Abstract
The bushy root-2 (brt-2) tomato mutant has twisting roots, and slower plant development. Here we used whole genome resequencing and genetic mapping to show that brt-2 is caused by a serine to cysteine (S75C) substitution in the DNA binding domain (DBD) of a heat shock factor class B (HsfB) encoded by SolycHsfB4a. This gene is orthologous to the Arabidopsis SCHIZORIZA gene, also known as AtHsfB4. The brt-2 phenotype is very similar to Arabidopsis lines in which the function of AtHsfB4 is altered: a proliferation of lateral root cap and root meristematic tissues, and a tendency for lateral root cap cells to easily separate. The brt-2 S75C mutation is unusual because all other reported amino acid substitutions in the highly conserved DBD of eukaryotic heat shock factors are dominant negative mutations, but brt-2 is recessive. We further show through reciprocal grafting that brt-2 exerts its effects predominantly through the root genotype even through BRT-2 is expressed at similar levels in both root and shoot meristems. Since AtHsfB4 is induced by root knot nematodes (RKN), and loss-of-function mutants of this gene are resistant to RKNs, BRT-2 could be a target gene for RKN resistance, an important trait in tomato rootstock breeding.Gene & accession numbersSolycHsfB4a - Solyc04g078770.
Highlights
Root architecture is plastic and important for water and mineral absorption, anchorage and storage (Nibau et al, 2008)
Compared to Ailsa Craig (AC), brt-2 roots exhibited a strongly decreased growth, which is already noticeable at the cotyledon stage (Fig. 1a)
The brt-2 mutant shows a perturbed root phenotype leading to delayed shoot development The brt-2 mutant is a member of the large monogenic mutant collection of Tomato Genetics Resource Center (TGRC) which contains more than 1000 mutant lines
Summary
Root architecture is plastic and important for water and mineral absorption, anchorage and storage (Nibau et al, 2008). Changes in root function and architecture have resulted in enhancements for crop production (Hammer et al, 2009; Siddiqui et al, 2021), and much has been achieved to understand the genetic regulation of root system architecture and development, in Arabidopsis (Motte et al, 2019). Breeding for improved root systems is of great interest for grafted vegetable production where elite scion genotypes with favourable aboveground traits are grafted onto rootstocks, especially in the solanaceous crops tomato, pepper and eggplant (Thompson et al, 2017). Breeding for tomato rootstocks requires an understanding of the genetic variation for root traits and the available germplasm resources that can be applied to rootstock breeding (Pico et al, 2017)
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