Mining and expression analysis of candidate genes involved in regulating the chilling requirement fulfillment of Paeonia lactiflora ‘Hang Baishao’

Jiaping Zhang, Danqing Li, Xiaohua Shi, Dong Zhang, Shuai Qiu, Jianfen Wei, Jiao Zhang, Jianghua Zhou, Kaiyuan Zhu, Yiping Xia

The artificial enlargement of the planting area and ecological amplitude of ornamentals for horticultural and landscape applications are significant. Herbaceous peony (Paeonia lactiflora Pall.) is a world-famous ornamental with attractive and fragrant flowers and is mainly planted in temperate and cool areas. Comparatively higher winter temperatures in the subtropical and tropical Northern Hemisphere result in a deficit of chilling accumulation for bud dormancy release, which severely hinders "The southward plantation of herbaceous peony". Studies on the dormancy, chilling requirement (CR) and relevant molecular mechanisms of peony are needed to enhance our ability to extend the range of this valuable horticultural species. Based on natural and artificial chilling experiments, and chilling hour (CH) and chilling unit (CU) evaluation systems, the lowest CR of 'Hang Baishao' was between 504.00 and 672.00 CHs and the optimal CR was 672.00 CHs and 856.08 CUs for achieving strong sprouting, growth and flowering performance. Transcriptome sequencing and gene identification by RNA-Seq were performed on 'Hang Baishao' buds during the dormancy and sprouting periods. Six gene libraries were constructed, and 66 temperature- and photoperiod-associated unigenes were identified as the potential candidate genes that may regulate or possibly determine CR characteristics. The difference in the expression patterns of SUPPRESSPOR OF OVEREXPRESSION OF CONSTANS1 (SOC1) between the winters of 2012-2013 and 2015-2016, and the difference of CR fulfillment periods also between these two winters represented the interesting congruent relationships. This correlation was also observed for WRKY DNA-BINDING PROTEIN 33 (WRKY 33). Combined with the results acquired from all of experiments, 'Hang Baishao' was confirmed to be a superb peony resource that have significantly low CR characteristics. The two genes of SOC1 and WRKY33 are likely involved in determining the CR amount and fulfillment period of 'Hang Baishao'. HEAT SHOCK PROTEIN, OSMOTIN and TIMING OF CAB EXPRESSION 1 also deserve attention for the CR research. This study could contribute to the knowledge of the deep factors and mechanisms that regulate CR characteristics, and may be beneficial for breeding new germplasms that have low CRs for landscape or horticulture applications in subtropical regions.