John N. Bell

Organization and differential activation of a gene family encoding the plant defense enzyme chalcone synthase in Phaseolus vulgaris

SummaryChalcone synthase (CHS) catalyzes the first and key regulatory step in the branch pathway of phenylpropanoid biosynthesis specific for synthesis of ubiquitous flavonoid pigments and UV protectants. In bean (Phaseolus vulgaris L.) and other members of the Leguminoseae, chalcone synthase is also involved in the synthesis of the isoflavonoid-derived phytoalexin antibiotics characteristic of this family. We have demonstrated that the haploid genome of bean contains a family of about six to eight CHS genes, some of which are tightly clustered. Treatment of bean cells with fungal elicitor activates several of these genes leading to the accumulation of at least five and probably as many as nine distinct CHS transcripts encoding a set of CHS isopolypeptides of Mr 42–43 kDa but with differing pI in the range pH 6–7. In elicited cells specific transcripts and encoded polypeptides are differentially induced with respect to both the extent and kinetics of accumulation. Wounding or infection of hypocotyl tissue also activates several CHS genes with marked differences in the pattern of accumulation of specific transcripts and encoded polypeptides in wounded compared to infected tissue or elicited cells, indicating operation of more than one cue for defense gene activation. Illumination induces accumulation of a different set of CHS transcripts including only one of the set hitherto demonstrated to be induced by biological stress. The organization and differential regulation of the CHS gene family in bean are discussed in relation to the functions of this enzyme in adaptative and protective responses to diverse enviromental stresses.

Organization, Structure and Activation of Plant Defence Genes

Plants exhibit natural resistance to disease which has been exploited by breeders to reduce crop losses and hence increase yield. Disease resistance involves not only static protection, but also inducible defence mechanisms including: (i) accumulation of host-synthesized phytoalexins; (ii) deposition of lignin-like material; (iii) accumulation of hydroxyproline-rich glycoproteins and (iv) increases in the activity of certain hydrolytic enzymes such as chitinase and glucanase [1]. Although the genetics, physiology and cytology of plant:pathogen interactions have been extensively studied, until recently relatively little was known at the biochemical level about how plants respond to infection to activate these defence responses.

Molecular response of plants to infection

Plants exhibit resistance to disease involving inducible defenses including phytoalexin and hydroxyproline-rich glycoprotein (HRGP) accumulation, lignin deposition and increased activity of certain hydrolytic enzymes. Treatment of suspension-cultured cells of Phaseolus vulgaris L. with fungal elicitors redirects RNA synthesis leading to induction of mRNAs encoding phytoalexin biosynthetic enzymes such as phenylalanine ammonia-lyase and chalcone synthase; the lignin precursor biosynthetic enzyme cinnamyl alcohol dehydrogenase and HRGP. Accumulation of defense mRNAs is also observed during race:cultivar specific interactions between the fungus Colletotrichum lindemuthianum and P. vulgaris hypocotyls. There are clear temporal and spatial differences in the pattern of mRNA accumulation between incompatible (host resistant) and compatible (host susceptible) interactions. The data suggest that specific activation of plant defense genes is a key early component in the sequence of events leading to expression of defense reponses during race:cultivar specific interactions and that an elicitation signal is transmitted intercellularly to pre-activate defense genes in hitherto uninfected tissue. These observations indicate a number of potential sites for biotechnological manipulation and enhancement of disease resistance.

Accumulation of Hydroxyproline-Rich Glycoprotein mRNAs in Biologically Stressed Cell Cultures and Hypocotyls

Hydroxyproline-rich glycoproteins (HRGPs) are structural components of plant cell walls and may also function in the processes of plant development, growth, and disease resistance [reviewed in 1,2]. In higher plants, these cell wall HRGPs consist of 35–45% hydroxyproline (Hyp) and are also relatively rich in serine, valine, tyrosine and lysine. A repeating pentapeptide sequence, Ser-(Hyp)4, further characterizes these unusual glycoproteins which are often referred to as “extensins”. Most of the hydroxyproline residues are O-glycosidically attached to short oligoarabinosides, while some of the serine residues are O-glycosidically linked to galactose.