Reactive Oxygen Species Generated by NADPH Oxidases Promote Radicle Protrusion and Root Elongation during Rice Seed Germination

Abstract

Seed germination is a complicated biological process that requires regulation through various enzymatic and non-enzymatic mechanisms. Although it has been recognized that reactive oxygen species (ROS) regulate radicle emergence and root elongation in a non-enzymatic manner during dicot seed germination, the role of ROS in monocot seed germination remains unknown. NADPH oxidases (NOXs) are the major ROS producers in plants; however, whether and how NOXs regulate rice seed germination through ROS generation remains unclear. Here, we report that diphenyleneiodinium (DPI), a specific NOX inhibitor, potently inhibited embryo and seedling growth—especially that of the radicle and of root elongation—in a dose-dependent manner. Notably, the DPI-mediated inhibition of radicle and root growth could be eliminated by transferring seedlings from DPI to water. Furthermore, ROS production/accumulation during rice seed germination was quantified via histochemistry. Superoxide radicals (O2−), hydrogen peroxide (H2O2) and hydroxyl radicals (•OH) accumulated steadily in the coleorhiza, radicle and seedling root of germinating rice seeds. Expression profiles of the nine typical NOX genes were also investigated. According to quantitative PCR, OsNOX5, 7 and 9 were expressed relatively higher. When seeds were incubated in water, OsNOX5 expression progressively increased in the embryo from 12 to 48 h, whereas OsNOX7 and 9 expressions increased from 12 to 24 h and decreased thereafter. As expected, DPI inhibits the expression at predetermined time points for each of these genes. Taken together, these results suggest that ROS produced by NOXs are involved in radicle and root elongation during rice seed germination, and OsNOX5, 7 and 9 could play crucial roles in rice seed germination. These findings will facilitate further studies of the roles of ROS generated by NOXs during seed germination and seedling establishment and also provide valuable information for the regulation of NOX family gene expression in germinating seeds of monocot cereals.

The signalling role of ROS in the regulation of seed germination and dormancy

“…They transfer electrons across the plasma membrane from cytoplasmic NADPH to molecular oxygen to produce superoxide in the apoplast which is rapidly dismutated to H 2 O 2 . The activity of NADPH oxidases has been frequently proposed to be involved in ROS production during seed imbibition (eg [72] [24,32,33,46]) and, similarly to other plant systems, NOXs could play a role in sensing environmental conditions favourable for germination.…”

“…ROS have also been shown to be produced in cell walls of growing embryos, at the time of radicle protrusion [91], including for non-endospermic seed species [42]. Depending on the species, it has been proposed that cell wall peroxidases, NADPH oxidases or polyamine oxidases were involved in ROS production in the apoplast of elongating cells of the embryonic axis [72,92,37,93,46,94]. Independently of the mechanisms of production, there is growing evidence showing that apoplastic ROS production is necessary for initiating cell elongation in the growing radicle.…”

NADPH Oxidases: The Vital Performers and Center Hubs during Plant Growth and Signaling

“…AtRbohB is a major producer of •O 2 in germinating seeds, and inhibition of the •O 2 production by diphenylene iodonium (DPI) leads to a delay in seed germination of Arabidopsis and cress [48]. In rice, OsNOX5, 7 and 9 might play crucial roles in radicle and root elongation during seed germination by regulating ROS production [49]. Similarly, •O 2 produced by NADPH oxidase also regulates seed germination and seedling growth in barley [50,51].…”

Expression atlas of Selaginella moellendorffii provides insights into the evolution of vasculature, secondary metabolism and roots

“…To investigate the role of the core 171 gene families that are root-specific in all vascular plants (Supplemental Table 6), we extracted S. moellendorffii root-specific genes belonging to these families and used CoNekT-Plants to create a custom network (https://conekt.sbs.ntu.edu.sg/custom_network/). Functional analysis of this network revealed groups of genes involved in root biology, such as transferases and oxidases [84][85][86] , transporters to mediate root signalling and nutrient uptake 87,88 , auxininduced genes important for lateral root growth 89,90 , regulation of transcription, protein degradation, cell wall modification, and flavonoid biosynthesis. Functional enrichment analysis of the root-specific genes in all five species revealed bins corresponding to glutathione S transferase, phenylpropanoids, biotic stress, cytochrome P450, various transporters, cell wall modification, hormone metabolism, and others ( Figure 6E).…”