Natarajan had quit Congress yesterday after accusing Gandhi of interference in the Environment ministry during her stint.
Kumar, while replying to the questions of media persons, said that nobody interfered in the official decisions taken by her during her tenure.
"It is a political conspiracy. Such type of allegations are totally baseless. Why she kept mum then? If she had some issues, she should have said earlier. This shows that she is speaking such things for her own political benefits," Kumar said.
The senior Congress leader said Gandhi was a "very sensitive leader" and he worked for the upliftment of poor and tribal communities.
Attacking Narendra Modi, Kumar said the Prime Minister had not been able to fulfil any of the promises he had made, including the black money stashed in tax heavens overseas, in the run to Lok Sabha elections.