Brassinosteroid signaling may regulate the germination of axillary buds in ratoon rice

Xu et al. BMC Plant Biology (2020) 20:76 https://doi.org/10.1186/s12870-020-2277-x RESEARCH ARTICLE Open Access Brassinosteroid signaling may regulate the germination of axillary buds in ratoon rice Huibin Xu1,2,3,4,5,6,7†, Ling Lian1,2,3,4,5,6,7†, Fuxiang Wang1,2,3,4,5,6,7†, Jiahuan Jiang1,2,3,4,5,6,7, Qiang Lin1,2,3,4,5,6,7, Hongguang Xie1,2,3,4,5,6,7, Xi Luo1,2,3,4,5,6,7, Yongsheng Zhu1,2,3,4,5,6,7, Chuanying Zhuo8, Jinlan Wang1,2,3,4,5,6,7, Huaan Xie1,2,3,4,5,6,7*, Zhaowei Jiang1* and Jianfu Zhang1,2,3,4,5,6,7* Abstract Background: Rice ratooning has traditionally been an important component of the rice cropping system in China. However, compared with the rice of the first harvest, few studies on factors effecting ratoon rice yield have been conducted. Because ratoon rice is a one-season rice cultivated using axillary buds that germinate on rice stakes and generate panicles after the first crop’s harvest, its production is mainly affected by the growth of axillary buds. The objectives of this study were to evaluate the sprouting mechanism of axillary buds to improve the ratoon rice yield. Results: First, we observed the differentiation and growth dynamics of axillary buds at different nodes of Shanyou 63, and found that they differentiated from bottom to top before the heading of the mother stem, and that they developed very slowly. After heading they differentiated from top to bottom, and the ones on the top, especially the top 2nd node, developed much faster than those at the other nodes. The average length and dry weight of the axillary buds were significantly greater than those at other nodes by the yellow ripe stage, and they differentiated into pistils and stamens by 6 d after the yellow ripe stage. The morphology of vegetative organs from regenerated tillers of Shanyou 63 also suggested the superior growth of the upper buds, which was regulated by hormones, in ratoon rice. Furthermore, a comprehensive proteome map of the rice axillary buds at the top 2nd node before and after the yellow ripe stage was established, and some proteins involved in steroid biosynthesis were significantly increased. Of these, four took part in brassinosteroid (BR) biosynthesis. Thus, BR signaling may play a role in the germination of axillary buds of ratoon rice. Conclusions: The data provide insights into the molecular mechanisms underlying BR signaling, and may allow researchers to explore further the biological functions of endogenous BRs in the germination of axillary buds of ratoon rice. Background Rice (Oryza sativa L.) represents an important food crop worldwide and can produce panicle-bearing secondary tillers (i.e., ratoons) following harvest [1]. The production of a second rice crop in one cropping season is known as ratooning. The ratoon crop develops by regenerating rice tillers from nodal buds of the stubble that is left behind after the first seasonal rice harvest [2, 3]. In areas where adequate water is available after the main season, rice ratooning can be practiced as an alternative to double * Correspondence: ; ; † Huibin Xu, Ling Lian and Fuxiang Wang contributed equally to this work. 1 Rice Research Institute, Fujian Academy of Agricultural Sciences, Fuzhou 350019, Fujian, China Full list of author information is available at the end of the article cropping [4]. It requires fewer inputs, such as fertilizers, than the main crop and can often provide a net profit for the grower when the main crop barely recovers input costs [5]. In China, ratoon rice dates back 1700 years [6, 7]. In the 1930s to 1940s, studies of the morphological development of ratooning tillers began, and a number of ratoon rice varieties were bred in China in the 1960s and 1970s [8]. Since the 1980s, with the breeding of a number of new hybrid rice varieties having high regeneration capacities, ratooning rice has become a new cropping system promoted in large areas of southern China. In 1997, the planting area of ratoon rice expanded to 7.5 × 105 hm2 in China [6, 7]. From 1998 to 2002, super high-yielding demonstration area of ratoon rice was established in Youxi County, Sanming City, Fujian Province, China, and its accumulated planting area covered 1097.73 hm2 in 5 years. © The Author(s). 2020 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Xu et al. BMC Plant Biology (2020) 20:76 The average yield was 10,664 kg/hm2 in the first season and 6537 kg/hm2 in the ratooning season, leading to a total annual yield of 17,201 kg/hm2 [9]. There are 1.07 × 107 hm2 single cropping paddy fields in southern China, with 3.3 × 106 hm2 being suitable for planting ratoon rice. With a ratoon rice yield of 17,201 kg/hm2, this area could produce 5.68 × 1010 kg of rice per year. This is of great significance for grain production and its strategic development in China, which feeds 22% of the world’s population with 7% of the world’s cultivated land. Ratooning rice is the rice tillers which regenerate from the nodal buds of stubble [10]. The new tillers are regenerated from the first or main crop that is harvested in first season [3]. The physiological parameters are different between main and ratoon crop. For instance, panicle development and heading of ratoon rice is less than that of main rice. Low yield of ratoon crop usually depends on the reduction in number of productive tillers and short growth duration [10]. In the main rice crop, sugars and starches accumulated in leaves and culm are translocated to grain after flowering. The hypothesis is that carbohydrates accumulate in the culms again near main crop maturity, which may make preparations for ratoon growth and development. Therefore, effective application of ratoon-specific agronomic measures may provide a great potential for yield of ratooning rice [3]. The factors affecting the growth and development of ratoon rice include the varieties developed, water and fertilizer management, stubble height, plant protective practices, and external environmental factors, such as temperature and light, which affect the yield of ratoon rice [10]. Among these factors the most important is varietal development. Previous research focused on the effects of external factors, such as planting date, water and fertilizer management, and other environmental factors, on ratoon crop production. Although the varietal and stubble height effects on the yield of ratoon rice have been studied, their potentials and their impacts were limited [ (...truncated)