Daniela Paula de Toledo Thomazella, Kyungyong Seong, Rebecca Mackelprang, Douglas Dahlbeck, Yu Geng, Upinder S. Gill, Tiancong Qi, Julie Pham, Priscila Giuseppe, Clara Youngna Lee, Arturo Ortega, Myeong-Je Cho, Samuel F. Hutton, and Brian Staskawicz

PNAS July 6, 2021 118 (27) e2026152118

Significance

Susceptibility (S) genes are plant genes that facilitate pathogen infection. Inactivation of S genes has been considered a promising strategy to obtain broad-spectrum and durable resistance in crops. We characterized two orthologs of the Arabidopsis S gene DMR6 in tomato: SlDMR6-1 and SlDMR6-2. We show that SlDMR6-1, but not SlDMR6-2, is associated with plant immunity. Remarkably, Sldmr6-1 mutants display enhanced resistance to bacterial, oomycete, and fungal pathogens. This phenotype correlates with increased levels of the defense hormone salicylic acid (SA) and enhanced transcriptional activation of plant immune responses. We also demonstrate that SlDMR6-1 and SlDMR6-2 convert SA into its inactive form, 2,5-DHBA, indicating that they play a role in SA homeostasis.

Abstract

Plant diseases are among the major causes of crop yield losses around the world. To confer disease resistance, conventional breeding relies on the deployment of single resistance (R) genes. However, this strategy has been easily overcome by constantly evolving pathogens. Disabling susceptibility (S) genes is a promising alternative to R genes in breeding programs, as it usually offers durable and broad-spectrum disease resistance. In Arabidopsis, the S gene DMR6 (AtDMR6) encodes an enzyme identified as a susceptibility factor to bacterial and oomycete pathogens. Here, we present a model-to-crop translational work in which we characterize two AtDMR6 orthologs in tomato, SlDMR6-1 and SlDMR6-2. We show that SlDMR6-1, but not SlDMR6-2, is up-regulated by pathogen infection. In agreement, Sldmr6-1 mutants display enhanced resistance against different classes of pathogens, such as bacteria, oomycete, and fungi. Notably, disease resistance correlates with increased salicylic acid (SA) levels and transcriptional activation of immune responses. Furthermore, we demonstrate that SlDMR6-1 and SlDMR6-2 display SA-5 hydroxylase activity, thus contributing to the elucidation of the enzymatic function of DMR6. We then propose that SlDMR6 duplication in tomato resulted in subsequent subfunctionalization, in which SlDMR6-2 specialized in balancing SA levels in flowers/fruits, while SlDMR6-1 conserved the ability to fine-tune SA levels during pathogen infection of the plant vegetative tissues. Overall, this work not only corroborates a mechanism underlying SA homeostasis in plants, but also presents a promising strategy for engineering broad-spectrum and durable disease resistance in crops.

See: https://www.pnas.org/content/118/27/e2026152118

Figure 1: Identification and expression analyses of AtDMR6 orthologs in tomato. (A) Phylogenetic tree of the 2-ODD in plants. The 2-ODD homologs collected from 13 different plant species were used to infer the tree. The clades that contain functionally known 2-ODD members, including DMR6, DLO, FLS, ANS, FNS, and F3H, are annotated in different colors. (B) Phylogenetic tree of the DMR6 and DLO clades. The DMR6 and DLO clades were zoomed in from the phylogenetic tree given in A. The blue dots on the nodes indicate bootstrap values ≥ 70. (C) Up-regulation of SlDMR6-1 gene in response to different pathogens: X. gardneri (P = 0.0012), P. syringae (FDR = 2.69E-95), P. capsici (P = 0.00419), and M. perniciosa (FDR_12h = 6E-4, FDR_24h = 3.27E-7, FDR_48h = 0.045). SlDMR6-2 was not expressed or did not appear among the DEGs under the tested conditions. FDRs and P values are represented by yellow and gray asterisks, respectively. ns, not significant, P/FDR ≥ 0.05; *P/FDR < 0.05; **P/FDR < 0.01; ***P/FDR <0.001. Gene-expression values for P. syringae, P. capsici, and M. perniciosa were obtained from public transcriptome data (43⇓–45) and were differentially normalized. X. gardneri gene expression was obtained by qPCR (SI Appendix, Materials and Methods).