The correct answer is (4) B only – low protein despite high mRNA is best explained by codon usage differences between the host plant and the heterologous transgene.

Interpreting the scenario

• All lines use the same CaMV 35S promoter, yet:

  • Protein levels of the transgene are low in every line.

  • mRNA levels are high but differ between lines.

  • T-DNA copy number varies between lines.

This points to a post-transcriptional/translation problem common to all lines (explaining uniformly low protein), plus normal line‑to‑line variation in T‑DNA integration (explaining variable mRNA).

Statement-by-statement analysis

(A) “Variations in the number of T-DNA inserts … is due to more number of host cells getting infected with the T-DNA” – Incorrect

• Each independent plant comes from a single transformed cell; copy number differences arise from:

  • how many T‑DNA molecules integrate into that cell’s genome (co-integration, concatemerization, replication before integration), and

  • integration pattern, not simply “more host cells” being infected in that event.​

• Therefore A does not correctly explain copy-number variation between independent events.

(B) “Low expression levels of the transgenic protein … could be due to codon usage variations between the host plant and the heterologous source of the transgene” – Correct

• If the transgene comes from a species with very different codon usage bias, plant tRNAs for those codons may be scarce.

• Translation then becomes inefficient; ribosomes stall or elongate slowly, giving low protein levels even when mRNA is abundant.​

• Codon optimization is a standard strategy to fix exactly this problem, so B is a valid explanation.

(C) “The coding sequence of the transgene contained sequences that destabilized the transgene mRNA” – Inconsistent with data

• Destabilizing motifs (e.g., AU‑rich elements, cryptic splice sites) would reduce mRNA stability, leading to low mRNA levels.

• Here, transgene mRNA levels are explicitly stated to be high, so widespread mRNA destabilization is not supported.

(D) “Variation in copy number of T-DNA … is due to variation in the promoter used to express the transgene” – Incorrect

• All constructs use the same CaMV 35S promoter, so promoter variation cannot explain different T‑DNA copy numbers.

• Copy number variation is determined by transformation and integration events, not by which promoter is cloned into the T‑DNA.​

Why “B only” is the correct option

• Only B gives a mechanism that:

  • applies to all lines (a common translation bottleneck causing globally low protein), and

  • is consistent with high mRNA but low protein.

• A and D misattribute copy-number variation, and C contradicts the observation of high mRNA.

Therefore, the combination that represents all correct statements is option (4) B only.

SEO‑oriented introduction (for article use)

When a transgene driven by the strong CaMV 35S promoter produces high mRNA but uniformly low protein across independent transgenic plants, the most likely explanation is a codon usage mismatch between the heterologous gene and the host plant. Suboptimal codons slow or impair translation, so even abundant mRNA yields little protein, while differences in T‑DNA copy number or promoter sequence do not inherently cause this specific high‑mRNA/low‑protein pattern.