Catalase associated with antagonistic changes of abscisic acid and gibberellin response, biosynthesis and catabolism is involved in eugenol-inhibited seed germination in rice.

KEY MESSAGE: The inhibitory effect of eugenol on rice germination is mediated by a two-step modulatory process: Eugenol first regulates the antagonism of GA and ABA, followed by activation of catalase activity. The natural monoterpene eugenol has been reported to inhibit preharvest sprouting in rice. However, the inhibitory mechanism remains obscure. In this study, simultaneous monitoring of GA and ABA responses by the in vivo GA and ABA-responsive dual-luciferase reporter system showed that eugenol strongly inhibited the GA response after 6 h of imbibition, whereas eugenol significantly enhanced the ABA response after 12 h of imbibition. Gene expression analysis revealed that eugenol significantly induced the ABA biosynthetic genes OsNCED2, OsNCED3, and OsNCED5, but notably suppressed the ABA catabolic genes OsABA8ox1 and OsABA8ox2. Conversely, eugenol inhibited the GA biosynthetic genes OsGA3ox2 and OsGA20ox4 but significantly induced the GA catabolic genes OsGA2ox1 and OsGA2ox3 during imbibition. OsABI4, the key signaling regulator of ABA and GA antagonism, was notably induced before 12 h and suppressed after 24 h by eugenol. Moreover, eugenol markedly reduced the accumulation of H2O2 in seeds after 36 h of imbibition. Further analysis showed that eugenol strongly induced catalase activity, protein accumulation, and the expression of three catalase genes. Most importantly, mitigation of eugenol-inhibited seed germination was found in the catc mutant. These findings indicate that catalase associated with antagonistic changes of ABA and GA is involved in the sequential regulation of eugenol-inhibited seed germination in rice.

Direct and Knock-on Effects of Water Stress on the Nutrient Contents of Triticum aestivum (Poales: Poaceae) and Population Growth of Rhopalosiphum padi (Hemiptera: Aphididae).

To ascertain the direct effects of water stress upon wheat plants (Triticum aestivum L.) and how these effects, in turn, influence the population growth of the bird cherry-oat aphid (Rhopalosiphum padi L.), we conducted a physiological analysis of wheat seedlings grown under three different watering regimes and subsequently determined the population parameters of the aphid using the age-stage, two-sex life table. A significantly higher content of free amino acids and soluble sugars were observed in wheat seedlings exposed to drought stress compared to seedlings that were well-watered and those that were grown under waterlogged conditions. Extended phloem salivation and stylet penetration with shorter duration of sustained ingestion from phloem was observed in an electrical penetration graph (EPG) of R. padi on drought-stressed wheat seedlings. This suggested that the aphid's feeding activity, as well as nutrient intake, were impeded. The significantly higher percentage of essential amino acids found in wheat seedlings grown under waterlogged conditions promoted significantly higher fecundity and intrinsic rate of increase in R. padi populations compared to aphids fed on drought-treated or well-watered wheat seedlings. Our findings suggest that wheat seedling responses to water stress involve changes in sap composition that are responsible for altering the aphids' nutrient intake and consequently affect their population growth. From a grower's perspective, extending wheat cultivation in a rice-wheat rotation paddy field during the winter season may not be economically profitable if the fields are chronically waterlogged, since this may potentially lead to a higher infestation of cereal aphids.

Analysis by virus induced gene silencing of the expression of two proline biosynthetic pathway genes in Nicotiana benthamiana under stress conditions.

Proline accumulation is responsible for stress adaptation in many plants. To distinguish the involvement of two proline synthetic pathways, the virus induced gene silencing (VIGS) system that silenced the expression of genes encoding Δ(1)-pyrroline-5-carboxylate synthetase (P5CS; EC:1.5.1.12) and ornithine-δ-aminotransferase (OAT; EC 2.6.1.13) was performed, separately or concomitantly, in four-week-old Nicotiana benthamiana. Leaf discs of VIGS-treated tobacco were subjected to the treatment of drought, abscisic acid (ABA), or polyethylene glycol (PEG). The treated leaf discs were then collected for the determination of mRNA, chlorophyll, proline and polyamine level. Under drought stress or PEG treatment, most proline accumulation was inhibited in P5CS-silenced plants and only a small portion was inhibited in OAT-silenced plants under drought stress and no inhibition was observed under PEG treatment. Under ABA treatment, proline accumulation was inhibited completely in P5CS-silenced plants but unaffected in OAT-silenced plants. The degradation of chlorophyll was enhanced in P5CS-silenced plants but retarded in OAT-silenced plants under PEG treatment. Under ABA treatment, the degradation of chlorophyll was unaffected in both P5CS-silenced and OAT-silenced plants. The increase of polyamine level was unaffected in P5CS-silenced plants but increased in OAT-silenced plants under PEG treatment. Under ABA treatment, the increase of polyamine level was unaffected in P5CS-silenced plants but the polyamine level was increased later in OAT-silenced plants. Therefore, P5CS plays a major role in proline accumulation under drought, PEG, or ABA treatment, while OAT plays a minor role in drought or PEG treatment and does not participate in ABA treatment. OAT appears to have a close relationship with the regulation of polyamine levels in PEG and ABA treatments.