Phytohormones signaling pathways and ROS involvement in seed germination
Oracz and Karpiński, Front. Plant Sci. (2016) https://doi.org/10.3389/fpls.2016.00864
Phytohormones and reactive oxygen species (ROS) are major determinants of the regulation of development and stress responses in plants. During life cycle of these organisms, signaling networks of plant growth regulators and ROS interact in order to render an appropriate developmental and environmental response. In plant’s photosynthetic (e.g., leaves) and non-photosynthetic (e.g., seeds) tissues, enhanced and suboptimal ROS production is usually associated with stress, which in extreme cases can be lethal to cells, a whole organ or even an organism. However, controlled production of ROS is appreciated for cellular signaling. Despite the current progress that has been made in plant biology and increasing number of findings that have revealed roles of ROS and hormonal signaling in germination, some questions still arise, e.g., what are the downstream protein targets modified by ROS enabling stimulus-specific cellular responses of the seed? Or which molecular regulators allow ROS/phytohormones interactions and what is their function in seed life? In this particular review the role of some transcription factors, kinases and phosphatases is discussed, especially those which usually known to be involved in ROS and hormonal signal transduction under stress in plants, may also play a role in the regulation of processes occurring in seeds. The summarized recent findings regarding particular ROS- and phytohormones-related regulatory proteins, as well as their integration, allowed to propose a novel, possible model of action of LESION SIMULATING DISEASE 1, ENHANCED DISEASE SUSCEPTIBILITY 1, and PHYTOALEXIN DEFICIENT 4 functioning during seeds life.
Figure 1. The proposed model of action of LESION SIMULATING DISEASE 1 (LSD1), ENHANCED DISEASE SUSCEPTIBILITY 1–PHYTOALEXIN DEFICIENT 4 (EDS1–PAD4) and other molecular regulators: (A) during seed germination in light, as well as in foliar tissues exposed to (B) optimal or (C) suboptimal conditions. LSD1 is a negative regulator of cell death and suppresses EDS1–PAD4 in laboratory conditions. This regulatory hub integrates signals induced by reactive oxygen species (ROS) and phytohormones [abscisic acid (ABA), ethylene (ET), and salicylic acid (SA)] during plants growth and development. It is highlighted that the mechanism of action of LSD1 and EDS1–PAD4 is conditionally regulated. In seeds imbibing in the presence of light, LSD1 suppresses EDS1–PAD4, thus it prevents cell death and promotes cell wall remodeling required for cell elongation of radicle during germination (A). In plants growing in optimal conditions the inhibition of EDS1–PAD4 by LSD1 results in positive modulation of photosynthesis, cell wall remodeling, elongation, and division of cells crucial for proper growth and development (B). The perturbations in photosynthetic tissue caused by suboptimal conditions lead to limitations of LSD1 suppression effect on EDS1–PAD4, resulting in induction of stress response and acclimation achieved by modulation of stomatal closure, photorespiration, cell wall lysis and cell death (C). The dotted lines indicate possible interactions (not yet confirmed). The size of letters and thickness of arrows reflects to the effect (increase/decrease) of particular interactions between regulators. (Oracz and Karpiński, Front. Plant Sci., (2016) https://doi.org/10.3389/fpls.2016.00864
Funding: Krystyna Oracz is very thankful for a financial support from the HOMING PLUS/2012-5/10 grant of the Foundation for Polish Science and Stanisław Karpiński is grateful for afinancial support from the Opus 6 (UMO-2013/11/B/NZ3/00973) and Maestro 6 (UMO-2014/14/A/NZ1/00218) projects granted by the Polish National Science Centre.