Overexpression of the Brassica rapa bZIP Transcription Factor, BrbZIP-S, Increases the Stress Tolerance in Nicotiana benthamiana

SIMPLE SUMMARY: Energy homeostasis plays a crucial role in regulating plant defense responses. In this study, we characterized BrbZIP-S (S(1)-bZIP from Brassica rapa) as a key modulator of energy metabolism, including sugar and proline metabolism. In addition, plants overexpressing BrbZIP-S exhibited increased tolerance to darkness and heat stress, suggesting that BrbZIP-S regulates plant stress responses through a complex network mediated by abscisic acid, sugar, and proline. ABSTRACT: In higher plants, S(1)-basic region-leucine zipper (S(1)-bZIP) transcription factors fulfill crucial roles in the physiological homeostasis of carbon and amino acid metabolisms and stress responses. However, very little is known about the physiological role of S(1)-bZIP in cruciferous vegetables. Here, we analyzed the physiological function of S(1)-bZIP from Brassica rapa (BrbZIP-S) in modulating proline and sugar metabolism. Overexpression of BrbZIP-S in Nicotiana benthamiana resulted in delayed chlorophyll degradation during the response to dark conditions. Under heat stress or recovery conditions, the transgenic lines exhibited a lower accumulation of H(2)O(2), malondialdehyde, and protein carbonyls compared to the levels in transgenic control plants. These results strongly indicate that BrbZIP-S regulates plant tolerance against dark and heat stress. We propose that BrbZIP-S is a modulator of proline and sugar metabolism, which are required for energy homeostasis in response to environmental stress conditions.