Epidermal Cell Wall Biogenesis with Emphasis on Cuticular Layer Formation during Direct Somatic Embryogenesis in Rice

Abstract

Epidermal cell wall biogenesis with emphasis on cuticular layer formation during direct somatic embryogenesis in rice was examined by transmission electron microscopy. The first phenomenon noticed in the early globular stage of somatic embryos was a high activity of cell wall formation. In this stage, many endoplasmic reticula, Golgi bodies and their vesicles were present and epidermal cells were covered by fibrillar material. Cutin was initially deposited on the epidermal cell wall of late globular or early elongate stage embryos. In the late elongate stage, a lamellated cuticular layer formed, and in the late scutellar stage, a more homogenous continuous cuticular layer formed. The cuticular layer disappeared gradually from the scutellum cell wall as the somatic embryo germinated. The disappearance of the cuticular layer was similar to that of the zygotic embryo. There is no cuticular layer on the scutellar epithelial cell wall of the immature zygotic embryo 5 days after germination. Our observations lead to the conclusion that one of the structural and physiological markers of direct somatic embryogenesis is the transformation of fibrillar material into the cuticular layer.     

Improvement of Direct Somatic Embryogenesis in Rice by Selecting the Optimal Developmental Stage of Explant and Applying Desiccation Treatment

Summary

By selecting the optimal developmental stage of zygotic embryos used as expiants and applying desiccation treatment, we improved direct somatic embryogenesis in rice scutellum from two cultivars, Nipponbare and Sasanishiki. Zygotic embryos isolated 14-17, 21, 28-30 and 35-40 d after anthesis (DAA) from Nipponbare and 14-17, 18-21, 28-30 and 40-42 DAA from Sasanishiki were cultured on the embryo induction medium (EIM). Then they were transferred to embryo maturation medium (EMM) and germinated on the embryo germination medium (EGM). Only immature zygotic embryo isolated 14-17 DAA from Nipponbare and Sasanishiki could develop somatic embryos that germinated. Explants from embryos at other developmental stages could develop somatic embryos only until the elongating or scutellar stage. They enlarged and formed callus without further development. The EIM and EMM consisted of N6 macronutrients, B5 micronutrients, and B5 vitamins, supplemented with 0.1 g L^-1 casein hydrolysate, 1.5 g L^-1 proline and 1 g L^-1 MES buffer. EGM consisted of MS macro- and micronutrients and MS vitamins without organic supplement. In addition, 2 mg L^-1 2,4-D was added to EIM, 1 mg L^-1 2,4-D to EMM and 0.01 mg L^-1 zeatin to EGM. Developmental processes of somatic embryos derived from the expiants were observed by scanning electron microscopy. Desiccation treatment of maturing somatic embryo was proved to produce fully mature somatic embryos capable of germinating vigorously.     

霜疫霉侵染后荔枝果皮结构变化的扫描电镜观察

霜疫霉病严重危害荔枝果实的产量和质量,其病原菌是霜疫霉(Phytophthora litchii Chen ex Ko et al.)。以4×10~4cfu/mL霜疫霉孢子悬浮液分别接种抗病品种‘黑叶’和感病品种‘桂味’的果实,通过扫描电子显微镜观察接种0、1、6、12、24、48、72、96 h后荔枝外果皮显微结构的变化。扫描电镜观察发现,‘黑叶’果皮的龟裂片平整,主要组成单元为微小乳突状结构,其中间微微凸起且结构致密紧实;‘桂味’果皮的裂片谷较深,主要组成单元为微小蜂窝状结构,其中间凹陷且壁薄挺立。随着贮藏时间的延长,2个品种外果皮均出现不同程度的损伤,但‘黑叶’比‘桂味’损伤较轻。用相同浓度霜疫霉孢子接种后,‘桂味’果皮附着孢子的数量是‘黑叶’的近5倍,说明‘桂味’果皮显微结构更易于霜疫霉孢子的附着和侵染。接种霜疫霉后,病原菌侵染加剧荔枝外果皮结构的崩解,但‘黑叶’外果皮崩解速度较慢。在‘黑叶’上霜疫霉的孢子萌发、菌丝生长和孢子囊形成都晚于‘桂味’,说明其对霜疫霉的生长有一定的抑制作用。‘黑叶’和‘桂味’果皮显微结构的差异是其对霜疫霉表现不同抗性的重要原因。     

Changes in the thermal and structural properties of maize starch during nixtamalization and tortilla-making processes as affected by grain hardness

The objective of this work was to evaluate the changes in the thermal and structural properties of maize starch during nixtamalization and the tortilla-making process and their relationship with grain hardness. Three maize types with varying hardness (hard, intermediate, soft) were processed by three nixtamalization processes (classic, traditional and ecological). Starch from the three maize types showed an A-type pattern and two endotherms corresponding to gelatinization and melting of the Type I amylose-lipid complexes. After cooking and steeping, in intermediate and soft grains the partial gelatinization and the annealing affected the starch properties and promoted the formation of amylose-lipid complexes. These effects were not observe in hard grains. The increase in melting enthalpy and the intensity of the peak 2θ∼20° from nixtamal to tortillas demonstrated the formation of amylose-lipid complexes. A third endotherm above 114 °C in some treatments of nixtamal and tortilla starch demonstrated the transformation of some amylose-lipid complexes in a most ordered structures (Type II complexes). The V-type polymorph structure found in native starch, nixtamal, and tortilla corresponds to a coexistence of Type I and Type II complexes. Formation of amylose-lipid complexes in tortillas had a partial effect on decreasing starch retrogradation (r = −0.47, P < 0.05).