Phytohormone and Putative Defense Gene Expression Differentiates the Response of ‘Hayward’ Kiwifruit to Psa and Pfm Infections

Abstract

Pseudomonas syringae pv. actinidiae (Psa) and Pseudomonas syringae pv. actinidifoliorum (Pfm) are closely related pathovars infecting kiwifruit, but Psa is considered one of the most important global pathogens, whereas Pfm is not. In this study of Actinidia deliciosa ‘Hayward’ responses to the two pathovars, the objective was to test whether differences in plant defense responses mounted against the two pathovars correlated with the contrasting severity of the symptoms caused by them. Results showed that Psa infections were always more severe than Pfm infections, and were associated with highly localized, differential expression of phytohormones and putative defense gene transcripts in stem tissue closest to the inoculation site. Phytohormone concentrations of jasmonic acid (JA), jasmonate isoleucine (JA-Ile), salicylic acid (SA) and abscisic acid were always greater in stem tissue than in leaves, and leaf phytohormones were not affected by pathogen inoculation. Pfm inoculation induced a threefold increase in SA in stems relative to Psa inoculation, and a smaller 1.6-fold induction of JA. Transcript expression showed no effect of inoculation in leaves, but Pfm inoculation resulted in the greatest elevation of the SA marker genes, PR1 and glucan endo-1,3-beta-glucosidase (β-1,3-glucosidase) (32- and 25-fold increases, respectively) in stem tissue surrounding the inoculation site. Pfm inoculation also produced a stronger response than Psa inoculation in localized stem tissue for the SA marker gene PR6, jasmonoyl-isoleucine-12-hydrolase (JIH1), which acts as a negative marker of the JA pathway, and APETALA2/Ethylene response factor 2 transcription factor (AP2 ERF2), which is involved in JA/SA crosstalk. WRKY40 transcription factor (a SA marker) was induced equally in stems by wounding (mock inoculation) and pathovar inoculation. Taken together, these results suggest that the host appears to mount a stronger, localized, SA-based defense response to Pfm than Psa.

Early Pathogen Recognition and Antioxidant System Activation Contributes to Actinidia arguta Tolerance Against Pseudomonas syringae Pathovars actinidiae and actinidifoliorum

“…Limited bacterial progression within plant tissues is regarded as a tolerance trait. However, until now very few long-term disease assessments under the same controlled environmental conditions have been performed to compare Psa-and Pfm-inoculated A. chinensis and A. arguta plants, hindering the full comprehension of bacterial infection and plant tolerance mechanisms (Wurms et al, 2017a). Datson et al (2015) reported that in A. arguta plants stab-inoculated with a Psa strain, the endophytic bacterial population was lower and more restricted to the site near the inoculation zone when compared with A. chinensis, even four weeks after inoculation.…”

“…Although SAM is the precursor of not only ET but also of polyamines, due to its key regulatory role during the first step of ET biosynthesis it is frequently used as reporter for ET metabolism (Zhao et al, 2017). Genes involved in ET biosynthesis, such as AP2/ERF, EIN2 and SAM, were found to be upregulated just 2 dpi in A. chinensis plants inoculated with Psa (Wurms et al, 2017a;Wurms et al, 2017b, Nunes da Silva et al, 2019. LOX1 increased activity was also previously reported in A. chinensis and A. arguta Psa-and Pfm-inoculated plants, and is regarded as a strategy used by the pathogen to antagonize plant defense mechanisms (Cellini et al, 2014;Nunes da Silva et al, 2019).…”

“…The basal plant resistance mechanisms, also called innate immunity, are the first line of defense against various pathogens, including bacteria, and are triggered via plant cells pattern recognition receptors, which recognize specific pathogen-associated molecular patterns characteristic to each pathogen ( Wurms et al., 2017a ; Michelotti et al., 2018 ; Wang et al., 2018 ). As such, microorganism recognition by host plants involves a complex physiological and molecular reprogramming that encompasses overregulation of pathogenesis-related genes (PR genes) and defense-related proteins ( Michelotti et al., 2018 ).…”

“…Despite being key players in plant defense against pathogens, how phenolic compounds are regulated in Psa- and Pfm-infected kiwifruit plants is still poorly understood. It is clear that the interaction between kiwifruit plants and Psa and Pfm is highly complex, with multiple bacterial factors and signalling events occurring simultaneously in the host plant tissues, which ultimately defines the susceptibility or tolerance of the plant exposed to the pathogen ( Petriccione et al., 2015 ; Wurms et al., 2017a ; Wang et al., 2018 ; Tahir et al., 2019).…”

Identification and Analysis of NBS-LRR Genes in Actinidia chinensis Genome

“…The AP2 transcription factors and ERF transcription factors belong to the AP2/ERF superfamily, which has been proven to participate in various resistance responses [ 44 , 45 ].In the study of P. syringae, which causes various diseases in crops including kiwifruit, the expression of specific AP2/ERF transcription factors enhanced disease resistance in tobacco and Arabidopsis, respectively [ 46 , 47 , 48 ]. Wurms et al found that Psa infection caused an upregulation of AP2/ERF2 transcription factor in kiwifruit [ 49 ].…”