Molecular identification and functional characterization of an environmental stress responsive glutaredoxin gene ROXY1 in Quercus glauca

Quercus glauca is a valuable natural resource with both economic and ecological values. It is one of the dominant forest tree species widely distributed in Southern China. As a perennial broadleaf plant, Q. glauca inevitably encounters numerous stresses from environment. Glutaredoxins (GRXs) are a kind of small oxidoreductases that play an important role in response to oxidative stress. CC-type GRXs also known as ROXYs are specific to land plants. In this study, we isolated a CC-type GRX gene, QgROXY1, from Q. glauca. Expression of QgROXY1 is induced by a variety of environmental stimuli. QgROXY1 protein localizes to both cytoplasm and nucleus; whereas the nucleus localized QgROXY1 could physically interact with the basic region/leucine zipper motif (bZIP) transcription factor AtTGA2 from Arabidopsis thaliana. Transgenic A. thaliana ectopically expressing QgROXY1 is hypersensitive to exogenously applied salicylic acid. Induction of plant defense gene is significantly impaired in QgROXY1 transgenic plants that results in enhanced susceptibility to infection of Botrytis cinerea pathogen, indicating the evolutionary conserved function among ROXY homologs in weedy and woody plants. This is the first described function for the ROXYs in tree plants. Through this case study, we demonstrated the feasibility and efficacy of molecular technology applied to characterization of gene function in tree species.

Photoperiodic control of reproductive arrest in the oak-inhabiting spider mite Schizotetranychus brevisetosus (Acari: Tetranychidae)

Populations of Schizotetranychus brevisetosus Ehara (Acari: Tetranychidae), which live on the evergreen oak (Quercus glauca), survive the coldest months as either adult females or winter eggs. Adult females comprise the majority of the population in early November and oviposit from late November to early March. Most winter eggs hatch by late March, and adults of the next generation emerge in April. This species is considered an egg-diapausing species, but the environmental cues that regulate female reproductive arrest and resumption are mostly unknown. We investigated the photoperiodic responses of autumn reproductive arrest in 10 populations collected from different elevations in Shikoku, Japan. All populations showed long-day responses to critical daylength (CDL) around 12.2 h light (12.2L) at 20 °C, though there was no linear relationship between CDL and altitude. This result explains the steep decline in the proportion of summer eggs in November. Nonreproductive females developed under 10L at 20 °C commenced oviposition 14.3-20.6 days after transferring to 15L. This long pre-oviposition period explains the reduction in eggs before winter reproduction and suggests shallow adult diapause. Eggs thus obtained hatched in 12.9-15.3 days, similarly to summer eggs. Therefore, egg diapause in S. brevisetosus is much shallower than in species on deciduous hosts, which lay their winter eggs in early autumn to hatch at leaf flush in spring. The reproductive arrest and short hatching period may be an adaptation allowing egg-laying in midwinter, when predation pressure is low.

Production of winter eggs in Schizotetranychus brevisetosus (Acari: Tetranychidae) inhabiting evergreen Japanese blue oak

The overwintering pattern of parasitic herbivorous arthropods is closely related to host phenology, because defoliation imposes strong selection pressures on various developmental stages. This relationship has been well studied in populations of spider mites (Acari: Tetranychidae) on deciduous hosts, but is little studied in populations on evergreen hosts, probably because their leaves are always available. However, spring defoliation may also influence the life cycle. We studied the overwintering pattern of Schizotetranychus brevisetosus (Acari: Tetranychidae), a specialist on evergreen oak, Quercus glauca, in Kochi, Japan. Only adult females and their eggs (winter eggs) survived the coldest months. We also observed a conspicuous seasonal change in egg colour and size: December winter eggs were 1.7× larger than September summer eggs, suggesting the winter eggs are diapausing. Adult females produced summer eggs until November and winter eggs from late November until they disappeared in March. The winter eggs hatched in early March. The immature stages developed in mid- to late March, when 39% of old leaves fell, some of which carried hundreds of immature individuals. Therefore, even in evergreen hosts, defoliation places potentially strong selection pressure on the immature stages. Despite this, S. brevisetosus appears not to change its egg hatching season to avoid the defoliation season, so it may have evolved a behavioural adaptation to escape falling leaves. The existence of several species with similar lifestyles suggests the importance of host phenology and predation pressure for evolution of the overwintering pattern.

Effect of pre-germination treatments on seed physiology and germination of central Himalayan oaks?

The continuous decline in regeneration of two important species of central Himalayan oak, namely Quercus glauca and Q. leucotrichophora, is of great concern. A study was therefore, carried out to improve germination ability of these species using various presoaking treatments. Seeds of both the species lost viability following storage; tetrazolium staining pattern and germination capacity of seeds following different period of storage at 4 °C and 20 °C indicated retainment of viability for a period of 12 months at 4 °C. Of the various physical, chemical and plant growth regulator treatments examined to improve seed germination, only KNO3 1.0 % was found to be effective. Seeds scarified at the chalazal end exhibited significant improvement in germination in both the species (94.4 % compared to 56.7 % in control in Q. glauca and 82.7 % compared to 64.0 % in control in Q. leucotrichophora). The results of this study impart simple methods to improve seed germination for developing nurseries for commercial purposes.