Transcriptome analysis of the model grass Lolium temulentum exposed to green leaf volatiles

Abstract

Background Forage and turf grasses are routinely cut and grazed upon throughout their lifecycle. When grasses are cut or damaged, they rapidly release a volatile chemical cocktail called green leaf volatiles (GLV). Previously we have shown that mechanical wounding or exposure to GLV released from cut grass, activated a Lt 46 kDa mitogen-activated protein kinase (MAPK) within 3 min and a 44 kDa MAPK within 15–20 min in the model grass species Lolium temulentum ( Lt ). Currently very little is known concerning the perception, signaling or molecular responses associated with wound stress in grasses. Since GLV are released during wounding, we wanted to investigate what genes and signaling pathways would be induced in undamaged plants exposed to GLV. Results RNA-Seq generated transcriptome of Lolium plants exposed to GLV identified 4308 up- and 2794 down-regulated distinct differentially-expressed sequences (DES). Gene Ontology analysis revealed a strong emphasis on signaling, response to stimulus and stress related categories. Transcription factors and kinases comprise over 13% of the total DES found in the up-regulated dataset. The analysis showed a strong initial burst within the first hour of GLV exposure with over 60% of the up-regulated DES being induced. Specifically sequences annotated for enzymes involved in the biosynthesis of jasmonic acid and other plant hormones, mitogen-activated protein kinases and WRKY transcription factors were identified. Interestingly, eleven DES for ferric reductase oxidase, an enzyme involved in iron uptake and transport, were exclusively found in the down-regulated dataset. Twelve DES of interest were selected for qRT-PCR analysis; all displayed a rapid induction one hour after GLV exposure and were also strongly induced by mechanical wounding. Conclusion The information gained from the analysis of this transcriptome and previous studies suggests that GLV released from cut grasses transiently primes an undamaged plant’s wound stress pathways for potential oncoming damage, and may have a dual role for inter- as well as intra-plant signaling. Electronic supplementary material The online version of this article (10.1186/s12870-019-1799-6) contains supplementary material, which is available to authorized users.

Transcriptome Analysis of Wounding in the Model Grass Lolium temulentum

“…As shown in Table 2 , genes encoding many of the enzymes in the JA biosynthetic pathway [ 20 , 71 , 72 , 73 ], including phospholipase (34 up (13 phospholipase A), 17 down), lipoxygenases (24 up, 12 down), allene oxide synthase (6 up, 0 down), allene oxide cyclase (1 up, 1 down), and 12-oxophytodienoic acid reductase (8 up, 1 down), were predominantly found in the upregulated DEG dataset as compared to the downregulated dataset. Many of these same genes were also upregulated in response to GLVs [ 42 ], and a similar upregulation of JA biosynthesis genes has been reported in Arabidopsis in response to wounding [ 4 ]. Furthermore, methyl jasmonate has been shown to induce defense responses in gymnosperms via terpenoid biosynthesis, the development of traumatic resin ducts, and polyphenolic parenchyma cells [ 74 , 75 , 76 , 77 , 78 , 79 , 80 , 81 ].…”

“…The initial read counts and percent alignments for expression libraries generated from control and wounded Lt plants are summarized in Table 1 . The Lolium transcriptome previously used to assemble the GLV transcriptome [ 42 ] was used as a reference transcriptome to assemble the wound transcriptome and is shown in Table S1 . The results of wounding were evaluated by comparing the values derived for the differential expression of sequences (fragments per kilobase million (FPKM)) between the wounded plants and their corresponding untreated controls at the 1, 2, 6, 12, and 24 h time points.…”

“…The results of wounding were evaluated by comparing the values derived for the differential expression of sequences (fragments per kilobase million (FPKM)) between the wounded plants and their corresponding untreated controls at the 1, 2, 6, 12, and 24 h time points. As with the GLV transcriptome [ 42 ], the false discovery rate (FDR) was set at ≤0.05 and the p -value was ≤0.01; and we converted “0” values for FPKM to 0.05, where appropriate. The results of the analyses were separated into upregulated and downregulated datasets.…”

“…Volatile compounds are another important class of wound signaling molecules [ 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 ]. These plant volatiles may prime or enhance plants’ wound response via inter- and/or intra-plant signaling [ 30 , 31 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 ]. When grasses are mechanically damaged, they release a volatile chemical blend called green leaf volatiles (GLVs) into the surrounding atmosphere [ 32 , 43 ].…”

“…Furthermore, an analysis of selected genes found to be upregulated in the GLV transcriptome showed strong induction by mechanical wounding. The analysis of the Lt GLV transcriptome combined with previous studies suggests that GLVs released from cut grasses not only affect the injured plants, but can also transiently prime wound stress pathways in nearby undamaged plants, possibly alerting them to potential oncoming damage [ 42 ]. While the analysis of the GLV transcriptome gives insight into the perception, potential signaling pathways, and networks used to respond to wound stress, it lacks information on the processes that the plants utilize to effect repair and regrowth of lost tissue, and the type of defense-related proteins and compounds they produce.…”