Transcriptional profiling of defense responses to Botrytis cinerea infection in leaves of Fragaria vesca plants soil-drenched with β-aminobutyric acid

Abstract

Grey mold caused by the necrotrophic fungal pathogen Botrytis cinerea can affect leaves, flowers, and berries of strawberry, causing severe pre- and postharvest damage. The defense elicitor β-aminobutyric acid (BABA) is reported to induce resistance against B. cinerea and many other pathogens in several crop plants. Surprisingly, BABA soil drench of woodland strawberry (Fragaria vesca) plants two days before B. cinerea inoculation caused increased infection in leaf tissues, suggesting that BABA induce systemic susceptibility in F. vesca. To understand the molecular mechanisms involved in B. cinerea susceptibility in leaves of F. vesca plants soil drenched with BABA, we used RNA sequencing to characterize the transcriptional reprogramming 24 h post-inoculation. The number of differentially expressed genes (DEGs) in infected vs. uninfected leaf tissue in BABA-treated plants was 5205 (2237 upregulated and 2968 downregulated). Upregulated genes were involved in pathogen recognition, defense response signaling, and biosynthesis of secondary metabolites (terpenoid and phenylpropanoid pathways), while downregulated genes were involved in photosynthesis and response to auxin. In control plants not treated with BABA, we found a total of 5300 DEGs (2461 upregulated and 2839 downregulated) after infection. Most of these corresponded to those in infected leaves of BABA-treated plants but a small subset of DEGs, including genes involved in ‘response to biologic stimulus‘, ‘photosynthesis‘ and ‘chlorophyll biosynthesis and metabolism’, differed significantly between treatments and could play a role in the induced susceptibility of BABA-treated plants.

Secondary Metabolites and Their Role in Strawberry Defense

“…Also, the regulation of terpene biosynthetic genes upon pathogen infection in strawberry is underrepresented in the literature. Terpene synthases and (−)-germacrene D synthases are two of the many terpene biosynthetic enzymes, the genes of which are highly upregulated upon infection by different pathogens in strawberry [ 14 , 15 , 22 , 23 , 24 ]. (−)-germacrene D synthase (GDS) catalyzes the reaction to convert farnesyl diphosphate, a precursor of sesquiterpenes, to (−)-germacrene D. Sesquiterpenes, a class of terpenes having a 15 carbon-atom (15C) backbone in their structure, are one of the important metabolites synthesized in response to plant invasion by pathogens.…”

“…(−)-germacrene D synthase (GDS) catalyzes the reaction to convert farnesyl diphosphate, a precursor of sesquiterpenes, to (−)-germacrene D. Sesquiterpenes, a class of terpenes having a 15 carbon-atom (15C) backbone in their structure, are one of the important metabolites synthesized in response to plant invasion by pathogens. For example, the expression of several terpene synthase genes was induced in strawberry after inoculation with the following fungal pathogens: Botrytis cinerea , Colletotrichum gloeosporioides , and Phytophthora cactorum [ 14 , 15 , 22 , 23 , 25 , 26 , 27 ].…”

“…This includes the biosynthesis of defense-related proteins, secondary metabolites, and cell-wall reinforcement. The activation of plant defenses after pathogen perception is mediated by well-known plant defense hormones such as salicylic acid (SA) and jasmonic acid (JA) [ 14 , 15 , 33 ]. The accumulation of SA and JA in the plant leads to increased levels of the pathogenesis-related (PR) proteins that help in resisting the pathogen spread and thereby offer protection to the plant [ 15 , 34 ].…”

“…The activation of plant defenses after pathogen perception is mediated by well-known plant defense hormones such as salicylic acid (SA) and jasmonic acid (JA) [ 14 , 15 , 33 ]. The accumulation of SA and JA in the plant leads to increased levels of the pathogenesis-related (PR) proteins that help in resisting the pathogen spread and thereby offer protection to the plant [ 15 , 34 ]. PR genes are a part of the inducible plant defense responses that are highly responsive to various stresses such as cold, salinity, dehydration, UV light, wounding, and pathogen attack.…”

“…This article focuses on three major specialized metabolites: terpenoids, flavonoids, and allergens, and discusses their phylogenetic and regulatory aspects, including in the context of beneficial microbes. We particularly focus on the diploid strawberry F. vesca due to the wide availability of its genomic resources [ 9 , 10 , 11 , 12 ], well-understood molecular mechanisms of defense, and relatively high number of available metabolomic and transcriptomic datasets [ 13 , 14 , 15 , 16 ], compared to the octoploid strawberry.…”