Induced Resistance by Ascorbate Oxidation Involves Potentiating of the Phenylpropanoid Pathway and Improved Rice Tolerance to Parasitic Nematodes

Redox signalling in plant-nematode interactions: Insights into molecular crosstalk and defense mechanisms

Plant-parasitic nematodes, specifically cyst nematodes (CNs) and root-knot nematodes (RKNs), pose significant threats to global agriculture, leading to substantial crop losses. Both CNs and RKNs induce permanent feeding sites in the root of their host plants, which then serve as their only source of nutrients throughout their lifecycle. Plants deploy reactive oxygen species (ROS) as a primary defense mechanism against nematode invasion. Notably, both CNs and RKNs have evolved sophisticated strategies to manipulate the host's redox environment to their advantage, with each employing distinct tactics to combat ROS. In this review, we have focused on the role of ROS and its scavenging network in interactions between host plants and CNs and RKNs. Overall, this review emphasizes the complex interplay between plant defense mechanism, redox signalling and nematode survival tactics, suggesting potential avenues for developing innovative nematode management strategies in agriculture.

Silencing of a Nicotiana benthamiana ascorbate oxidase gene reveals its involvement in resistance against cucumber mosaic virus

MAIN CONCLUSION

Silencing of an ascorbate oxidase (AO) gene in N. benthamiana enhanced disease severity from cucumber mosaic virus (CMV), showing higher accumulation and expansion of the spreading area of CMV. A Nicotiana benthamiana ascorbate oxidase (NbAO) gene was found to be induced upon cucumber mosaic virus (CMV) infection. Virus-induced gene silencing (VIGS) was employed to elucidate the function of AO in N. benthamiana. The tobacco rattle virus (TRV)-mediated VIGS resulted in an efficient silencing of the NbAO gene, i.e., 97.5% and 78.8% in relative quantification as compared to the control groups (TRV::eGFP- and the mock-inoculated plants), respectively. In addition, AO enzymatic activity decreased in the TRV::NtAO-silenced plants as compared to control. TRV::NtAO-mediated NbAO silencing induced a greater reduction in plant height by 15.2% upon CMV infection. CMV titer at 3 dpi was increased in the systemic leaves of NbAO-silenced plants (a 35-fold change difference as compared to the TRV::eGFP-treated group). Interestingly, CMV and TRV titers vary in different parts of systemically infected N. benthamiana leaves. In TRV::eGFP-treated plants, CMV accumulated only at the top half of the leaf, whereas the bottom half of the leaf was "occupied" by TRV. In contrast, in the NbAO-silenced plants, CMV accumulated in both the top and the bottom half of the leaf, suggesting that the silencing of the NbAO gene resulted in the expansion of the spreading area of CMV. Our data suggest that the AO gene might function as a resistant factor against CMV infection in N. benthamiana.

The phenylalanine ammonia-lyase inhibitor AIP induces rice defence against the root-knot nematode Meloidogyne graminicola

The phenylalanine ammonia-lyase (PAL) enzyme catalyses the conversion of l-phenylalanine to trans-cinnamic acid. This conversion is the first step in phenylpropanoid biosynthesis in plants. The phenylpropanoid pathway produces diverse plant metabolites that play essential roles in various processes, including structural support and defence. Previous studies have shown that mutation of the PAL genes enhances disease susceptibility. Here, we investigated the functions of the rice PAL genes using 2-aminoindan-2-phosphonic acid (AIP), a strong competitive inhibitor of PAL enzymes. We show that the application of AIP can significantly reduce the PAL activity of rice crude protein extracts in vitro. However, when AIP was applied to intact rice plants, it reduced infection of the root-knot nematode Meloidogyne graminicola. RNA-seq showed that AIP treatment resulted in a rapid but transient upregulation of defence-related genes in roots. Moreover, targeted metabolomics demonstrated higher levels of jasmonates and antimicrobial flavonoids and diterpenoids accumulating after AIP treatment. Furthermore, chemical inhibition of the jasmonate pathway abolished the effect of AIP on nematode infection. Our results show that disturbance of the phenylpropanoid pathway by the PAL inhibitor AIP induces defence in rice against M. graminicola by activating jasmonate-mediated defence.

Modulation of ethylene and ROS-scavenging enzymes by multifarious plant growth-promoting endophytes in tomato (Solanum lycopersicum) plants to combat Xanthomonas -induced stress

The purpose of the current study was to explore root endophytes- Priestia megaterium T3 and Bacillus cereus T4 from Moringa olefiera for the suppression of leaf spot disease in tomato plants challenged with Xanthomonas vesicatoria. Both strains had plant growth-stimulating characteristics including auxin production, solubilization of inorganic phosphate and zinc complexes, and production of ammonia, siderophore, as well as hydrolytic enzymes. An agar well diffusion and fluorescence viability assay have validated the antibacterial effect of the cell-free culture supernatant of strains T3 and T4. Liquid chromatography-mass spectrometry (LC-MS) profiling has identified the secondary metabolites in the cell-free supernatant of strains T3 and T4. The bio-priming of tomato seeds with a consortium of T3 and T4 strains has significantly declined ethylene (by 0.61-fold) and hydrogen peroxide (H2O2, 0.64-fold) concentration thus, maintaining a lower content of ROS-induced malondialdehyde (MDA, 0.91-fold) as compared to control counterparts. Consequently, the leaf spot disease severity was reduced by ∼70% in consortium-treated tomato plants in contrast to their pathogen-challenged control. The consortia (T3+T4) treatment has facilitated induced systemic resistance by enhancing enzymatic activities of phenylalanine ammonia-lyase (PAL), peroxidase (PO), polyphenol oxidase (PPO), catalase (CAT), and ascorbate oxidase (AO) to detoxify the excessive Xanthomonas-induced ROS accumulation in tomato plants. Conclusively, bacterial endophytes modulate X. vesicatoria-induced ROS response and ethylene levels in tomato plants. The current findings indicate that plant growth-promoting endophytic bacterial strains hold the potential to sustainably enhance plant growth and suppress bacterial leaf spot disease in tomato plants.

Priming seeds for the future: Plant immune memory and application in crop protection

Plants have evolved adaptive strategies to cope with pathogen infections that seriously threaten plant viability and crop productivity. Upon the perception of invading pathogens, the plant immune system is primed, establishing an immune memory that allows primed plants to respond more efficiently to the upcoming pathogen attacks. Physiological, transcriptional, metabolic, and epigenetic changes are induced during defense priming, which is essential to the establishment and maintenance of plant immune memory. As an environmental-friendly technique in crop protection, seed priming could effectively induce plant immune memory. In this review, we highlighted the recent advances in the establishment and maintenance mechanisms of plant defense priming and the immune memory associated, and discussed strategies and challenges in exploiting seed priming on crops to enhance disease resistance.