measured during grain filling. In Yayou 2, strong-potential grains (SPGs) accumulated dry weight much earlier than weak-potential grains (WPGs), but this difference was not obvious for Yanjing 2. Carbon dioxide enrichment imposed after heading significantly stimulated dry matter accumulation of WPGs of Yayou 2, but had little influence on WPGs of Yanjing 2 and SPGs of both cultivars. Leaf sheath dry matter decreased steadily in both cultivars during early stages of grain filling and accumulated during the later stages. Carbon dioxide enrichment increased leaf sheath dry matter. Dry matter accumulated by grains was linearly related to the increases in endosperm cell numbers and the activities of sucrose synthase (SS) and ADP-glucose pyrophosphorylase (AGPase) in SPGs and WPGs of both cultivars. Application of either 6-benzyladenine or abscisic acid had no significant influences on both endosperm cell number and grain dry matter accumulation. These results suggest that grain sink strength, determined by both cell numbers, SS and AGPase activities in the endosperm control the dry matter accumulation of grains.

Two tropical woody species, Acacia confusa Merrill and Litsea glutinosa (Lour.) C.B. Robinson, were grown under controlled conditions in PVC pipes filled with John Innes No. 2 soil. To investigate root distribution, physiological characteristics and hydraulic conductivity, four soil treatments were imposed-well-watered and noncompacted (control), well-watered and compacted; unwatered and noncompacted, and unwatered and compacted. In L. glutinosa, rooting depth and root elongation were severely restricted when soil bulk density increased from around 1.12 to 1.62 g cm(-3), whereas soil compaction had little effect on these parameters in A. confusa. As soil drying progressed, root water potential and osmotic potential declined more slowly in L. glutinosa than in A. confusa. Both the soil drying and compaction treatments significantly stimulated the accumulation of root abscisic acid (ABA) in both species. Soil drying damaged the root cell membrane of A. confusa, but had little influence on the root cell membrane of L. glutinosa. Soil drying had a greater effect on root hydraulic conductivity (L(p)) in L. glutinosa than in A. confusa, whereas the effect of soil compaction on L(p) was less in L. glutinosa than in A. confusa. Soil drying enhanced the effects of soil compaction on root L(p). We conclude that soil drying and compaction have large species-specific effects on the distribution, growth and physiology of roots. The relationships of these root properties to the species' ability to tolerate unfavorable soil conditions were examined.