A scientific review published in Plant Cell Reports documents how mitogen-activated protein kinase signaling cascades operate as central regulatory networks controlling fruit quality, ripening, and postharvest resilience. Across agricultural production, fruit quality represents an essential commercial benchmark and breeding target composed of interconnected traits such as pigmentation, firmness, sweetness, aromatic flavor, and storage longevity. According to author Ahmed Alabd, these agronomic characteristics are coordinated not by isolated genetic elements, but through intracellular signaling networks that translate external environmental signals and endogenous developmental cues into biochemical responses. Within these signaling systems, mitogen-activated protein kinase cascades function through a tiered, highly conserved three-kinase mechanism comprising a MAPKK kinase, a MAPK kinase, and a downstream MAPK. Sequential activation occurs through phosphorylation, wherein MAPKKKs activate MAPKKs, which in turn phosphorylate MAPKs, enabling them to recognize and phosphorylate structural proteins, metabolic enzymes, and transcription factors. While earlier academic reviews broadly examined MAPK signaling in general plant growth and stress pathways, this work synthesizes recent molecular investigations across horticultural crops to clarify how these kinase modules dictate trait-specific outcomes, resolve physiological trade-offs, and offer targets for crop enhancement.

The review details the molecular architecture of the canonical three-tiered cascade and demonstrates how MAPKs phosphorylate diverse transcription factor families to modulate gene expression. During this signaling process, activated MAPKs recognize specific downstream targets, such as NAC, MYB, basic leucine zipper, and WRKY transcription factors, altering their stability, transcriptional activity, or protein interactions. For example, experimental evidence demonstrates that MAPK3 and MAPK6 phosphorylate NAC transcription factors to alter ethylene-mediated fruit softening programs, while MAPK3, MPK4, and MPK14 modulate MYB regulators to govern anthocyanin biosynthesis and ripening progression. Downstream basic leucine zipper family members act as substrates for MPK6, MPK6-3, and MAPK11-3, linking cellular signaling directly to fruit maturation and pigment production. Beyond direct phosphorylation of transcriptional regulators, MAPKs engage in physical protein-protein interactions and indirect associations, such as MPK14 interacting with Brassinazole-resistant factors, MPK4 and MPK8 communicating with ethylene response factors, MAPK8 associating with auxin response factors, and MPK6 interacting with MYC transcription factors. Direct phosphorylation also extends to structural enzymes, as observed in strawberry where FvMAPK3 targets the chalcone synthase enzyme FvCHS1.

Investigating the physiological coordination of quality traits, the review emphasizes that MAPK signaling governs pigment biosynthesis and sugar accumulation by integrating external light, temperature, and internal carbohydrate cues. In apple, light exposure triggers two distinct signaling axes: the MdMAPK4-MdMYB1 module and the MdMAPK6-MdHY5 pathway. Activated MdMPK4 physically interacts with and phosphorylates the transcription factor MdMYB1, stabilizing the protein to activate anthocyanin biosynthetic genes, while MdMPK6 phosphorylates MdHY5, increasing its binding affinity for promoters of anthocyanin-associated genes. Conversely, dark conditions stimulate MdMAPK4 to phosphorylate the ethylene response factor MdERF17, which regulates chlorophyll degradation during light-to-dark transitions, demonstrating that a single kinase can route separate environmental signals into distinct fruit coloration outcomes. Temperature cues similarly dictate pigmentation in strawberry, where low temperature activates FvMAPK3, which subsequently suppresses anthocyanin accumulation by phosphorylating FvMYB10 to reduce its transcriptional activity, phosphorylating FvCHS1 to accelerate its proteasomal degradation, and phosphorylating FvWRKY50 to diminish its activation of anthocyanin-related genes. In eggplant, high light triggers SmMPK4.1 to repress anthocyanin accumulation by altering SmMYB75-driven activation of chalcone synthase expression.

Carbohydrate signaling intersects with these pigment networks to coordinate sugar metabolism and coloration simultaneously in ripening fruit tissues. In cultivated strawberry, sucrose exposure induces FvMAPK6, which directly phosphorylates the repressor transcription factors FvMYB44.1 and FvMYB44.2. This specific phosphorylation event reduces both the stability and the transcriptional activity of these repressors, thereby relieving their suppression of downstream target genes including the chalcone isomerase gene FvCHI, the sucrose-phosphate synthase gene FvSPS3, and the sugar transporter gene FvSWEET1. By de-repressing these metabolic targets, FvMAPK6 synchronizes anthocyanin synthesis with sucrose accumulation. Complementary genetic studies reveal that transient overexpression of FaMAPK5 and FaMAPK10 in strawberry fruit elevates anthocyanin, total sugar, and glucose contents. However, the author cautions that MAPK pathways frequently exhibit pleiotropic effects that impose physiological trade-offs. For instance, while FvMAPK6 enhances fruit quality through accelerated accumulation of anthocyanins and sugars, its activity concurrently reduces overall fruit yield and vegetative growth. Exploiting MAPK cascades for crop improvement consequently demands precision to balance desired quality traits against potential agronomic penalties caused by broad kinase activation across non-target tissues.

Postharvest softening and flavor development represent another major domain governed by MAPK phosphorylation cascades, as textural breakdown directly restricts postharvest shelf life, transportability, and storage capacity. In banana fruit, MaMPK2 physically interacts with and phosphorylates the basic leucine zipper regulator MabZIP93, stimulating transcriptional activation of cell-wall-modifying genes, including MaPL2, MaPE1, MaXTH23, and MaXGT1. Similarly, in apple, ethylene induction stimulates MdMAPK3 to phosphorylate the transcription factor MdNAC72, suppressing its transcriptional repression of the polygalacturonase gene MdPG1 and accelerating ethylene-mediated softening. In fig, FcMAPK4 phosphorylates FcNOR, which subsequently activates FcERF5 to upregulate the polygalacturonase FcPG12. Conversely, certain kinases function as postharvest preservation brakes: in banana, MaMPK14 phosphorylates the brassinosteroid-regulated factors MaBZR1 and MaBZR2, strengthening their transcriptional repression of the cell-wall-modifying genes MaEXP2, MaPL2, and MaXET5, while also phosphorylating MaMYB4 to enhance repression of MaACS1, MaXTH5, MaPG3, and MaEXPA15. Regarding aroma and flavor metabolism, FvMAPK6 in strawberry phosphorylates FvJAZ12, which alleviates the repression of FvMYC2 and promotes the expression of lipoxygenase 3, an enzyme involved in jasmonate biosynthesis and flavor-associated metabolic pathways.

Fruit ripening and postharvest senescence represent genetically programmed processes directed by transcription factor networks that rely on MAPK-dependent phosphorylation to modulate hormonal signaling and oxidative dynamics. In banana, MaMAPK11-3 phosphorylates MabZIP74, attenuating its repression of the ethylene biosynthesis genes MaACO1 and MaACO4, while MaMPK6-3 phosphorylates MabZIP21 to stimulate the expression of ripening genes, and MaMAPK1 enhances MaNAC29-mediated activation of MbACO2. In tomato, ripening progression is restrained by SlMAPK4, which phosphorylates SlWRKY6 to downregulate the senescence and ripening-associated genes SlSGR1 and SlSAG12. In apple, MdMAPK6 phosphorylates MdWRKY9 specifically at the Tyr394 residue, increasing its stability and activity to counter the repressor MdERF5L, which relieves suppression of the ethylene synthesis gene MdACS1 and accelerates ripening. Flesh tissue of papaya exhibits marked CpMAPK20 induction upon ethephon exposure, linking the kinase to ethylene-driven postharvest maturation. Conversely, postharvest fruit senescence and superficial browning are driven in apple by MdMPK6/02G, which phosphorylates and stabilizes MdWRKY70L, promoting the downstream expression of MdSAG101 and MdZAT12, triggering reactive oxygen species accumulation that causes cellular decay.

In addition to developmental regulation, MAPK cascades provide critical immune defense signaling against postharvest fungal pathogens that cause severe decay and economic losses during storage. In tomato fruit, SlMPK8 phosphorylates the transcription factor SlERF.C1, which enhances ERF binding to the PR1 and PR1a gene promoters, boosting pathogenesis-related gene expression and conferring resistance against Botrytis cinerea. Similarly, application of yeast cell wall extracts and beta-glucan induces SlMAPK3, driving transcriptional activation of SlPti5 and SlERF1 to induce PR defense genes like SlCHI9, SlPR1, and SlPR5, whereas functional inhibition of SlMPK1, SlMPK2, and SlMPK3 sharply increases susceptibility to Botrytis cinerea. In walnut, JrMAPK3 phosphorylates JrWRKY22, enhancing promoter binding and expression of the pathogenesis-related genes JrPR1 and JrGLU to protect fruit against Colletotrichum gloeosporioides anthracnose infection. In apple, chemical priming with acibenzolar-S-methyl stimulates Penicillium expansum resistance by inducing MdMAPK2, MdMAPK4, and MdMAPKK1 while downregulating MdMAPK3, and MdMAPKKK1 promotes defense against Botryosphaeria dothidea when phosphorylated by the receptor-like cytoplasmic kinase MdBSK1.

Hormone-mediated and chemically induced defense pathways in stone and subtropical fruits also rely heavily on intact kinase signaling cascades. In peach fruit, PpMAPKK5 physically interacts with the basic leucine zipper factor PpTGA1, driving the upregulation of salicylic acid-responsive pathogenesis-related genes to defend postharvest tissues against Rhizopus stolonifer. Furthermore, priming peach fruit with beta-aminobutyric acid activates PpMAPK1, which coordinates with the vesicle-trafficking protein SNARE13 in an NPR1-dependent manner to induce salicylic acid-responsive defense transcripts including PpNPR1, PpPR1, PpBGL2, and PpPR5. In red pitaya fruit, gamma-aminobutyric acid treatment elevates resistance against Gilbertella persicaria by modulating the expression of HcMAPK1, HcMAPK7, HcMAPKK2, HcMAPKK4, and HcMAPKKK1. Strawberry defense responses are similarly controlled by specific kinases: overexpression of FaMAPK19 enhances resistance against Botrytis cinerea, whereas antisense suppression compromises immunity, and FaMAPK5 provides comparable defense protection against fungal infection. These combined findings demonstrate that MAPKs reinforce postharvest defenses through the transcriptional reprogramming of PR genes and the coordination of defense hormone pathways.

While terminal MAPKs have received substantial research attention, the review highlights that complete MAPKKK-MAPKK-MAPK cascades are beginning to be resolved across several fruit species. In banana, an ethylene-induced MaMKK1-MaMPK6-3 and MaMPK11-4 module phosphorylates terminal kinases at conserved pTEpY amino acid motifs, enabling them to activate the transcription factor MabZIP21 during ripening. In postharvest peach fruit, the PpMAPKKK3-PpMAPKK5-PpMAPK1 module orchestrates beta-aminobutyric-acid-induced pattern-triggered immunity against Rhizopus stolonifer. In strawberry, the FvMKK4-FvMAPK3 cascade represses low-temperature anthocyanin accumulation by destabilizing FvWRKY50 through ubiquitination and suppressing FvMYB10 transcriptional activity. Concurrently, the opposing FvMKK4-FvMAPK6 cascade phosphorylates FvRIF, which positively regulates FvMYB10 expression and structural biosynthetic genes to promote fruit ripening, demonstrating how distinct cascade modules achieve antagonistic regulation over identical downstream transcription factors. Beyond ripening and coloration, strawberry FaMAP3K interacts with FaERF13 to modulate organic acid accumulation; ethylene disrupts this protein-protein interaction, relieving transcriptional repression on FaKAT and reducing total fruit acidity during ripening, while tomato SlMKK4 governs pollen development through gene networks tied to auxin and sugar metabolism.

Despite recent discoveries, the review emphasizes that our functional understanding of MAPK cascades remains incomplete and largely restricted to a few model crops such as apple, banana, and strawberry. Most crop-specific MAPKs and their upstream MAPKKKs and MAPKKs remain uncharacterized, and the molecular mechanisms governing MAPK substrate specificity across different environmental conditions are poorly understood. The author proposes that future investigations must integrate single-cell resolution approaches to map the spatiotemporal dynamics of MAPK signaling across distinct fruit cell layers and developmental stages. Furthermore, systematic phosphoproteomics coupled with transcriptomics, metabolomics, and proteomics will be necessary to uncover direct downstream phosphorylation substrates. To translate these molecular insights into crop breeding, the review highlights precision genome-editing tools, specifically base editing and prime editing, to introduce phospho-mimetic or phospho-dead mutations directly into validated MAPK phosphorylation sites. Rather than relying on blunt whole-gene knockouts that trigger undesirable pleiotropic trade-offs, such targeted molecular edits offer a promising avenue to optimize fruit appearance, flavor, texture, shelf life, and disease resilience.

Subject of Research: Regulation of fruit quality traits and postharvest physiology by MAPK signaling cascades

Article Title: MAPK cascades: key regulators of fruit quality traits

Article References: Alabd, A. (2026). MAPK cascades: key regulators of fruit quality traits. Plant Cell Reports, 45(11), Article 354. https://doi.org/10.1007/s00299-026-04004-3

Image Credits: AI Generated

DOI: 10.1007/s00299-026-04004-3

Keywords: MAPK cascades, fruit quality, anthocyanin biosynthesis, fruit softening, fruit ripening, postharvest pathology, plant signaling, plant biotechnology, crop genetics, MAPK, cascades, regulators