Transcriptional reprogramming of the bud-mutation loquat YongLu emerging freezing resistant

Abstract Background: Freezing seriously affects loquat, an originally subtropical fruit. Here, a wide-spread cultivar loquat bud mutation ‘Yonglu’ (YL) was found to be more resistant to freezing than the parental ‘Ruantiaobaisha’ (RT), especially the fruits. The freezing resistance mechanism of YL was analyzed by physiological and transcriptomic methods. Results: After freezing treatment, only the fruits of YL showed a significant accumulation of proline (which increased by 1.5-fold). Interestingly, abscisic acid (ABA) accumulation in YL showed no difference after freezing treatment. Moreover, the stomatal density, area, and apertures of YL significantly decreased. All these results suggested that the proline content and stomatal closure contributed to the enhanced freezing tolerance. Further, the transcriptional profiles of freezing tissues were obtained and compared. Functional enrichment analysis showed that aside from basic nutrients and energy metabolisms, such as citrate cycle and oxidative phosphorylation, the fruits and flowers of YL have different systems to enhance freezing tolerance, suggesting a progressive freezing-resistant network. Signaling transduction of nearly all hormones, including ABA, was significantly differentially expressed. Several key genes, such as PP2C and SnRK2, which are crucial for transducing ABA signals, were significantly down- and upregulated, respectively. But the genes of neither ABA biosynthesis nor ABA transduction pathway significantly differentially expressed between two freezing varieties, suggesting they contribute limit in enhancing freezing tolerance of YL. Conclusion: Concludingly, the bud mutation YL loquat has stronger freezing tolerance than its parental RT due to the increasing proline and more effective regulation of stomatal closure. Furthermore, the bud mutation YL loquat reprogrammed its entire transcriptional profiles to enhance their abilities to adapt to freezing stress. However, ABA biosynthesis, ABA transduction pathway and CBF pathway, which are common reasons in cold response, were not the main reason for the increasing freezing tolerance of YL. There must be unknown regulation mechanisms for the bud mutation to freezing resistant.