Which among the following statements is incorrect?

The correct answer is Option (4) → Bt toxin gene has been cloned from the nematode and been expressed in plants to provide resistance to insects without the need for insecticides.

The statements cover two ways of making pest-resistant plants: Bt toxin genes from a bacterium against insects, and RNA interference against a nematode. The incorrect one gives the wrong source for the Bt toxin gene.

Option (1) B. thuringiensis forms protein crystals during a particular phase of their growth. — A correct statement. B. thuringiensis forms protein crystals during a particular phase of growth. These crystals contain the toxic insecticidal protein.

Option (2) RNAi takes place in all eukaryotic organisms as a method of cellular defense. — A correct statement. RNAi is a method of cellular defence in all eukaryotic organisms. A complementary dsRNA silences a specific mRNA.

Option (3) Using Agrobacterium vectors, nematode-specific genes were introduced into the host plant. — A correct statement. To protect tobacco from Meloidegyne incognitia, nematode-specific genes were put into the plant with Agrobacterium vectors.

Option (4) Bt toxin gene has been cloned from the nematode and been expressed in plants to provide resistance to insects without the need for insecticides. — An incorrect statement. The Bt toxin gene is cloned from the bacterium Bacillus thuringiensis, not from a nematode. The nematode is the pest in the RNAi method.

NCERT Extract:
Biotechnological Applications in Agriculture – “… Bt toxin is produced by a bacterium called Bacillus thuringiensis (Bt for short). Bt toxin gene has been cloned from the bacteria and been expressed in plants to provide resistance to insects without the need for insecticides; in effect created a bio-pesticide.”
Biotechnological Applications in Agriculture – “… B. thuringiensis forms protein crystals during a particular phase of their growth. These crystals contain a toxic insecticidal protein.”
Biotechnological Applications in Agriculture – “… RNAi takes place in all eukaryotic organisms as a method of cellular defense. … Using Agrobacterium vectors, nematode-specific genes were introduced into the host plant (Figure 10.2).”

Figure 10.2 Host plant-generated dsRNA triggers protection against nematode infestation: (a) Roots of a typical control plants; (b) transgenic plant roots 5 days after deliberate infection of nematode but protected through novel mechanism