SlMAPK3, a key mitogen-activated protein kinase, regulates the resistance of cherry tomato fruit to Botrytis cinerea induced by yeast cell wall and β-glucan

Highlights

• Yeast cell wall component as MAMPs stimulated disease resistance of cherry tomato fruit.

• The resistance mechanism is associated with induction of related defense genes expression.

• SlMAPK3 played as a key upstream signaling kinase responding yeast cell wall induction.

• SlMAPK3 could directly regulate the expression of specific transcription factors and PR genes.

Abstract

Induced disease resistance of fruit by bio-based compounds is a promising strategy to control fruit decay. This research was aimed at studying the resistance of cherry tomato fruit to Botrytis cinerea induced by the yeast cell wall component from Saccharomyces cerevisiae and investigate a role of MAPKs in regulating the resistance response. The disease resistance of cherry tomato fruit was effectively enhanced by yeast cell wall and β-glucan. The expression of SlMAPK3 (but not of SlMAPK1 and SlMAPK2) was significantly increased by yeast cell wall and β-glucan and reached peak at 1 h. The yeast cell wall component also induced high expression of PR genes (SlPR1, SlPR5 and SlCHI9) and the transcription factors (SlERF1 and SlPti5) that specifically bind to the promoter of PR genes. The expression of PR genes (SlPR1, SlPR5 and SlCHI9) peaked after 24 h (at 24 or 48 h). The peak of SlERF1 and SlPti5 gene expression mostly appeared at around 4 h. It supposed to be a chronological order in the peaks of gene expression profile among SlMAPK3, PR genes and transcription factors. U0126 (1,4-diamino-2,3-dicyano-1,4-bis(o-amino-phenylmercapto)butadiene) significantly inhibited transcription of SlMAPK3, PR genes and transcription factors. The yeast cell wall and β-glucan could not induce high expression of SlMAPK3 and the downstream genes of SlMAPK3 in the U0126 treatment. These findings indicated that the yeast cell wall component that acts as microbe associated molecular patterns (MAMPs) could effectively induce disease resistance in cherry tomato fruit after harvest. The mechanism of induced resistance was associated with the expression of SlMAPK3 and defense-related genes. SlMAPK3, as an important upstream signaling kinase, had a direct regulatory effect on the downstream transcriptional factors (SlERF1 and SlPti5) to activate the expression of PR genes in the yeast cell wall component-induced immune responses in cherry tomato fruit.

Introduction

Cherry tomato (Solanum lycopersicum var. cerasiforme) is consumed around the world for its nutritional value and great taste. However, cherry tomato fruits are highly susceptible to various pathogenic fungi, Botrytis cinerea, Alternaria alternata and Rhizopus stolonifera, especially in china; large amounts of cherry tomato fruit are spoiled during postharvest stage (Tang et al., 2019; Wang et al., 2008). Although the most common method to control the postharvest decay remains the application of synthetic chemical fungicides, the potential toxicity and negative impact on both human health and the environment limit the continued use of chemical fungicides. Therefore, finding safe, cheap and eco-friendly alternatives to synthetic fungicides is urgently needed to control postharvest diseases of fruits.

Induction of fruit disease resistance using bio-based compounds represents a promising and alternative strategy to chemical pesticides for controlling postharvest disease (Burketova et al., 2015; Romanazzi et al., 2016). Numerous compounds, including phytohormones (salicylic acid, methyl jasmonate and γ-aminobutyric acid) and antagonistic yeasts (eg. Cryptococcus laurentii, Yarrowia lipolytica), have been reported to induce resistance against pathogens and were efficient in controlling postharvest diseases (Guo et al., 2014; He et al., 2017; Yang et al., 2017; Zhang et al., 2017).

Plant innate immune system recognizes the microbe- or pathogen-associated molecular patterns (MAMPs or PAMPs) and plant defense is induced by pathogens. MAMPs, as commonly conserved structures, include lipopolysaccharides (the principal component of the outer membrane of gram-negative bacteria), flagellin (the protein subunit of the bacterial flagellar filament) and chitin (the main structural component of cell walls in fungi) (Zipfel and Felix, 2005). MAMPs induce the early plant defense responses, including the reactive oxygen species burst and generation of nitric oxide (NO), activation of calcium-dependent protein kinases, and the mitogen activated protein kinase (MAPK) signaling cascades. Then, MAMPs induce the late resistance responses, includes production of antimicrobial compounds (phytoalexins, alkaloids, flavonoids, and others), deposition of callose in the cell wall, and high expression of pathogenesis-related (PR) genes (He et al., 2007; Nurnberger, 2004).

Activation of MAPK cascades is one of the earliest signaling response pathways after plants get infected by pathogens, subsequently regulating the biosynthesis of phytoalexins and expression of defense genes (Meng and Zhang, 2013). In tomato, MAPK1/2/3 were involved in the resistance of tomato plant to bacterial wilt (Chen et al., 2009) and herbivorous insects (Kandoth et al., 2007). MAPK1/2/3 were also involved in mediating the disease resistance of tomato fruits against B. cinerea (Zheng et al., 2014).

The antagonistic yeasts can induce disease resistance by enhancing the activity of defense enzymes and inducing transcription of defense-related genes in citrus (Guo et al., 2014), apple (Zhang et al., 2017), pear (Yu et al., 2007), and cherry tomato (Guo et al., 2016; Lai et al., 2018). The yeast cell wall acted as MAMPs to induce disease resistance in Arabidopsis thaliana and Brassica rapa against bacterial and fungal pathogens by inducing the early activation of jasmonate/ethylene and the late activation of salicylic acid pathways and enhanced expression of several plant defense-related genes (Narusaka et al., 2015). The yeast cell wall also enhanced transcription of PR genes and increased activities of defense-related enzymes in pear fruit, enhancing disease resistance against Penicillium expansum (Sun et al., 2018). However, disease resistance of postharvest fruit induced by yeast cell wall has not been tested broadly; in addition, the activation and regulatory effect of MAPKs in the resistance response induced by yeast and yeast cell wall have not been well explored yet.

Therefore, the objectives of the present study were to (1) assess whether the yeast cell wall and its main component, β-glucan, induce the resistance against B. cinerea in cherry tomato fruit; (2) analyze effects of the yeast cell wall component on the gene expression of MAPK1/2/3, defense-related PRs gene and transcription factors; (3) investigate the regulation of MAPK1/2/3 in the resistance response induced by yeast cell wall and the regulatory effect of MAPKs in the expression of defense-related genes using U0126 (1,4-diamino-2,3-dicyano-1,4-bis(o-amino-phenylmercapto)butadiene), a specific inhibitor of MAPKK that blocks the MAPK pathway.