Suppression of Hydroxycinnamate Network Formation in Cell Walls of Rice Shoots Grown under Microgravity Conditions in Space

Abstract

Network structures created by hydroxycinnamate cross-links within the cell wall architecture of gramineous plants make the cell wall resistant to the gravitational force of the earth. In this study, the effects of microgravity on the formation of cell wall-bound hydroxycinnamates were examined using etiolated rice shoots simultaneously grown under artificial 1 g and microgravity conditions in the Cell Biology Experiment Facility on the International Space Station. Measurement of the mechanical properties of cell walls showed that shoot cell walls became stiff during the growth period and that microgravity suppressed this stiffening. Amounts of cell wall polysaccharides, cell wall-bound phenolic acids, and lignin in rice shoots increased as the shoot grew. Microgravity did not influence changes in the amounts of cell wall polysaccharides or phenolic acid monomers such as ferulic acid (FA) and p-coumaric acid, but it suppressed increases in diferulic acid (DFA) isomers and lignin. Activities of the enzymes phenylalanine ammonia-lyase (PAL) and cell wall-bound peroxidase (CW-PRX) in shoots also increased as the shoot grew. PAL activity in microgravity-grown shoots was almost comparable to that in artificial 1 g-grown shoots, while CW-PRX activity increased less in microgravity-grown shoots than in artificial 1 g-grown shoots. Furthermore, the increases in expression levels of some class III peroxidase genes were reduced under microgravity conditions. These results suggest that a microgravity environment modifies the expression levels of certain class III peroxidase genes in rice shoots, that the resultant reduction of CW-PRX activity may be involved in suppressing DFA formation and lignin polymerization, and that this suppression may cause a decrease in cross-linkages within the cell wall architecture. The reduction in intra-network structures may contribute to keeping the cell wall loose under microgravity conditions.

Persistence of plant hormone levels in rice shoots grown under microgravity conditions in space: its relationship to maintenance of shoot growth

“…We have previously reported that the activity of cell wall‐bound peroxidase increased less in microgravity‐grown shoots than in artificial 1 g ‐grown shoots and that the increases in expression levels of some class III peroxidase genes were also reduced under microgravity conditions in space (Wakabayashi et al ). The treatment with ethylene has been shown to enhance the activity of peroxidase in plant organs by inducing peroxidase proteins (Shimokawa , Abeles et al ).…”

“…On the other hand, transcriptome analysis in Arabidopsis has shown that the transcript levels of ACC synthase and ACC oxidase were increased under hypergravity conditions (Martzivanou andHampp 2003, Tamaoki et al 2009). We have previously reported that the activity of cell wall-bound peroxidase increased less in microgravity-grown shoots than in artificial 1 g-grown shoots and that the increases in expression levels of some class III peroxidase genes were also reduced under microgravity conditions in space (Wakabayashi et al 2015). The treatment with ethylene has been shown to enhance the activity of peroxidase in plant organs by inducing peroxidase proteins (Shimokawa 1985, Abeles et al 1988).…”

Modification of growth anisotropy and cortical microtubule dynamics in Arabidopsis hypocotyls grown under microgravity conditions in space

“…The morphology of shoots has been examined in various space experiments. Wheat coleoptiles, pea epicotyls and azuki bean epicotyls oriented in the abaxial direction (Heathcote et al , Ueda et al , Soga et al ), rice coleoptiles bent in the adaxial direction (Hoson et al , Wakabayashi et al ), cucumber hypocotyls elongated straight in predetermined direction (Takahashi et al ), whereas Arabidopsis hypocotyls elongated in various directions (Figs and , Hoson et al ). These results indicate that the growth pattern of shoots under microgravity conditions in space varies by plant species and by the type of organs.…”

Anthocyanin and Phenolic Acid Profiles in Purple, Red and Non-Pigmented Rice during Germination

“…With the progress of germination, the bound phenolic acid fraction increased, in conjunction with new cell wall matrix formed during growth of seedling tissues (Shahidi and Yeo, 2016). The activity of cell wall peroxidases, the enzyme which liberates phenolic acid, is induced by germination (Wakabayashi et al, 2015). Also, an increase in bound phenolic acid in non-pigmented rice was shown to be positively correlated with cell wall peroxidase activity after 4 days of germination (Cho and Lim, 2018).…”