Involvement of abscisic acid and salicylic acid in signal cascade regulating bacterial endophyte‐induced volatile oil biosynthesis in plantlets of Atractylodes lancea

Abstract

The enormous biological diversity of endophytes, coupled with their potential to enhance the production of bioactive metabolites in plants, has driven research efforts focusing on endophytes. However, limited information is available on the impacts of bacterial endophytes on plant secondary metabolism and signaling pathways involved. This work showed that an endophytic Acinetobacter sp. ALEB16, capable of activating accumulation of plant volatile oils, also induced abscisic acid (ABA) and salicylic acid (SA) production in Atractylodes lancea. Pre-treatment of plantlets with biosynthetic inhibitors of ABA or SA blocked the bacterium-induced volatile production. ABA inhibitors suppressed not only the bacterium-induced volatile accumulation but also the induced ABA and SA generation; nevertheless, SA inhibitors did not significantly inhibit the induced ABA biosynthesis, implying that SA acted downstream of ABA production. These results were confirmed by observations that exogenous ABA and SA reversed the inhibition of bacterium-induced volatile accumulation by inhibitors. Transcriptional activities of genes in sesquiterpenoid biosynthesis also increased significantly with bacterium, ABA and SA treatments. Mevalonate pathway proved to be the main source of isopentenyldiphosphate for bacterium-induced sesquiterpenoids, as assessed in experiments using specific terpene biosynthesis inhibitors. These results suggest that Acinetobacter sp. acts as an endophytic elicitor to stimulate volatile biosynthesis of A. lancea via an ABA/SA-dependent pathway, thereby yielding additional insight into the interconnection between ABA and SA in biosynthesis-related signaling pathways.

Endophytic Bacterium-Triggered Reactive Oxygen Species Directly Increase Oxygenous Sesquiterpenoid Content and Diversity in Atractylodes lancea

“…The inhibitors or ROS scavenger were sprayed on plant leaves until dripping wet to allow a 1-day infiltration period before bacterial inoculation or oxidant application (13). The oxidant solutions (200 l) were sprayed on plantlets, which flowed from leaf surfaces to plant roots.…”

“…lancea plantlets and growth conditions. A. lancea aseptic tissue culture plantlets were established as previously described (13,18). Buds were collected from cultivated A. lancea and washed under running water, after which all procedures were conducted aseptically.…”

“…All inhibitors, oxidants, and ROS scavenger, including MEV (30 M), FOS (200 M), H 2 O 2 (2, 5, or 10 mM), sodium hypochlorite (NaClO; 0.2, 0.5, or 1 M), and AsA (1,000 ppm), were dissolved or diluted in doubledistilled water (13). MEV was previously converted to a water-soluble sodium salt as described by Hagen and Grunewald (22).…”

“…strain AL12, can efficiently increase oxygenous sesquiterpenoid accumulation in A. lancea (11)(12)(13). These endophytes can activate signaling molecules, such as hydrogen peroxide (H 2 O 2 ), salicylic acid, and jasmonic acid (14,15), which increase the expression of genes encoding key enzymes involved in sesquiterpenoid biosynthesis pathways (13), promoting the formation of sesquiterpenoid hydrocarbon scaffolds in A. lancea. However, the mechanisms by which endophytes promote the oxidation of sesquiterpenoid hydrocarbon scaffolds and increase oxygenous sesquiterpenoid accumulation are unknown.…”

“…It is worth noting that P. fluorescens ALEB7B can increase oxygenous sesquiterpenoid accu-mulation in A. lancea more efficiently than other endophytes, such as Acinetobacter sp. strain ALEB16 (13).…”