The correct statement is (4) Opines are a source of nitrogen for Agrobacterium cells.

Explanation of each option

1. None of the virulence genes of Agrobacterium tumefaciens are expressed constitutively – Incorrect

• The vir regulon is generally inducible by plant signals, but at least some vir genes (for example virA and virG or their basal promoters) show low‑level or constitutive expression to sense plant signals and regulate the system.​

• Therefore it is wrong to say that “none” of the vir genes are constitutively expressed.

2. Integration of T-DNA with the nuclear genome of plant cells occurs only by homologous recombination – Incorrect

• Multiple studies show that in plants T‑DNA usually integrates by illegitimate (non‑homologous) recombination, often accompanied by small deletions or filler DNA at junctions.​

• Homologous recombination is rare in plants, so saying it occurs “only by homologous recombination” is incorrect.

3. Host plant genes do not play any role in Agrobacterium-mediated transfer of T-DNA into plant cells – Incorrect

• Host genes are involved at many steps: recognition, endocytosis or membrane passage, nuclear import, chromatin remodeling, and DNA repair/integration; plant mutants in these pathways show altered transformation efficiency.​

• Thus host plant genes clearly play roles in T‑DNA transfer and integration.

4. Opines are a source of nitrogen for Agrobacterium cells – Correct

• Opines are low‑molecular weight compounds synthesized in transformed plant tumors; they are secreted into the gall and specifically catabolized by Agrobacterium as important carbon and nitrogen (and sometimes phosphorus or sulfur) sources.​

• Many Agrobacterium strains can grow in media where particular opines are the sole carbon and/or nitrogen source, making this statement accurate.

SEO‑oriented introduction (for article use)

In Agrobacterium–plant interactions, opines produced by transformed plant cells function as specialized nutrients that the bacterium can use as carbon and nitrogen sources, giving Agrobacterium a selective advantage inside the tumor. In contrast, vir genes can have basal expression, T‑DNA usually integrates by illegitimate rather than homologous recombination, and host plant genes are essential for successful T‑DNA transfer and integration, so only the statement about opines is correct in this multiple‑choice question.