A new class of oxidosqualene cyclases directs synthesis of antimicrobial phytoprotectants in monocots

Abstract

Here we show the novel oxidosqualene cyclase AsbAS1 catalyzes the first committed step in the synthesis of antifungal triterpenoid saponins that accumulate in oat roots. We also demonstrate that two sodium azide-generated saponin-deficient mutants of oat, which define the Sad1 genetic complementation group, are defective in the gene encoding this enzyme and provide molecular genetic evidence indicating a direct link between AsbAS1, triterpenoid saponin biosynthesis, and disease resistance. Orthologs of AsbAS1 are absent from modern cereals and may have been lost during selection, raising the possibility that this gene could be exploited to enhance disease resistance in crop plants.

A Serine Carboxypeptidase-Like Acyltransferase Is Required for Synthesis of Antimicrobial Compounds and Disease Resistance in Oats

“…Previously, we have shown that Sad1 and Sad2 are expressed specifically in the root tip cells (Haralampidis et al, 2001;Qi et al, 2006). RNA gel blot analysis indicated that expression of SCPL1 was also root specific ( Figure 2B).…”

“…The synthesis of avenacins is tightly regulated and is restricted to the epidermal cells of the root tip (Haralampidis et al, 2001). Previously, we have shown that Sad1 and Sad2 are expressed specifically in the root tip cells (Haralampidis et al, 2001;Qi et al, 2006).…”

“…Avenacins are antimicrobial triterpene glycosides (saponins) that are synthesized in the roots and confer broad-spectrum disease resistance to soil-borne pathogens (Hostettmann and Marston, 1995;Papadopoulou et al, 1999). The ability to synthesize avenacins is restricted to the genus Avena and has evolved since the divergence of oats from other cereals and grasses (Haralampidis et al, 2001;Qi et al, 2004Qi et al, , 2006Mylona et al, 2008). There are four structurally related avenacins, A-1, B-1, A-2, and B-2 (Hostettmann and Marston, 1995) (Figure 1A).…”

“…Avenacin-deficient mutants of A. strigosa are compromised in disease resistance, indicating that avenacins provide protection against microbial attack (Papadopoulou et al, 1999). We have cloned the gene for the first committed step in the pathway, Saponin deficient 1 (Sad1), which encodes the oxidosqualene cyclase enzyme b-amyrin synthase (Haralampidis et al, 2001) and the gene for a second early pathway step, Sad2, which encodes a novel cytochrome P450 (Qi et al, 2006). Sad1 and Sad2 are adjacent genes that lie ;70 kb apart in the A. strigosa genome (Qi et al, 2006).…”

Gene Clusters for Secondary Metabolic Pathways: An Emerging Theme in Plant Biology

“…Unlike operons, the genes within these clusters are transcribed separately. The five plant clusters are for the synthesis of cyclic hydroxamic acids in maize (Frey et al, 1997(Frey et al, , 2003(Frey et al, , 2009von Rad et al, 2001;Jonczyk et al, 2008), triterpenes in oat (Avena sativa) and Arabidopsis (the avenacin and thalianol clusters, respectively; Papadopoulou et al, 1999;Haralampidis et al, 2001;Qi et al, 2004Qi et al, , 2006Field and Osbourn, 2008;Mylona et al, 2008;Mugford et al, 2009), and diterpenes in rice (Oryza sativa; the momilactone and phytocassane clusters; Sakamoto et al, 2004;Wilderman et al, 2004;Shimura et al, 2007;Swaminathan et al, 2009). These clusters are diverse in organization and function and all appear to have evolved independently (Field and Osbourn, 2008;Frey et al, 2009;Osbourn and Field, 2009;Swaminathan et al, 2009).…”