Molecular mechanism of BjCHI1-mediated plant defense against Botrytis cinerea infection

ABSTRACT

Plant chitinases are a group of proteins associated with defense against pathogen attack. BjCHI1 is the first characterized chitinase containing two chitin binding domains (CBDs). Investigations have shown that BjCHI1 inhibits growth of fungal phytopathogens and agglutinates Gram-negative bacteria. Our recent studies revealed that expression of BjCHI1 mRNA is largely induced upon infection of Botrytis cinerea via a R2R3-MYB transcription factor BjMYB1 interacting with a W box-like element (Wbl-4) in the BjCHI1 promoter. The enhanced expression pattern of BjMYB1 was similar to that of BjCHI1 and associated with resistant phenotype against B. cinerea. These findings suggest that BjCHI1 is involved in host defense against fungal attack through interaction with BjMYB1. Here, we review the recent studies on BjCHI1 and propose a model of BjCHI1-mediated plant defense against fungal attack.

Plants are constantly exposed to a large group of pathogens. Botrytis cinerea is the second most important plant fungal pathogen which infects more than 200 plant species and causes massive economic losses in agriculture.^1,2 To defense pathogen attack, plants have evolved mechanisms apperceiving pathogens and activating resistance genes to protect themselves.³ Chitinases, a class of pathogenesis-related proteins, play a major role in plant defense against pathogen attack. They inhibit growth of fungal pathogens via catalyzing the random cleavage of the internal β-1,4 glycosidic linkages in chitin, an important structural component of fungal cell walls.⁴ In the last decade, significant advances have been made in understanding the mechanisms and molecular interactions of chitinases in relation to immune response regulation.⁵ BjCHI1, the first characterized chitinase with two CBDs identified in Brassica juncea, has been extensively investigated.^6-12 We here review the recent studies on BjCHI1 and propose a model of BjCHI1-mediated plant defense against fungal attack.

Fungal inductivity of BjCHI1 and fungus-responsive element in the BjCHI1 promoter

As a major constituent of fungal cell walls, chitin has been used as a fungal elicitor in research on plant defense against fungi.^13-17 We examined mRNA expression of BjCHI1 in B. juncea seedlings and found that the transcription level was strongly enhanced upon treatments with either chitin elicitor (hexa-N-acetylchitohexaose) or B. cinerea infection, suggesting that some chitin-responsive cis-acting element exists in the BjCHI1 promoter (BjC-P).^11,12 We further cloned BjC-P and investigated its fungal inductivity, resulting in identification of the core fungus-responsive element Wbl-4 (GTAGTGACTCAT, −668 to −657) in BjC-P.¹¹

Transcription factor BjMYB1 regulates the expression of BjCHI1

To further investigate the regulation mechanism of BjCHI1-mediated plant defense against B. cinerea, we adopted the core fungus-inducible element Wbl-4 as a target and performed yeast one-hybrid screening of a B. juncea cDNA library, resulting in isolation of a R2R3-MYB transcription factor designated as BjMYB1.¹² Further investigations revealed that BjMYB1 upregulated the activity of the BjC-P promoter by interacting with the Wbl-4 element.¹¹ Coincidentally, BjMYB1 exhibited an induced expression pattern similar to that of BjCHI1 in B. juncea plants upon infection of B. cinerea. Moreover, heterogeneous overexpression of BjMYB1 significantly elevated the resistance of transgenic Arabidopsis thaliana to the fungus B. cinerea.¹² These findings implicated that BjCHI1 functions in defense against fungal attack via BjMYB1′s binding to the Wbl-4 element in the BjC-P promoter and activating expression of BjCHI1.

A possible model of BjCHI1-mediated plant defense against fungal attack

It is known that receptors for chitin oligosaccharides are plasma membrane-localized lysine motif (LysM) proteins such as OsCEBiP and OsCERK1 in rice, AtLYK5 and AtCERK1 in Arabidopsis.^13,18-22 In rice, OsCEBiP and OsCERK1 form an active receptor complex for recognizing chitin oligosaccharides.^23,24 Similarly, the LYK5-CERK1 complex recognizes chitin in Arabidopsis.^13,25 OsCERK1 and AtCERK1 phosphorylate the cytoplasmic kinases OsRLCK185 and PBL27, respectively. Subsequently, phosphorylated OsRLCK185 and PBL27 connect chitin perception to MAPK activation.^19,26,27 Since BjCHI1 and BjMYB1 can be activated by treatment of chitin oligosaccharides, we speculate that the same type chitin receptor gene exists in B. juncea. Indeed, a partial cds encoding the LysM domain receptor-like kinase 1 (GenBank: AB826463.1) from B. juncea was found in the database of NCBI (https://www.ncbi.nlm.nih.gov/nuccore/AB826463.1). Predicted homolog (LOC103869159) of chitin elicitor receptor kinases also exists in genome of B. rapa that evolutionally provided the AA genome of B. juncea. Based on this information and the findings from studies on BjCHI1 and BjMYB1, we here propose a model of BjCHI1-mediated plant defense against fungal attack (Fig. 1). In this model, chitin oligosaccharides released from fungal cell walls were recognized by the lysine motif-containing receptor complex (PRR complex) formed with homologs of OsCEBiP/OsCERK1 or AtLYK5/AtCERK1. The PRR complex then recruits homolog of OsRLCK185 or AtPBL27 to activate mitogen-activated protein kinase (MAPK) cascade. The defense signaling was then transduced downstream through unknown signal molecules to activate the R2R3-type transcriptional factor protein BjMYB1. Induced BjMYB1 enters into the nucleus and binds to the Wbl4 element in the promoter BjC-P to activate the expression of BjCHI1. Accumulated BjCHI1 facilitates the cleavage of chitin in the fungal cell walls, resulting in defense to the fungal infection.

Figure 1.

A model of BjCHI1-mediated fungal defense.

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.

Funding

This work was supported by the National Natural Science Foundation of China (31470367 and 31071484).