No abstract

Vachellia (formerly Acacia) trees are native to arid environments in Africa and the Arabian Peninsula, where they often support the local animal and plant communities acting as keystone species. The aim of this study was to examine whether oil pollution affected the central metabolism of the native keystone trees Vachellia tortilis (Forssk.) and V. raddiana (Savi), as either adults or seedlings. The study was conducted in the Evrona Nature Reserve, a desert ecosystem in southern Israel where two major oil spills occurred in 1975 and in 2014. Leaf samples were collected to analyze the central metabolite profiles from oil-polluted and unpolluted adult trees and from Vachellia seedlings growing in oil-polluted and unpolluted soils in an outdoor setup. We found that oil pollution had a stronger effect on one-year-old seedlings than on adult trees, reducing the levels of amino acids, sugars, and organic acids. While adult trees are mildly affected by oil pollution, the effects on young seedlings can cause a long-term reduction in the population of these keystone desert trees, ultimately threatening this entire ecosystem.

Number of tables: 2; number of figures: 7 22 Word count: 6,146 23 Number of supplementary tables: 8; number of supplementary figures: 1 24 HIGHLIGHT 25 A combined transcriptomic and metabolomic profiling of Setaria viridis leaves response to aphid 26 and caterpillar infestation identifies the genes related to the biosynthesis of serotonin and their 27 function in defense. 28 29 ABSTRACT 30 Setaria viridis (green foxtail millet), a short life-cycle C4 plant in the Gramineae, serves as a 31 resilient crop that provides good yield even in dry and marginal land. Although S. viridis has 32 been studied extensively in the last decade, its defense responses, in particular the chemical 33 defensive metabolites that protect it against insect herbivory, are unstudied. To characterize S. 34 viridis defense responses, we conducted transcriptomic and metabolomic assays of plants 35 infested with aphids and caterpillars. Pathway enrichment analysis indicated massive 36 transcriptomic changes that involve genes from amino acid biosynthesis and degradation, 37 secondary metabolites and phytohormone biosynthesis. The Trp-derived metabolite serotonin 38 was notably induced by insect feeding. Through comparisons with known rice serotonin 39 biosynthetic genes, we identified several predicted S. viridis Trp decarboxylases and cytochrome 40 P450 genes that were up-regulated in response to insect feeding. The function of one Trp 41 decarboxylase was validated by ectopic expression and detection of tryptamine accumulation in 42 Nicotiana tabacum. To validate the defensive properties of serotonin, we used an artificial diet 43 assay to show reduced Rhopalosiphum padi aphid survival with increasing serotonin 44 concentrations. This demonstrated that serotonin is a defensive metabolite in S. viridis and is 45 fundamental for understanding the adaptation of it to biotic stresses. 46 47 KEY WORDS 48 Insect herbivore, metabolite profile, RNAseq, Rhopalosiphum padi, Serotonin, Setaria viridis, 49 transcriptome, tryptophan 50 51 ABBREVIATIONS 52 S. viridis, Setaria viridis; R. padi, Rhopalosiphum padi; TDC, Trp decarboxylases; T5H, 53 tryptamine 5-hydroxylase; 54 55 K, Miyagawa H. 2011. Probing the role of tryptophan-derived secondary metabolism in defense responses against Bipolaris oryzae infection in rice leaves by a suicide substrate of tryptophan decarboxylase. Phytochemistry 72, 7-13. Joung J-G, Corbett AM, Fellman SM, Tieman DM, Klee HJ, Giovannoni JJ, Fei Z. 2009. Plant MetGenMAP: an integrative analysis system for plant systems biology. Plant Physiology 151, 1758 LP-1768.

Autophagy, an intracellular process that facilitates the degradation of cytoplasmic materials, plays a dominant role in plant fitness and immunity. While autophagy was shown to be involved in plant response to fungi, bacteria, and viruses, its role in response to insect herbivory is as yet unknown. In this study, we demonstrate a role of autophagy in plant defense against herbivory using Arabidopsis thaliana and the green peach aphid, Myzus persicae. Following six hours of aphid infestation of wildtype plants, we observed high expression of the autophagy-related genes ATG8a and ATG8f, as well as NBR1 (Next to BRCA1 gene 1), a selective autophagy receptor. Moreover, the number of autophagosomes detected by the overexpression of GFP-fused ATG8f in Arabidopsis increased upon aphid infestation. Following this, atg5.1 and atg7.2 mutants were used to study the effect of autophagy on aphid reproduction and feeding behavior. While aphid reproduction on both mutants was lower than on wildtype, feeding behavior was only affected by atg7.2 mutants. Moreover, upon aphid feeding, the Phytoalexin-deficient 4 (PAD4) defense gene was upregulated in wildtype plants but not affected in the mutants. By contrast, the hydrogen peroxide content was much higher in the mutants relative to wildtype, which might have disturbed aphid reproduction and interfered with their feeding. Additionally, an analysis of the phloem sap metabolite profile revealed that atg7.2 mutant plants have lower levels of amino acids and sugars. These findings, together with the high hydrogen peroxide levels, suggest that aphids might exploit the plant autophagy mechanism for their survival.