CRISPR/Cas9 mutagenesis of the Melon Prv resistance gene generated a temperature-dependent autoimmune allele

The majority of plant disease resistance (R) genes encode nucleotide binding-leucine
rich repeat (NLR) proteins. In melon, the dominant Prv gene encodes an NLR protein
and provides resistance to papaya ringspot virus (PRSV). Following map-based
cloning of the Prv gene in our lab, we validated it by CRISPR/Cas9 mutagenesis of a
PRSV resistant melon genotype, and obtained a few inactivated alleles. The mutant
progeny became susceptible, proving the function of the candidate gene. One of the
Prv mutated alleles, prvΔ154, encodes a truncated product, and displayed an extreme
dwarf phenotype. This was accompanied by leaf lesions and cell death, high salicylic
acid levels and defense gene expression. The autoimmune phenotype, which was
recessive with respect to the wild type and to other mutated alleles, was temperature-
dependent, being cured under high temperatures. It is possible that Prv and other
NLRs in melon are adjusted to lower temperatures and it will be interesting to further
investigate the response of melons to PRSV at different temperature regimes.