Classification of mutations by functional consequence:

• Failure of expression of an allele causes complete or partial loss of the gene product's normal activity, termed a loss-of-function mutation (null/amorphic): (A)-(III).
• point mutation that does not alter the amino acid due to genetic code degeneracy is a silent (synonymous) mutation: (B)-(I).
• An altered gene product that acts antagonistically to the wild-type allele and disrupts normal multimeric protein function exerts a dominant-negative effect: (C)-(IV).
• mutation that confers a novel phenotype or elevated/constitutive enzymatic activity is a gain-of-function mutation: (D)-(II).

Bacterial pathogenesis entry mechanisms:

• Unlike fungal pathogens, plant pathogenic bacteria cannot mechanically breach or enzymatically penetrate the intact cuticular barrier directly. They depend entirely on natural openings (stomata, hydathodes, lenticels) or wounds caused by insect feeding, hail, or cultivation: Assertion (A) is true.
• The vast majority of plant pathogenic bacterial genera (Pseudomonas, Xanthomonas, Ralstonia, Erwinia, Agrobacterium) belong to Gram-negative proteobacteria: Reason (R) is true.
• However, being Gram-negative does not explain why bacteria require wounds (entry limitation is due to lack of appressoria/cuticle-penetrating mechanical structures, regardless of Gram stain reaction).