(A) Complement the mutant with X or Y and analyze the phenotype in each case is the correct approach.

Option Analysis

Complementation tests determine if mutations in genes X and Y cause the reduced SAR phenotype independently or additively. Introducing wild-type copies reveals functionality: restoration by X alone indicates Y mutation sufficiency; by Y alone indicates X sufficiency; by both suggests dual necessity; neither suggests other causes.

Option (B) tests only joint action, missing individual roles.

Option (C) analyzes F2 segregation for one gene typically (3:1), but two unlinked genes yield 9:7—ambiguous without prior linkage knowledge.

Option (D) checks expression levels, not causal mutations.

Systemic acquired resistance (SAR) mutants with homozygous mutations in genes X and Y challenge CSIR NET aspirants in plant genetics and molecular biology. This key CSIR NET question tests understanding of complementation experiments to pinpoint if the reduced SAR phenotype stems from mutation in either or both genes.

SAR Mechanism Basics

Systemic acquired resistance (SAR) activates broad-spectrum immunity post-pathogen attack via signals like salicylic acid. Mutants like npr1 show defective SAR; sequencing reveals genes X and Y mutations, demanding precise genetic tests.

Complementation Test Explained

Complementation introduces wild-type gene copies into the double homozygous mutant (x/x y/y). Phenotype restoration confirms causality:

• X alone restores: Y mutation drives phenotype.

• Y alone: X drives it.

• Both needed: Additive/epistatic effects.

• Neither: Unrelated cause.

This directly tests “either or both,” making (A) optimal for SAR mutant analysis.

Why Other Options Fail

• (B) Joint complementation ignores singles.

• (C) Wild-type cross segregates 9:7 for two genes but can’t isolate individual contributions without markers.

• (D) Expression differs but proves no causation.