The Morphological Structure, Physiological and Biochemical Changes During Sorus Development of Saccharina japonica

Abstract

The reproductive characteristics of some hybrids from crosses of cultivated strains with wild populations are more similar to their wild parents. These hybrids form sorus twice a year in spring and autumn, unlike the conventional cultivars, which formed sorus once a year in summer. The S. japonica seedling industry begins in August in the north of China. However, hybrids form sorus in September or later. Therefore, these hybrids cannot be used as parental stock in the cultivation of summer seedlings in the north of China, hindering the promotion and application of these hybrids with excellent traits. Unfortunately, very few studies have focused on the induction and mechanism of sorus formation in kelp. It was of great significance to explore artificial induction technology for sorus formation of kelp hybrids and ensure the timely formation of sorus when the summer seedling cultivation based on an understanding of the biological process of sorus development. At present, research on the biological processes and characteristics of hybrid kelp sorus were limited. This study investigated the hybrid variety "Yudai No. 1". Discs from the kelp sporophytes were cultured in inflatable bottles. The sorus development process was divided into five stages (SA~SE) based on the appearance and morphological changes of the sorus. Samples for each stage were collected separately. The appearance, morphology, and tissue structure changes during the formation and development of sorus were systematically observed. Changes in the physiological and biochemical characteristics at different stages were also quantitatively studied. During sorus development, the surface of the sporophyte was smooth at stage SA, frosted in stage SB, noticeably protruded at stage SC, the cuticle at the apex of the paraphysis cells broken at stage SD, and the cuticle was smooth again in stage SE. The process was accompanied by the protrusion of epidermal cells (SB), the elongation of paraphysis cell (SC), the differentiation and development of sporoblast (SC, SD), and the formation and release of zoospores (SE). The cells (paraphysis cell and sporoblast) varied significantly and were constantly elongating at all stages (P < 0.05). The cells were especially elongated during the stage of zoospore formation and release (SE), zoospore cells were nearly 1-fold longer than the zoospores that were not released at stage SD. During the development of S. japonica sorus, the accumulation of nitrogen by sorus continued to increase, and there was little change after reaching the maximum level at stage SD. The formation of sorus was accompanied by the accumulation of nutrients. The protein content increased significantly in the early stages of sorus development and decreased at the stage of zoospore release. The protein content was significantly higher in the SC stage than that at stage SA (P < 0.05). Subsequently, the decline began after the SC stage, indicating the development of the sorus was the main biological activity, and the metabolic level was gradually reduced. Unlike that in previous studies, we identified a significant increase in the chlorophyll content during sorus development, which probably ensured all zoospores include chloroplasts. Meanwhile, hydrogen peroxide (H2O2) and superoxide anions showed similar trends of initially increasing at the beginning of sorus formation and decreasing in the later stages. Changes in the H2O2 content were highly significant in sorus formation. There were differences in the activity of different antioxidant enzymes in the process of sorus formation, among which superoxide dismutase (SOD) activity had a general downward trend, while ascorbate peroxidase (APX), peroxidase (POD), and catalase (CAT) showed a trend of rising in the early stages and then declining in the later stages. Moreover, POD, APX, and CAT activity had the significantly lowest levels at stage SC, SD, and SE, whereas the maximum levels of POD and APX were at stage SB, and maximum CAT levels were at stage SD. However, the malondialdehyde (MDA) content did not vary significantly during the whole development process. SOD activity gradually decreased throughout development, and the H2O2 content continued to increase, suggesting kelp sorus development may require hydrogen peroxide involvement. The activities of various antioxidant enzymes changed dynamically at different stages of sporangia development, and accurately regulated the oxygen species (ROS). The ROS increase in the process of sorus development did not harm any cells and ROS participated as a signaling molecule in the molecular regulation process of sorus development. In sorus development, the activity of RuBP carboxylase (RubisCO) initially decreased at stage SB and SC and then increased. There was no significant variation in the plant malate dehydrogenase activity. This study deepened the understanding of the hybrid kelp sorus formation process, physiological, and biochemical characteristics, and provided a theoretical basis for the artificial induction of hybrid kelp sorus formation in the future.

Keywords

• saccharina japonica
• sorus
• development process
• morphological structure
• physiology and biochemistry