Figure 2.

Molecular regulation mechanisms of cell wall (CW) remodeling related to plant drought tolerance. Receptor-like kinases (RLKs) such as leucine-rich repeat receptor-like kinases (LRRKs), wall-associated kinases (WAKs), and extracellular proteins, including extracellular leucine-rich repeat-containing extension 1(LRX1) perceive drought-triggered CW damage or composition and/or structural alterations to orchestrate CW stress signaling, which is mediated via mitogen-activated protein kinases (MAPKs). The MAPK cascade regulate turgor maintenance through vacuolar invertase, and together with NACs and MYB transcription factors, regulate the activation of downstream CW remodeling-involved genes. The CWI maintenance pathway crosstalk with ROS and phytohormonal signaling pathways to actuate plant drought tolerance. Plasma membrane (PM)-localized proline-rich extension-like receptor kinase 4 (PERK4) interacts with pectin and stimulates Ca²⁺ channels, resulting in cytosolic Ca²⁺ burst, which alters intracellular and extracellular pH, and trigger NADPH oxidase (NOX)-reliant reactive oxygen species (ROS) accumulation. The Ca²⁺ influx evokes respiratory burst oxidase homologues, causing ROS accumulation in the cell wall. This, together with peroxidase (PRX) activity, facilitates oxidative crosslinking of extensins and pectin accumulation, stiffening the cell wall. On the other hand, limited PRX activity or H₂O₂ generation trigger OH•− radicals accumulation, increased severing of sugar bonds in polysaccharides, and eventual CW loosening. THESEUS1 (THE1) perceives cellulose reduction-induced structural defects in CW, whereas COBRA is a PM-localized and GPI-anchored protein crucial for cellulose microfibrils alignment. ANX1/2, ANXUR1/2 (closest homologs of FER in Arabidopsis thaliana) modulate pollen tube rupture which encompass rapid alterations in CW composition and architecture (Ringli, 2010). Complete black arrows denote cell wall integrity (CWI) maintenance signaling/mechanisms, blue lines relate to phytohormonal regulation and lignification mechanisms already discussed in detail in other sections, red lines relate to reactive oxygen species (ROS) signaling, whereas black dotted lines imply crosstalk among these pathways. ABA, abscisic acid; CP, cytoplasm; EXPAs, expansins; FER1/2, FERONIA 1/2; GRP3, Glycine-rich protein 3; LACs, laccases; JA, jasmonic acid; ROP-GEF, Rho-of-plants-guanosine nucleotide exchange factors; SA, salicylic acid; SHR, SHORT-ROOT transcription factor; SUB, SUBERMAN transcription factor. The illustration is based on (Ringli, 2010; Novaković et al., 2018; Gigli-Bisceglia et al., 2020; Bacete et al., 2022) and others discussed in text.