Cell Wall Integrity Signaling in Fruit Ripening

Abstract

Plant cell walls are essential structures for plant growth and development as well as plant adaptation to environmental stresses. Thus, plants have evolved signaling mechanisms to monitor the changes in the cell wall structure, triggering compensatory changes to sustain cell wall integrity (CWI). CWI signaling can be initiated in response to environmental and developmental signals. However, while environmental stress-associated CWI signaling has been extensively studied and reviewed, less attention has been paid to CWI signaling in relation to plant growth and development under normal conditions. Fleshy fruit development and ripening is a unique process in which dramatic alternations occur in cell wall architecture. Emerging evidence suggests that CWI signaling plays a pivotal role in fruit ripening. In this review, we summarize and discuss the CWI signaling in relation to fruit ripening, which will include cell wall fragment signaling, calcium signaling, and NO signaling, as well as Receptor-Like Protein Kinase (RLKs) signaling with an emphasis on the signaling of FERONIA and THESEUS, two members of RLKs that may act as potential CWI sensors in the modulation of hormonal signal origination and transduction in fruit development and ripening.

Keywords: FERONIA; THESEUS; cell wall integrity; fruit ripening.

Figure 1

Proposed diagram showing CWI signaling in fruit ripening. Fruit growth and development are accompanied by cell wall degradation, which starts from cell separation resulting from pectin degradation. The cell wall degradation triggers several signaling systems, including RLK-associated signaling, OGs signaling and Ca²⁺ signaling, NO signaling, etc. Among the RLK-associated signals, CrRLK1Ls, particularly FER, and THE, have been established to play pivotal roles in both hormone signal production and transduction. OGs and Ca²⁺ signaling may potentially play important roles in the initiation of fruit ripening via or not via modulation of hormone signaling. NO potentially acts as a signal downstream of OGs signals and is proposed to be linked with multiple signaling pathways, among which the ethylene biosynthesis pathway has been well characterized. *, apoplastic space of tricells, showing cell separation and wall degradation.The wall model is based on a study by Zhang et al. [6] RLK, Receptor Like Protein kinase; OGs, oligosaccharides; FER, Feronia; THE, Theseus. SnRK2, sucrose nonfermenting 1-related protein kinase 2, PYR/PYL, ABA receptor; ABI1/2, protein phosphatase 2C; RopGEF, guanine nucleotide exchange factors; RAC/ROP, small GTPase; ROS, reactive oxygen species; CAT, catalase; NO, nitric oxide; SAMS, S-adenosyl-L-methionine synthetase; ACC, 1-aminocyclopropane-1-carboxylic acid; ACS, ACC synthetase; ACO, ACC oxidase; ETH, ethylene; JA, jasmonic acid; COI1, coronatine-insensitive 1; JAZ, jasmonate ZIM-domain; MAPK, mitogen-activated protein kinase; CDPK, Ca²⁺ dependent protein kinase.