Hypersensitivity-Related Genes HSR201 and HSR203J Are Regulated by Calmodulin-Binding Protein 60-Type Transcription Factors and Required for Pathogen Signal-Induced Salicylic Acid Synthesis

Peroxisomal Localization of Benzyl Alcohol O-Benzoyltransferase HSR201 is Mediated by a Non-canonical Peroxisomal Targeting Signal and Required for Salicylic Acid Biosynthesis

The phytohormone salicylic acid (SA) regulates plant responses to various types of environmental stress, particularly pathogen infections. We previously revealed that the benzyl alcohol O-benzoyltransferase HSR201 was required for pathogen signal-induced SA synthesis, and its overexpression together with NtCNL, encoding a cinnamate-coenzyme A ligase, was sufficient for the production of significant amounts of SA in tobacco. We herein examined the subcellular localization of HSR201 and found that it fused to a yellow fluorescent protein localized in peroxisomes. Most peroxisomal matrix proteins possess peroxisomal targeting signal type-1 (PTS1) located at the extreme C-terminus or PTS2 located at the N-terminus; however, a bioinformatics analysis failed to identify similar signals for HSR201. Deletion and mutation analyses of HSR201 identified one essential (extreme C-terminal Leu460) and three important (Ile455, Ile456 and Ala459) amino acid residues for its peroxisomal localization. The virus-induced gene silencing (VIGS) of PEX5, a PTS1 receptor, but not PEX7, a PTS2 receptor, compromised the peroxisomal targeting of HSR201 in Nicotiana benthamiana. When overexpressed with NtCNL, HSR201 mutants with reduced or non-peroxisomal targeting induced lower SA levels than the wild type; however, these mutations did not affect the protein stability or activity of HSR201. VIGS of the HSR201 homolog compromised pathogen signal-induced SA accumulation in N. benthamiana, which was complemented by the HSR201 wild type, but not the mutant with non-peroxisomal targeting. These results suggest that the peroxisomal localization of HSR201 is mediated by a non-canonical PTS1 and required for SA biosynthesis.

CBP60b clade proteins are prototypical transcription factors mediating immunity

Transcriptional reprogramming is critical for plant immunity. Several calmodulin (CaM)-binding protein 60 (CBP60) family transcription factors (TFs) in Arabidopsis (Arabidopsis thaliana), including CBP60g, systemic acquired resistance deficient 1 (SARD1), CBP60a, and CBP60b, are critical for and show distinct roles in immunity. However, there are additional CBP60 members whose function is unclear. We report here that Arabidopsis CBP60c-f, 4 uncharacterized CBP60 members, play redundant roles with CBP60b in the transcriptional regulation of immunity responses, whose pCBP60b-driven expression compensates the loss of CBP60b. By contrast, neither CBP60g nor SARD1 is interchangeable with CBP60b, suggesting clade-specific functionalization. We further show that the function of CBP60b clade TFs relies on DNA-binding domains (DBDs) and CaM-binding domains, suggesting that they are downstream components of calcium signaling. Importantly, we demonstrate that CBP60s encoded in earliest land plant lineage Physcomitrium patens and Selaginella moellendorffii are functionally homologous to Arabidopsis CBP60b, suggesting that the CBP60b clade contains the prototype TFs of the CBP60 family. Furthermore, tomato and cucumber CBP60b-like genes rescue the defects of Arabidopsis cbp60b and activate the expression of tomato and cucumber SALICYLIC ACID INDUCTION DEFICIIENT2 (SID2) and ENHANCED DISEASE SUSCEPTIBILITY 1 (EDS1) genes, suggesting that immune response pathways centered on CBP60b are also evolutionarily conserved. Together, these findings suggest that CBP60b clade TFs are functionally conserved in evolution and positively mediate immunity.

OsACA9, an Autoinhibited Ca2+-ATPase, Synergically Regulates Disease Resistance and Leaf Senescence in Rice

Calcium (Ca2+) is a versatile intracellular second messenger that regulates several signaling pathways involved in growth, development, stress tolerance, and immune response in plants. Autoinhibited Ca2+-ATPases (ACAs) play an important role in the regulation of cellular Ca2+ homeostasis. Here, we systematically analyzed the putative OsACA family members in rice, and according to the phylogenetic tree of OsACAs, OsACA9 was clustered into a separated branch in which its homologous gene in Arabidopsis thaliana was reported to be involved in defense response. When the OsACA9 gene was knocked out by CRISPR/Cas9, significant accumulation of reactive oxygen species (ROS) was detected in the mutant lines. Meanwhile, the OsACA9 knock out lines showed enhanced disease resistance to both rice bacterial blight (BB) and bacterial leaf streak (BLS). In addition, compared to the wild-type (WT), the mutant lines displayed an early leaf senescence phenotype, and the agronomy traits of their plant height, panicle length, and grain yield were significantly decreased. Transcriptome analysis by RNA-Seq showed that the differentially expressed genes (DEGs) between WT and the Osaca9 mutant were mainly enriched in basal immune pathways and antibacterial metabolite synthesis pathways. Among them, multiple genes related to rice disease resistance, receptor-like cytoplasmic kinases (RLCKs) and cell wall-associated kinases (WAKs) genes were upregulated. Our results suggest that the Ca2+-ATPase OsACA9 may trigger oxidative burst in response to various pathogens and synergically regulate disease resistance and leaf senescence in rice.

Hen Egg White Lysozyme (HEWL) Confers Resistance to Verticillium Wilt in Cotton by Inhibiting the Spread of Fungus and Generating ROS Burst

Verticillium wilt is a soil-borne vascular disease caused by the fungal pathogen Verticillium dahliae. It causes great harm to upland cotton (Gossypium hirsutum) yield and quality. A previous study has shown that Hen egg white lysozyme (HEWL) exerts strong inhibitory activity against V. dahliae in vitro. In the current study, we introduced the HEWL gene into cotton through the Agrobacterium-mediated transformation, and the exogenous HEWL protein was successfully expressed in cotton. Our study revealed that HEWL was able to significantly inhibit the proliferation of V. dahlia in cotton. Consequently, the overexpression of HEWL effectively improved the resistance to Verticillium wilt in transgenic cotton. In addition, ROS accumulation and NO content increased rapidly after the V. dahliae inoculation of plant leaves overexpressing HEWL. In addition, the expression of the PR genes was significantly up-regulated. Taken together, our results suggest that HEWL significantly improves resistance to Verticillium wilt by inhibiting the growth of pathogenic fungus, triggering ROS burst, and activating PR genes expression in cotton.

Structural diversity and stress regulation of the plant immunity-associated CALMODULIN-BINDING PROTEIN 60 (CBP60) family of transcription factors in Solanum lycopersicum (tomato)

Cellular signaling generates calcium (Ca2+) ions, which are ubiquitous secondary messengers decoded by calcium-dependent protein kinases, calcineurins, calreticulin, calmodulins (CAMs), and CAM-binding proteins. Previous studies in the model plant Arabidopsis thaliana have shown the critical roles of the CAM-BINDING PROTEIN 60 (CBP60) protein family in plant growth, stress responses, and immunity. Certain CBP60 factors can regulate plant immune responses, like pattern-triggered immunity, effector-triggered immunity, and synthesis of major plant immune-activating metabolites salicylic acid (SA) and N-hydroxypipecolic acid (NHP). Although homologous CBP60 sequences have been identified in the plant kingdom, their function and regulation in most species remain unclear. In this paper, we specifically characterized 11 members of the CBP60 family in the agriculturally important crop tomato (Solanum lycopersicum). Protein sequence analyses revealed that three CBP60 homologs have the closest amino acid identity to Arabidopsis CBP60g and SARD1, master transcription factors involved in plant immunity. Strikingly, AlphaFold deep learning-assisted prediction of protein structures highlighted close structural similarity between these tomato and Arabidopsis CBP60 homologs. Conserved domain analyses revealed that they possess CAM-binding domains and DNA-binding domains, reflecting their potential involvement in linking Ca2+ signaling and transcriptional regulation in tomato plants. In terms of their gene expression profiles under biotic (Pseudomonas syringae pv. tomato DC3000 pathogen infection) and/or abiotic stress (warming temperatures), five tomato CBP60 genes were pathogen-responsive and temperature-sensitive, reminiscent of Arabidopsis CBP60g and SARD1. Overall, we present a genome-wide identification of the CBP60 gene/protein family in tomato plants, and we provide evidence on their regulation and potential function as Ca2+-sensing transcriptional regulators.

The Peroxisomal β-Oxidative Pathway and Benzyl Alcohol O-Benzoyltransferase HSR201 Cooperatively Contribute to the Biosynthesis of Salicylic Acid

The phytohormone salicylic acid (SA) regulates plant defense responses against pathogens. Previous studies have suggested that SA is mainly produced from trans-cinnamic acid (CA) in tobacco, but the underlying mechanisms remain largely unknown. SA synthesis is activated by wounding in tobacco plants in which the expression of WIPK and SIPK, two mitogen-activated protein kinases, is suppressed. Using this phenomenon, we previously revealed that HSR201 encoding benzyl alcohol O-benzoyltransferase is required for pathogen signal-induced SA synthesis. In this study, we further analyzed the transcriptomes of wounded WIPK/SIPK-suppressed plants and found that the expression of NtCNL, NtCHD and NtKAT1, homologous to cinnamate-coenzyme A (CoA) ligase (CNL), cinnamoyl-CoA hydratase/dehydrogenase (CHD) and 3-ketoacyl-CoA thiolase (KAT), respectively, is associated with SA biosynthesis. CNL, CHD and KAT constitute a β-oxidative pathway in the peroxisomes and produce benzoyl-CoA, a precursor of benzenoid compounds in petunia flowers. Subcellular localization analysis showed that NtCNL, NtCHD and NtKAT1 localize in the peroxisomes. Recombinant NtCNL catalyzed the formation of CoA esters of CA, whereas recombinant NtCHD and NtKAT1 proteins converted cinnamoyl-CoA to benzoyl-CoA, a substrate of HSR201. Virus-induced gene silencing of any one of NtCNL, NtCHD and NtKAT1 homologs compromised SA accumulation induced by a pathogen-derived elicitor in Nicotiana benthamiana leaves. Transient overexpression of NtCNL in N. benthamiana leaves resulted in SA accumulation, which was enhanced by co-expression of HSR201, although overexpression of HSR201 alone did not cause SA accumulation. These results suggested that the peroxisomal β-oxidative pathway and HSR201 cooperatively contribute to SA biosynthesis in tobacco and N. benthamiana.