Abstract:
Reactive oxygen species are important signaling molecules that influence many aspects of plant biology. One way in which ROS influence plant growth and development is by modifying intercellular trafficking through plasmodesmata (PD). Viruses have evolved to use plasmodesmata for their local cell-to-cell spread between plant cells, so it is therefore not surprising that they have found ways to modulate ROS and redox signaling to optimize plasmodesmata function for their benefit. This review examines how intracellular signaling via ROS and redox pathways regulate intercellular trafficking via PD during development and stress. The relationship between viruses and ROS-redox systems, and the strategies viruses employ to control PD function by interfering with ROS-redox in plants is also discussed.
Introduction
Reactive oxygen species (ROS) are oxygen-containing molecules that are highly reactive due to the presence of unpaired electrons. In plant cells, ROS are produced as natural byproducts of various metabolic processes (Mittler, 2017). While ROS are important for various signaling and regulatory processes in plants, excessive ROS levels can cause oxidative stress, leading to damage in cellular components such as DNA, proteins, and lipids (Mittler et al., 2022). Some common types of ROS include the superoxide radical (O2•-), hydrogen peroxide (H2O2), the hydroxyl radical (•OH), and singlet oxygen (1O2). Nitric oxide (NO˙) and peroxynitrite (ONOO−) are classified as reactive nitrogen species (RNS) and are capable of exacerbating oxidative stress (del Río and López-Huertas, 2016; Mittler, 2017).
To protect against oxidative stress, plants have evolved elaborate antioxidant defense systems that include thioredoxins (TRX) and enzymes like superoxide dismutase (SOD), catalase, peroxidases, and various non-enzymatic antioxidants like ascorbate and glutathione. These antioxidants help neutralize and regulate ROS levels, maintaining cellular redox balance and protecting the plant from oxidative damage (Mittler et al., 2022). Redox state, short for reduction-oxidation state, refers to the balance of electrons in a chemical reaction. It involves two processes: reduction (gain of electrons) and oxidation (loss of electrons). The connection between redox (reduction-oxidation) reactions and ROS lies in the transfer of electrons during chemical processes. Redox reactions involve the exchange of electrons between molecules, leading to the reduction and oxidation of species. When electrons escape from this redox process, particularly in the electron transport chains of photosynthesis or cellular respiration, they can interact with molecular oxygen, giving rise to ROS (Foyer and Hanke, 2022). Disruptions in redox balance can lead to cellular dysfunction and contribute to diseases (Mittler, 2017) (Mittler et al., 2022).
Each cell in a plant is surrounded by a cellulosic wall which hampers communication between plant cells that involves direct plasma membrane contacts or plasma membrane-facilitated ligand-receptor interactions that are commonly found in animals. Plants have instead evolved intercellular connections called plasmodesmata (PD) that permeate the cell wall to allow direct trafficking of metabolite and signaling molecules between connected cells (Azim and Burch-Smith, 2020). Beyond their roles in trafficking endogenous plant molecules, PD also provide a route for direct cell-to-cell conveyance of viruses during infection (Heinlein, 2015; Reagan and Burch-Smith, 2020). PD are not only routes for direct cell-to-cell signaling, but they also connect individual cells to the vasculature for systemic or long-distance transport in the plant. The last few years have seen an exciting resurgence in interest in the role of PD in long-distance signaling. There is accumulating evidence that many mRNA molecules can travel long distances in the plant from their sites of synthesis (transcription) to their sites of action to regulate developmental processes (Thieme et al., 2015). The role of PD in long-distance signaling via propagation of ROS and calcium signals is also garnering attention (Toyota et al., 2018; Fichman et al., 2021), although the role of PD is these processes may not be as straightforward as thought (Bellandi et al., 2022). This review focuses on local cell-to-cell movement and explore how ROS and redox impacts cell-to-cell communication. It also discusses how redox signaling is modified by plant viruses to manipulate PD for their local cell-to-cell movement during infection as one illustration of how redox mechanisms contribute to plant responses to biotic stress.
Organelles as ROS production sites
ROS are generated within plant cells in response to various stimuli, including biotic and abiotic stresses, hormone signaling, and developmental cues. Chloroplasts, the hub of photosynthesis in plant cells, play a crucial role in the generation of ROS. As part of photosynthesis, electrons can escape from the electron transport chain, leading to the formation of O2·− and H2O2. The production of ROS in chloroplasts involves both photosystems I and II, leveraging the surplus of photons harnessed by PSII and directing electrons towards molecular oxygen via PSI, (Foyer and Hanke, 2022; Li and Kim, 2022). In photosynthesizing cells, peroxisomes are required for photorespiration that occurs when ribulose-1,5-bisphosphate carboxylase/oxygenase (RubiscCO) favors O2 as a substrate instead of CO2. The phosphoglycolate produced by this reaction is transported from chloroplasts to peroxisomes where it is broken down and H2O2 is produced (Tripathy and Oelmuller, 2012). Peroxisomes are also involved in other metabolic processes, including fatty acid and polyamines breakdown and detoxification of harmful substances. Peroxisomal ROS are also produced as by-products during these processes (del Río and López-Huertas, 2016; Corpas et al., 2020). Thus, in photosynthesizing cells, the chloroplasts and peroxisomes together account for approximately a significant proportion of ROS production. Glyoxysomes, specialized peroxisomes in germinating plant cells, participate in ROS production while converting stored lipids to carbohydrates (Janku et al., 2019; De Bellis et al., 2020). As in all eukaryotes, mitochondria are the respiratory organelles in plant cells. Mitochondrial ROS (mtROS) are produced by electron leakage occurring within the electron transport system, which is influenced by the inhibition of particular sites within the electron transport chain (ETC) or the reduction state of ETC components, as substrates are metabolized (Moller, 2001; Huang et al., 2016). Together, chloroplasts, peroxisomes and mitochondria account for the bulk of ROS production in plant cells (Tripathy and Oelmuller, 2012).
Apart from these main sources or ROS, the plasma membrane (PM), endoplasmic reticulum (ER) and apoplast also produce ROS. The Respiratory Burst Oxidase Homologs (RBOHs), are transmembrane PM proteins that transfer electrons from cytosolic NADPH to molecular oxygen, leading to superoxide radicals’ production and their transformation into other ROS. RBOHs can also be regulated by protein kinases, such as calcium-dependent protein kinases (CDPKs) and mitogen-activated protein kinases (MAPKs), which can phosphorylate and activate RBOHs (Chapman et al., 2019) (Zandalinas and Mittler, 2018). The ER generates ROS during oxidative protein folding, driven by the formation of ROS due to the occasional incorrect formation of disulfide bonds and the NAD(P)H-dependent electron transport system using Cytochrome P450 (Cyt P450) (Sharma et al., 2012; Cao and Kaufman, 2014). The apoplast and cell wall are additional sources of ROS production, involving various factors, such as the respiratory burst, cell wall peroxidases, polyamine oxidases, and processes like cell wall remodeling and lignification. RBOHs release superoxide radicals into the apoplast, notably during stress responses or immune reactions (Schmidt et al., 2018).