The Apoplast: A Key Player in Plant Survival

Abstract

<jats:p>The apoplast comprises the intercellular space, the cell walls, and the xylem. Important functions for the plant, such as nutrient and water transport, cellulose synthesis, and the synthesis of molecules involved in plant defense against both biotic and abiotic stresses, take place in it. The most important molecules are ROS, antioxidants, proteins, and hormones. Even though only a small quantity of ROS is localized within the apoplast, apoplastic ROS have an important role in plant development and plant responses to various stress conditions. In the apoplast, like in the intracellular cell compartments, a specific set of antioxidants can be found that can detoxify the different types of ROS produced in it. These scavenging ROS components confer stress tolerance and avoid cellular damage. Moreover, the production and accumulation of proteins and peptides in the apoplast take place in response to various stresses. Hormones are also present in the apoplast where they perform important functions. In addition, the apoplast is also the space where microbe-associated molecular Patterns (MAMPs) are secreted by pathogens. In summary, the diversity of molecules found in the apoplast highlights its importance in the survival of plant cells.</jats:p>

Dampening the DAMPs: How Plants Maintain the Homeostasis of Cell Wall Molecular Patterns and Avoid Hyper-Immunity

“…Many other signals, besides ET, JA, and SA, are known to influence the growth/defense trade-off. ROS in the apoplast are among these signals as their concentration and homeostasis is critical for maintaining the correct balance between growth and defense (Farvardin et al, 2020). A low level of H 2 O 2 in the cell wall, for example, promotes growth and concomitantly suppresses the plant defense (Neuser et al, 2019).…”

“…This may occur upon local injury caused by biotic or abiotic stress or upon a localized loss of CWI during growth and development like, for example, during the formation of lateral roots (Peretto et al, 1992). It is known that H 2 O 2 is involved not only in the strengthening and repairing of the plant tissues (Farvardin et al, 2020) but also in signaling during both immunity and development (Mhamdi and Van Breusegem, 2018;Huang et al, 2019;Sies and Jones, 2020). Spatial distribution of different ROS and their finely-tuned balance drive plant morphogenetic processes (Mhamdi and Van Breusegem, 2018).…”

Highlighting reactive oxygen species as multitaskers in root development

“…In turn, apoplastic ROS are engaged in plant development and responses to extrinsic signals. These ROS regulate plant cell division and cell elongation rate, and hence the whole plant growth ( Podgórska et al., 2017 ); they are also implicated in regulating stomatal closure, cell wall lignification, and callose deposition in response to external signals ( Podgórska et al., 2017 ; Qi et al, 2017 ; Janku et al., 2019 ; Farvardin et al, 2020 ). Obviously, ROS signaling encompasses several pathways within and between various organelles and cellular compartments.…”

The intracellular ROS accumulation in elicitor‐induced immunity requires the multiple organelle‐targeted Arabidopsis NPK1‐related protein kinases

“…Among ROS, H 2 O 2 is the most stable species (half‐life of more than 1 ms) and, therefore, is considered as the predominant ROS involved in cellular signaling (Mhamdi & Van Breusegem, 2018). Many studies on plant immunity have focused on extracellular ROS (Farvardin et al, 2020; Qi, Wang, Gong, & Zhou, 2017), whereas much less is known about the dynamics and the role of intracellular ROS.…”