Expression of Biphenyl Synthase Genes and Formation of Phytoalexin Compounds in Three Fire Blight-Infected Pyrus communis Cultivars

Abstract

Pear (Pyrus communis) is an economically important fruit crop. Drops in yield and even losses of whole plantations are caused by diseases, most importantly fire blight which is triggered by the bacterial pathogen Erwinia amylovora. In response to the infection, biphenyls and dibenzofurans are formed as phytoalexins, biosynthesis of which is initiated by biphenyl synthase (BIS). Two PcBIS transcripts were cloned from fire blight-infected leaves and the encoded enzymes were characterized regarding substrate specificities and kinetic parameters. Expression of PcBIS1 and PcBIS2 was studied in three pear cultivars after inoculation with E. amylovora. Both PcBIS1 and PcBIS2 were expressed in ‘Harrow Sweet’, while only PcBIS2 transcripts were detected in ‘Alexander Lucas’ and ‘Conference’. Expression of the PcBIS genes was observed in both leaves and the transition zone of the stem; however, biphenyls and dibenzofurans were only detected in stems. The maximum phytoalexin level (~110 μg/g dry weight) was observed in the transition zone of ‘Harrow Sweet’, whereas the concentrations were ten times lower in ‘Conference’ and not even detectable in ‘Alexander Lucas’. In ‘Harrow Sweet’, the accumulation of the maximum phytoalexin level correlated with the halt of migration of the transition zone, whereby the residual part of the shoot survived. In contrast, the transition zones of ‘Alexander Lucas’ and ‘Conference’ advanced down to the rootstock, resulting in necrosis of the entire shoots.

Genes Involved in Stress Response and Especially in Phytoalexin Biosynthesis Are Upregulated in Four Malus Genotypes in Response to Apple Replant Disease

“…After fire blight infection, BIS1 and BIS2 were upregulated in leaf tissue. In contrast, BIS3 was strongly expressed in the stem, where it was spatially limited to the transition zone between healthy and necrotic tissue (Chizzali et al, 2016). In the present study, expression of BIS genes was also focused on the region affected by the biotic stress, the roots.…”

“…Loss-of-function experiments on other plant-pathogen interactions have demonstrated that reduced levels of phytoalexins lead to increased disease susceptibility (Jeandet et al, 2014). Examples are known from pea (Wu and VanEtten, 2004), soybean (Graham et al, 2007), sorghum (Ibraheem et al, 2010), pear (Chizzali et al, 2016), and Arabidopsis (Jeandet et al, 2013). However, there are also reports that high phytoalexin concentrations may be toxic to plant cells (Dixon et al, 1994;Rogers et al, 1996), which was also hypothesized by Weiß et al (2017b) for apple rootstocks.…”

The genome sequence of the wild tomato Solanum pimpinellifolium provides insights into salinity tolerance

“…Research into biphenyl phytoalexins is somewhat scarce, possibly due to the absence of B4H in the model organism Arabidopsis, with most studies restricted to the Malinae subtribe of the subfamily Amygdaloideae (e.g. apple and pear) (Kokubun et al, 1995;Hüttner et al, 2010;Sircar et al, 2015;Chizzali et al, 2016). As such, the observed presence and, indeed, expansion of B4H-related genes in S. pimpinellifolium, could be related to the increased pathogen resistance of S. pimpinellifolium and represents an interesting target for further studies.…”

A new enzymatic activity from elicitor‐treated pear cell cultures converting trans‐cinnamic acid to benzaldehyde

“…Considering the high economic importance of pears, there is an urgent need to develop new disease‐resistant cultivars. Pathogen‐infected pear plants are known to produce two special classes of defense metabolites, the biphenyls and the dibenzofurans (Chizzali et al ). Interestingly, only members of the Malinae subtribe, such as apple, pear and mountain‐ash, are capable of producing these specialized defense metabolites, which showed considerable antimicrobial activity against the fire‐blight bacterium Erwinia amylovora (Chizzali et al ).…”

“…The starter biphenyl scaffold is formed by condensation of one molecule of benzoyl‐CoA with three molecules of malonyl‐CoA to produce 3,5‐dihydroxybiphenyl in a reaction catalyzed by the enzyme BIPHENYL SYNTHASE (BIS; Liu et al ). BIS is a type III polyketide synthase, whose cDNA has been cloned and functionally characterized in Sorbus aucuparia , P. communis and Malus domestica (Liu et al , Chizzali et al , ). Although the biosynthesis of biphenyls is well understood, formation of the precursor molecule benzoyl‐CoA is not well defined.…”