Can double PPO mutations exist in the same allele and are such mutants functional?

Area of Science:

• Plant genetics
• Molecular biology
• Biochemistry

Background:

• Herbicide resistance is a growing agricultural problem.
• Protoporphyrinogen oxidase (PPO)-inhibiting herbicides are crucial for weed management.
• Target-site mutations in the PPX2 gene are the primary mechanism of resistance in Amaranthus spp.

Purpose of the Study:

• To investigate the co-occurrence of prevalent target-site mutations in the PPX2 gene.
• To understand the genetic basis of PPO-inhibiting herbicide resistance.
• To determine the physiological viability of combined mutations.

Main Methods:

• Cross-breeding of Amaranthus plants with specific PPX2 mutations (R128G, ΔG210, G339A).
• Pyrosequencing and E. coli-based Sanger sequencing of offspring PPX2 alleles.
• Analysis of mutation frequencies and allele co-occurrence patterns.

Main Results:

• The R128G and ΔG210 mutations cannot co-occur on the same PPX2 allele.
• Double mutations within a single allele lead to significant loss of enzyme activity.
• The ΔG210 and G399A mutations also showed no co-occurrence in the same allele.

Conclusions:

• Homozygous double-mutants for PPX2 mutations are not physiologically viable.
• Double-mutant plants can only exist in a heterozygous state.
• If two resistance mutations are present, they occur on separate PPX2 alleles.