Comparative proteomic analysis of two contrasting cultivars reveals the mechanism of sugar beet response to freezing stress

Sugar beet (Beta vulgaris L.) is an economic crop integrating grain and feed production, and is a major source of sucrose. Freezing stress can cause dehydration and necrosis of seedlings. Therefore, it is important to study the basic physiological and proteomic mechanisms of different sugar beet cultivars under freezing stress. In this study, compared with freezing-sensitive (SME) cultivar, the freezing-tolerant (TBP) cultivar showed higher photosynthetic rate, antioxidant enzyme activity, proline and abscisic acid levels, lower malondialdehyde and gibberellin levels and relative conductivity under freezing stress. Moreover, comparative proteomic analysis revealed that proteins mapped to photosynthesis, tryptophan synthesis, anthocyanin synthesis, tricarboxylic acid cycle, reactive oxygen species scavenging and sucrose metabolism pathways were more abundant in TBP, indicating that these metabolic pathways may be beneficial to improve freezing tolerance. In particular, compared with SME cultivar, proteins involved in tryptophan synthesis were found to be upregulated only in TBP cultivar under freezing stress, including phospho-2-dehydro-3-deoxyheptonate aldolase 1, probable inactive shikimate kinase like 1, and anthranilate synthase alpha subunit 2. In addition, quantitative real-time polymerase chain reaction analysis showed that mRNA expression of these three proteins was significantly upregulated in the freezing stress (-5℃) vs. control group in the TBP cultivar. Additionally, tryptophan content in TBP leaves increased significantly under freezing stress, while in SME it showed no significant change. Furthermore, the application of exogenous tryptophan solution significantly reduced the mortality rate of sugar beet seedlings (17-day-old) under freezing stress. This study provides a foundation for further functional analysis to fully understand the molecular mechanisms of adaptation to freezing stress in sugar beet.