No abstract

Accumulation of proline is trusted to be an adaptive response of plants against drought stress, and exogenous application of nitric oxide (NO) enhances proline accumulation in Cu-treated algae. In order to investigate whether NO works as a necessary signaling molecule in drought-induced proline accumulation in rice leaves, effects of drought stress on endogenous NO content and proline accumulation were studied in rice leaves, using sodium nitroprusside (SNP, a NO donor) and 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO, a NO scavenger). The results showed that drought treatment increased both endogenous NO and proline contents in rice leaves, while foliar spray of various concentrations of SNP failed to induce proline accumulation in the leaves of well-watered rice and foliar spray of cPTIO failed to inhibit proline accumulation in the leaves of drought-stressed rice. These results indicate that increase of endogenous NO is dispensable for proline accumulation in the leaves of rice under drought stress. Further studies indicate that exogenous application of NO alleviates drought-induced water loss and ion leakage by decreasing transpiration rate of rice leaves.

Nitrate reductase (NR) is an enzymatic source of nitric oxide (NO) in plants, and it needs Mo for the Mo-cofactor to be activated. Because NR-deficient mutants are not always available in some species, a cheap and simple pharmacological application of tungstate, which substitutes for Mo in the Mo-cofactor as a competitive antagonist, is widely used as a NR inhibitor in plant NO research. However, evidence indicates that tungstate not only inactivates NR but also inhibits other molybdate-dependent enzymes in plants. In addition, a number of investigations have shown that tungstate also inhibits root growth, affects cortical microtubule formation, and induces programmed cell death (PCD) in plants, just like other heavy metals do. Therefore, tungstate has been shown to exert many other effects that are not connected with the inhibition of NR activity. The origin and mechanism of using tungstate as a NR inhibitor in plants is reviewed here and the progress regarding tungstate toxicity to plants and the possible problems involved in using tungstate as a NR inhibitor in plant NO research are analysed. In summary, the use of tungstate as a NR inhibitor in plant NO research must be treated with caution, keeping in mind that it is not completely specific. It is necessary to search for more NR-deficient mutants and new, specific NR inhibitors. A combination of pharmacological and biochemical analysis with a genetic approach will be necessary in order to investigate the roles of NO in plants.

In general, the fertility and kernel weight of inferior spikelets of rice (Oryza Sativa L.) are obviously lower than those of superior spikelets, especially under abiotic stress. However, different responses to heat stress are seemed to show between the superior and inferior spikelet, and this response is scarcely documented that the intrinsic factors remain elusive. In order to reveal the mechanism underlying, two rice plants with different heat tolerance were subjected to heat stress of 40°C at anthesis. The results indicated that a greater decrease in fertility and kernel weight was observed in superior spikelets compared to inferior spikelets. This decrease was primarily ascribed to their different organ temperatures, in which the temperature of the superior spikelets was significantly higher than that of inferior spikelets. We inferred the differences in canopy temperature, light intensity and panicle types, were the primary reasons for the temperature difference between superior and inferior spikelets. Under heat stress, the fertility and kernel weight of superior and inferior spikelets decreased as the panicle numbers per plant were reduced, which was accompanied by significantly increasing the canopy temperatures. Thus, it was suggested that the rice plant with characteristic features of an upright growth habit and loose panicles might be more susceptible to heat stress resulting from their higher canopy and spikelets temperatures.

Heat stress at the pollen mother cell (PMC) meiotic stage leads to pollen sterility in rice, in which the reactive oxygen species (ROS) and sugar homeostasis are always adversely affected. This damage is reversed by abscisic acid (ABA), but the mechanisms underlying the interactions among the ABA, sugar metabolism, ROS and heat shock proteins in rice spikelets under heat stress are unclear. Two rice genotypes, Zhefu802 (a recurrent parent) and fgl (its near-isogenic line) were subjected to heat stress of 40°C after pre-foliage sprayed with ABA and its biosynthetic inhibitor fluridone at the meiotic stage of PMC. The results revealed that exogenous application of ABA reduced pollen sterility caused by heat stress. This was achieved through various means, including: increased levels of soluble sugars, starch and non-structural carbohydrates, markedly higher relative expression levels of heat shock proteins (HSP24.1 and HSP71.1) and genes related to sugar metabolism and transport, such as sucrose transporters (SUT) genes, sucrose synthase (SUS) genes and invertase (INV) genes as well as increased antioxidant activities and increased content of adenosine triphosphate and endogenous ABA in spikelets. In short, exogenous application of ABA prior to heat stress enhanced sucrose transport and accelerated sucrose metabolism to maintain the carbon balance and energy homeostasis, thus ABA contributed to heat tolerance in rice.