Emerging Roles of Mitogen-Activated Protein Kinase Signaling Pathways in the Regulation of Fruit Ripening and Postharvest Quality

Abstract

Fleshy fruit ripening is a unique biological process that involves dramatic changes in a diverse array of cellular metabolisms. The regulation of these metabolisms is essentially mediated by cellular signal transduction of internal (e.g., hormones) and external cues (i.e., environmental stimuli). Mitogen-activated protein kinase (MAPK) signaling pathways play crucial roles in a diverse array of biological processes, such as plant growth, development and biotic/abiotic responses. Accumulating evidence suggests that MAPK signaling pathways are also implicated in fruit ripening and quality formation. However, while MAPK signaling has been extensively reviewed in Arabidopsis and some crop plants, the comprehensive picture of how MAPK signaling regulates fruit ripening and quality formation remains unclear. In this review, we summarize and discuss research in this area. We first summarize recent studies on the expression patterns of related kinase members in relation to fruit development and ripening and then summarize and discuss the crucial evidence of the involvement of MAPK signaling in fruit ripening and quality formation. Finally, we propose several perspectives, highlighting the research matters and questions that should be afforded particular attention in future studies.

Keywords: MAPK; fruit ripening; postharvest quality; signal transduction.

Figure 1

Diagram showing the updated signaling components and pathways of MAPs kinases identified as being implicated in the regulation of fruit ripening and postharvest quality. Fruit ripening and postharvest quality are determined by internal and external signals or, alternatively, by hormonal and environmental signals. Fruit ripening and quality are essentially determined by complicated cellular metabolisms, such as color, sugar, acid, flavor, texture, etc. In addition, anti-pathogenic activity is also an important part of fruit quality, as it determines the postharvest shelf life of fruits. MAPK signaling mediates the internal/external signal-triggered regulation of fruit ripening and quality-associated metabolisms. ‘?’ denotes currently unidentified components/biological events; ‘→’ denotes activation; and a horizontal ‘⊥’ denotes inhibition. Plant species are as follows: ‘Sl’ for tomato ‘Solanum lycopersicum’; ‘Le’ for tomato ‘Lycopersicon esculentum’; ‘Ma’ for banana ‘Musa acuminata’; ‘Fv’ for strawberry ‘Fragaria vesca’; ‘Md’ for apple ‘Malus domestica’.

Figure 2

Diagram showing the updated pattern of cellular signal transduction of ET and ABA with emphasis on the involvement of MAPs kinase in Arabidopsis thaliana. (a) Signal transduction of ET. ET perception by its receptors acts to inhibit CTR1, a Raf-like MAP3K. There exist two signaling pathways downstream of CTR1, i.e., the EIN2 pathway, which acts to control the nuclear accumulation of the EIN3/EIL protein, the core TFs controlling the expression of ET-responsive genes; another is the MAPK signaling pathway, where the MKK7/9-MPK3/6 module acts to link CTR1 with EIN3/EIL. In addition, MPK3/6 act to stabilize ACS via phosphorylation, thereby regulating ET biosynthesis. Noteworthily, MPK3/6 can be also activated by a variety of biotic/abiotic signals. Consequently, the MAPK signaling pathway functions in both mediating ET signal transduction and biotic/abiotic signals modulate ET production. (b) Signal transduction of ABA. ABA perception by its receptors acts to inhibit PP2C, clade-A type-2C protein phosphatases. Besides directly targeting SnRK2s, PP2C may also directly target several MAPs kinases, such as MAP3K17/18. In addition, a large number of MAPs kinases, such as MKK1/3/4/5 and MPK1/2/3/6/7/9/12/14, were reported to be involved in ABA signaling, ultimately regulating a variety of biological processes, such as stomatal movement, seed germination, root growth and abiotic/biotic responses.