The most appropriate explanation is (4) Co-suppression effect of transgene.

Core concept: co-suppression in overexpression lines

When a regulatory gene X is overexpressed via a strong promoter, some transformants accumulate very high levels of X mRNA. In a subset of lines, this excessive, often multi‑copy expression can activate post‑transcriptional gene silencing (PTGS). The RNA silencing machinery recognizes the X transcripts as aberrant, generates siRNAs, and these siRNAs then degrade both the transgene mRNA and the endogenous gene X mRNA. This phenomenon is called co-suppression (or sense suppression) and leads to unexpectedly low expression of gene X in some lines even though the transgene is present and should be overexpressed.

In the question, 22 lines show high X expression (no silencing), while 8 lines show low X expression, which is best explained by co-suppression.

Option-by-option explanation

(1) Suppression effect of the transgene

This is a vague phrase and not a standard technical term. It does not clearly specify the mechanism. In exam context, the recognized mechanism for an overexpressed gene causing reduced expression of itself and its endogenous counterpart is co-suppression, not just “suppression effect.”

(2) Knock down effect of gene X

“Knockdown” usually refers to deliberate reduction of expression using antisense, RNAi, VIGS, etc. Here the construct was designed for overexpression, not for knockdown. Low expression in some lines is not because gene X itself is inherently “knocking down” its own expression; rather, the overexpressed transgene has unintentionally triggered RNA silencing.

(3) Gene silencing effect

This is conceptually close—RNA silencing is indeed occurring—but the question asks for one explanation and gives a more precise term in option (4). Simply saying “gene silencing” does not capture the key feature that both transgene and endogenous gene are silenced together.

(4) Co-suppression effect of transgene – CORRECT

• Co-suppression (sense suppression) occurs when an overexpressed sense transgene causes silencing of both the transgene and the homologous endogenous gene.

• It explains why some lines with the same construct show much lower expression: those particular insertion events (copy number, locus context) trigger the plant’s RNA silencing machinery.

Thus, the observation that 8 out of 30 lines show low expression of gene X despite carrying an overexpression construct is most accurately explained by co-suppression induced by the transgene.

SEO-oriented introduction (for article use)

In crop improvement programs that rely on overexpression of regulatory genes, it is common to see strong line‑to‑line variation, with some transgenic plants showing the expected high expression and others unexpectedly low expression of the same gene. This pattern is often due to co-suppression, a form of RNA silencing in which high transgene expression triggers degradation of both transgene and endogenous transcripts, reducing overall gene X expression in a subset of lines.