Missense mutation of a class B heat shock factor is responsible for the tomato bushy root-2 phenotype

Abstract

AbstractThebushy root-2(brt-2) tomato mutant has twisting roots, and slower plant development. Here we used whole genome resequencing and genetic mapping to show thatbrt-2is caused by a serine to cysteine (S75C) substitution in the DNA binding domain (DBD) of a heat shock factor class B (HsfB) encoded bySolycHsfB4a. This gene is orthologous to the ArabidopsisSCHIZORIZAgene, also known asAtHsfB4. Thebrt-2phenotype is very similar toArabidopsislines in which the function ofAtHsfB4is altered: a proliferation of lateral root cap and root meristematic tissues, and a tendency for lateral root cap cells to easily separate. Thebrt-2S75C mutation is unusual because all other reported amino acid substitutions in the highly conserved DBD of eukaryotic heat shock factors are dominant negative mutations, butbrt-2is recessive. We further show through reciprocal grafting thatbrt-2exerts its effects predominantly through the root genotype even throughBRT-2is expressed at similar levels in both root and shoot meristems. SinceAtHsfB4is induced by root knot nematodes (RKN), and loss-of-function mutants of this gene are resistant to RKNs,BRT-2could be a target gene for RKN resistance, an important trait in tomato rootstock breeding.Gene & accession numbersSolycHsfB4a-Solyc04g078770.