Nitrogen Supply Alters Rice Defense Against the Striped Stem Borer Chilo suppressalis

Plant nutrition status is closely associated with plant defense against insect herbivores. However, the way nitrogen supply regulates rice anti-herbivore is not clear. This study investigated the effects of low (LN, 0.3 mM) and high (HN, 3 mM) nitrate levels on rice resistance against the striped stem borer Chilo suppressalis (SSB), one of the major destructive rice pests. Seven-day-old rice seedlings were cultured with different nitrate levels for 30 days and then inoculated with third instars of SSB. LN significantly enhanced rice anti-herbivore defense and lowered the total nitrogen content in the plants, but increased the content of free amino acids after SSB infestation. Additionally, LN significantly increased the accumulation of phenolic acids and flavonoids, especially lignin, resulting in enhanced constitutive defense in SSB-infested plants. SSB feeding led to a rapid accumulation of secondary metabolites. HN application led to the accumulation of metabolites derived from cinnamic acid, p-coumaric acid, p-coumaric CoA, feruloyl CoA, and apigenin, while LN led to the accumulation of metabolites derived from 3-dehydroquinic acid, phenylalanine, acetyl CoA, and aspartic acid. Collectively, our finding suggests that nitrogen deficiency enhances rice anti-herbivore defense via constitutive defense by the accumulation of phenolic acids and flavonoids.

Elicitor Application in Strawberry Results in Long-Term Increase of Plant Resilience Without Yield Loss

For a first step integrating elicitor applications into the current IPM strategy increasing plant resilience against pests, we investigated repeated elicitor treatments in a strawberry everbearer nursery and cropping cycle under glass. During nursery methyl-jasmonate (MeJA), testing induction of defenses with plant bioassays was applied every 3 weeks. Thrips damage and reproduction by spider mites, whitefly and aphids were strongly reduced upon elicitor treatment. Subsequently, we applied MeJA every 3 weeks or based on scouting pests during a whole cropping cycle. Thrips leaf bioassays and LC-MS leaf metabolomics were applied to investigate the induction of defenses. Leaf damage by thrips was lower for both MeJA application schemes compared to the control except for the last weeks. While elicitor treatments after scouting also reduced damage, its effect did not last. Thrips damage decreased from vegetative to mature plants during the cropping cycle. At the end of the nursery phase, plants in the elicitor treatment were smaller. Surprisingly, growth during production was not affected by MeJA application, as were fruit yield and quality. LC-MS leaf metabolomics showed strong induction of vegetative plants decreasing during the maturation of plants toward the end of cultivation. Concurrently, no increase in the JA-inducible marker PPO was observed when measured toward the end of cultivation. Mostly flavonoid and phenolic glycosides known as plant defense compounds were induced upon MeJA application. While induced defense decreased with the maturation of plants, constitutive defense increased as measured in the leaf metabolome of control plants. Our data propose that young, relatively small plant stages lack constitutive defense necessitating an active JA defense response. As plants, mature constitutive defense metabolites seem to accumulate, providing a higher level of basal resistance. Our results have important implications for but are not limited to strawberry cultivation. We demonstrated that repeated elicitor application could be deployed as part of an integrated approach for sustainable crop protection by vertical integration with other management tactics and horizontal integration to control multiple pests concurrently. This approach forms a promising potential for long-term crop protection in greenhouses.

Arabidopsis CALMODULIN-BINDING PROTEIN 60b plays dual roles in plant immunity

Arabidopsis SYSTEMIC ACQUIRED RESISTANCE DEFICIENT 1 (SARD1) and CALMODULIN-BINDING PROTEIN 60g (CBP60g) are two master transcription factors that regulate many defense-related genes in plant immunity. They are required for immunity downstream of the receptor-like protein SUPPRESSOR OF NPR1-1, CONSTITUTIVE 2 (SNC2). Constitutive defense responses in the gain-of-function autoimmune snc2-1D mutant are modestly affected by either sard1 or cbp60g single mutants, but completely suppressed by the sard1 cbp60g double mutant. Here we report that CBP60b, another member of the CBP60 family, also functions as a positive regulator of SNC2-mediated immunity. Loss-of-function mutations of CBP60b suppress the constitutive expression of SARD1 and enhanced disease resistance in cbp60g-1 snc2-1D, whereas over-expression of CBP60b leads to elevated SARD1 expression and constitutive defense responses. In addition, transient expression of CBP60b in Nicotiana benthamiana activates the expression of the pSARD1::luciferase reporter gene. Chromatin immunoprecipitation assay further showed that CBP60b is recruited to the promoter region of SARD1, suggesting that it directly regulates SARD1 expression. Interestingly, knocking out CBP60b in the wild type background leads to ENHANCED DISEASE SUSCEPTIBILITY 1 (EDS1)-dependent autoimmunity, suggesting that CBP60b is required for the expression of a guardee/decoy or a negative regulator in immunity mediated by receptors carrying an N-terminal TIR (Toll-interleukin-1 receptor-like) domain.Significance statementArabidopsis SARD1 serves as a master transcription factor in plant immunity. In this study, we showed that CBP60b positively regulates SARD1 expression, and TIR signaling is activated when CBP60b is inactivated.

Toll/interleukin-1 Receptor Domain-Only Protein of Arabidopsis Thaliana AtTX14 2IR, Produced By Alternative Splicing, Can Induce Defense Responses

Toll/interleukin -1 receptor (TIR) domains, which have NAD+ cleavage activity, are used as signaling modules in NOD-like receptors for defense responses. It has been shown that TIR domains not only form homo- or heterodimers with TIR domain-containing proteins but also interact with various proteins. A previous study showed that overexpression of Arabidopsis thaliana (Arabidopsis) AtTX14, encoding an N-terminal TIR domain and a C-terminal domain with unknown function, resulted in dwarfism and constitutive defense signaling or autoimmunity. Transgenic Arabidopsis overexpressing AtTX14 displays enhanced defense responses and associated dwarf phenotypes at 28 °C compared with those at 22 °C, which differs from other mutant or transgenic Arabidopsis with constitutive defense responses. We found that AtTX14 is alternatively spliced to encode three different proteins, and the TIR domain itself can induce autoimmunity and elevated defense responses to the bacterial pathogen Pseudomonas syringae pv. tomato. In addition, we revealed that the transcription of AtTX14 is regulated by a positive feedback mechanism. With transient overexpression of three AtTX14 protein forms in tobacco leaves, providing a heterologous system free from the positive feedback of AtTX14 in Arabidopsis, we demonstrated that expression of a splicing variant encoding the TIR domain-only protein is sufficient to activate defense signaling. A deeper understanding of interaction networks involving AtTX14 will broaden our knowledge on how plant defense signaling is regulated in response to pathogen infection and ambient temperature changes.