Nitrogen (N) is essential for growth, development and defense but, how low N effects defense and the role of Trichoderma in enhancing defense under low nitrate is not known. Low nitrate fed Arabidopsis plants displayed reduced growth and compromised LAR & SAR response when infected with avirulent and virulent Pseudomonas syringae DC3000. These responses were enhanced in the presence of Trichoderma. The mechanism of increased LAR and SAR mediated by Trichoderma involve increased N uptake and enhanced protein levels via modulation of nitrate transporter genes. nrt2.1 mutant is compromised in LAR and SAR response suggesting a link between enhanced N transport and defense. Enhanced N uptake was mediated by Trichoderma elicited nitric oxide (NO). Low NO producing nia1,2 mutant and nsHb+ over expressing lines were unable to induce nitrate transporters and compromised defense in presence of Trichoderma under low N suggesting a signaling role of Trichoderma elicited NO. Trichoderma also induced SA and defense gene expression under low N. SA deficient NahG and npr1 mutants were compromised in LAR and SAR response mediated by Trichoderma. The mechanism of enhanced plant defense under low N mediated by Trichoderma involve NO, ROS, SA production and induction of NRT and SAR marker genes.### Competing Interest StatementThe authors have declared no competing interest.

Trichoderma asperelloides enhances local (LAR) and systemic acquired resistance (SAR) response under low nitrate nutrition in Arabidopsis