The tu8 mutation of Arabidopsis thaliana encoding a heterochromatin protein 1 homolog causes defects in the induction of secondary metabolite biosynthesis

The tu8 mutant of Arabidopsis thaliana (L.) Heynh. was previously described as deficient in pathogen-induced auxin and glucosinolate (GSL) accumulation, as well as in heat-induced accumulation of cytosolic Hsp90, the latter feature was accompanied by reduced thermotolerance at higher temperatures. The mutated gene was shown to be a novel allele of TERMINAL FLOWER2, encoding the only Arabidopsis homolog for heterochromatin protein 1 (Kim et al., 2004). In this report, we investigated the influence of heat stress on auxin and GSL content, as well as the accumulation of several secondary metabolites derived from the phenylpropanoid pathway, including anthocyanins and sinapine derivatives, in the mutant tu8. tu8 had less sinapine and sinapoyl esters compared to the wild type. in addition, the induction of sinapine by heat shock in Columbia was not found in tu8. Anthocyanins were also induced by heat stress in wild type plants, whereas tu8 showed only slight induction of these compounds and only at higher temperatures. GSLs were induced at higher temperatures in the wild type, but induction was absent in tu8. Transcript levels known to be involved in IAA/glucosinolate synthesis and metabolism (nitrilase and myrosinase) were examined and both showed developmental regulation, while only nitrilase mRNA levels differed between wild type and mutant seedlings. Treatment of Columbia and tu8 with jasmonic acid (JA), a known inducer of glucosinolates, showed differences between wild type and tu8 with respect to induction of individual GSLs and anthocyanins. However, the transcript level of the TU8/TFL2 gene after heat shock and jasmonate treatment did not change. Loss of function or altered function in the heterochromatin protein most likely lead to the pleiotropic phenotype observed for the tu8 mutant.