Biosynthesis of abscisic acid in fungi: identification of a sesquiterpene cyclase as the key enzyme in Botrytis cinerea

Abstract

While abscisic acid (ABA) is known as a hormone produced by plants through the carotenoid pathway, a small number of phytopathogenic fungi are also able to produce this sesquiterpene but they use a distinct pathway that starts with the cyclization of farnesyl diphosphate (FPP) into 2Z,4E-α-ionylideneethane which is then subjected to several oxidation steps. To identify the sesquiterpene cyclase (STC) responsible for the biosynthesis of ABA in fungi, we conducted a genomic approach in Botrytis cinerea. The genome of the ABA-overproducing strain ATCC58025 was fully sequenced and five STC-coding genes were identified. Among them, Bcstc5 exhibits an expression profile concomitant with ABA production. Gene inactivation, complementation and chemical analysis demonstrated that BcStc5/BcAba5 is the key enzyme responsible for the key step of ABA biosynthesis in fungi. Unlike what is observed for most of the fungal secondary metabolism genes, the key enzyme-coding gene Bcstc5/Bcaba5 is not clustered with the other biosynthetic genes, i.e., Bcaba1 to Bcaba4 that are responsible for the oxidative transformation of 2Z,4E-α-ionylideneethane. Finally, our study revealed that the presence of the Bcaba genes among Botrytis species is rare and that the majority of them do not possess the ability to produce ABA.

Abscisic acid dynamics, signaling, and functions in plants

“…It is synthesized in plants and some phytopathogenic fungi using two distinct pathways. Phytopathogenic fungi synthesize ABA through the mevalonate pathway with intermediates containing no more than 15 carbon atoms, which is also called the “direct pathway” (Hirai et al 2000; Izquierdo‐Bueno et al 2018; Takino et al 2018). Plants synthesize ABA using the carotenoid pathway, which is also known as the “indirect pathway.” This is initiated from the cleavage of a C 40 precursor known as β‐carotene (Nambara and Marion‐Poll 2005; Arc et al 2013).…”

The Multiple Facets of Plant–Fungal Interactions Revealed Through Plant and Fungal Secretomics

“…Furthermore, several phytopathogenic fungi, such as Cercospora rosicola, B. cinerea and M. oryzae have the ability to produce ABA through a biosynthetic pathway that is distinct from that of plants (Izquierdo-Bueno et al, 2018). Impairing ABA biosynthesis in M. oryzae dramatically reduces virulence (Spence et al, 2015).…”

Integration of a multi-step heterologous pathway in Saccharomyces cerevisiae for the production of abscisic acid

“…Knock-out experiments confirmed that bcaba123 are essential for ABA production in B. cinerea , whereas bcaba4 is involved in the pathway but not essential [19, 20]. It was hypothesized that the cyclisation of FPP requires a sesquiterpene cyclase (STC); however, none of the proteins in the gene cluster showed known STC motifs [19, 21]. A study of Izquierdo‐Bueno et al [21] identified a STC gene named bcaba5 , which is co-expressed with, but not located in the gene cluster.…”

“…It was hypothesized that the cyclisation of FPP requires a sesquiterpene cyclase (STC); however, none of the proteins in the gene cluster showed known STC motifs [19, 21]. A study of Izquierdo‐Bueno et al [21] identified a STC gene named bcaba5 , which is co-expressed with, but not located in the gene cluster. Bcaba5 knock-out mutants did not produce ABA [21].…”

“…A study of Izquierdo‐Bueno et al [21] identified a STC gene named bcaba5 , which is co-expressed with, but not located in the gene cluster. Bcaba5 knock-out mutants did not produce ABA [21]. However, Takino et al [22] evaluated the function of BcABA3 in vitro and showed that the enzyme can catalyse the cyclisation of FPP via an until then unknown reaction mechanism.…”