Abstract
Reactive oxygen species (ROS), like hydrogen peroxide (H2O2) and superoxide anion (O2•-), are important plant cell signaling molecules involved in diverse physiological processes, such as programmed cell death, development, cell elongation and hormonal signaling. Recently, much attention has been paid to the role of ROS in regulating plant root development. Two ROS, superoxide and hydrogen peroxide, were shown to exhibit a typical accumulation pattern in the Arabidopsis root apex and play distinct roles in root development.1 The latest study showed that UPBEAT1 (UPB1), a bHLH transcription factor, modulates the ROS balance by directly regulating the expression of a set of peroxidases, therefore, regulates the root cell proliferation and differentiation.2 In this addendum, we proposed a possible hypothesis that OsSPR1 maintained the mitochondria function to restrict H2O2 production in root apex for normal root development.
Key words: reactive oxygen species, hydrogen peroxide, cell elongation, meristem, mitochondria
Reactive oxygen species (ROS), like super-oxide anion (O2•-), hydrogen peroxide (H2O2), are by-products constantly produced during normal metabolic processes, such as photosynthesis, photorespiration and cellular respiration. High level of ROS can be very lethal for the plant cell integrity. However, at lower concentrations, ROS function in signaling pathways that regulate plant development in response to physiological and environmental cues. Superoxide anion generated by one-electron reduction of oxygen by the plasma membrane NADPH oxidase play important role for root growth and root hair development.3 The superoxide ion may be converted into H2O2 spontaneously which is also involved in many developmental and physiological process.4 Hydrogen peroxide can further be reduced by peroxidases to generated more reactive free radicals hydroxyl radical (OH). This radical is very important for cell elongation by cell wall loosening mechanism.5 Therefore, the maintenance of ROS homeostasis is crucial for plant development.
The root apex is a zone of active ROS production.1 It comprises cells in very different states within a short distance, including cell division, elongation and differentiation zones. Different ROS distribution pattern in root apex is reported in several plants include Arabidopsis,1,2 maize5 and rice.6 O2•- and H2O2 have both distinct accumulation zones and different roles in the extremity of the growing Arabidopsis.1 The latest study indicates that UPBEAT1 (UPB1), a bHLH transcription factor, modulates the ROS balance by directly regulating the expression of a set of peroxidases, therefore, regulates the root cell proliferation and differentiation for root growth.2 It was also proposed that maintenance of cellular proliferation requires an accumulation of O2•-, whereas cellular differentiation requires elevated H2O2 levels.2
The Osspr1 mutant was identified as a short root mutant with altered iron content in shoot and elevated H2O2 levels in the root tip. OsSPR1 encodes a novel mitochondrial protein with the Armadillo-like repeat domain. It is well known that the mitochondria are one of the major sources for ROS production in plants. Therefore, we reasoned that OsSPR1 directly acts in mitochondria of root meristem cells to affect respiratory electron chain for restricting the H2O2 production, which can maintain the ROS balance in root apex to direct normal root growth. While, in the Osspr1 mutant, high content of H2O2 are generated from dysfunctional mitochondria as indicated by altered alternative oxidases (AOXs) expression levels. It altered ROS balance in the root apex and ultimately lead to a quick cellular differentiation with inhibited root cell elongation. However, we could not exclude other possibilities, such as OsSPR1 involved in the iron transport into the mitochondria to control H2O2 production, since a higher root iron concentration was detected in OsSPR1 overexpression lines. Although the Osspr1 mutant root exhibited a normal iron concentration, the cellular distribution of iron in Osspr1 root apex cells might still be altered. To determine the exact function of OsSPR1 in rice, detailed analysis of the mitochondria function, the ROS production and cellular iron distribution in osspr1 still need to be investigated.
Acknowledgments
We thank Dr. keke Yi (Institute of Virology and Biotechnology, Zhejiang Academy of Agricultural Sciences) for critical reading and valuable suggestion. This work was supported by the Zhejiang Academy of Agricultural Sciences.
Abbreviations
- ROS
reactive oxygen species
- AOX
alternative oxidase
- H2O2
hydrogen peroxide
- bHLH
basic helix-loop-helix
- OH
hydroxyl radical
- O2•-
superoxide anion
- RAM
root apical meristem
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