Sugar transport played a more important role than sugar biosynthesis in fruit sugar accumulation during Chinese jujube domestication.

MAIN CONCLUSION: Sugar transport, including the symplasmic pathway in plasmodesmata and apoplasmic pathway mediated by sugar transporters, accelerated sugar accumulation in cultivated jujube, while sugar metabolism-related genes played weak roles in jujube domestication. The fruit of Chinese jujube (Ziziphus jujuba Mill.) is high in sugar concentration. By contrast, wild type-sour jujube (Z. jujuba Mill. var. spinosa Hu) contains markedly less sugar. It is unknown whether sugar transport or sugar metabolism drove sugar accumulation during jujube domestication. Using a combination of ultrastructural observations, phylogenetic analysis, testing for soluble sugars, and transcriptional analysis, the sugar accumulation mechanism was studied in the developmental stages of cultivated jujube and sour jujube. Our results indicate that the symplasmic transport pathway in plasmodesmata is present in cultivated jujube, but not in sour jujube. Sugar transporter genes have higher frequencies of duplication than sugar metabolism-related genes. Gene expression patterns indicate that sugar transporter genes, especially ZjSUT2, ZjSWEET1, ZjSWEET7, ZjSWEET11, ZjSTP3, and ZjSTP13a, rather than sugar metabolism-related genes showed higher expression levels in cultivated jujube versus sour jujube during fruit sugar accumulation. These findings suggest that sugar transport, including apoplasmic and symplasmic transport, rather than sugar biosynthesis, is associated with the difference in sugar accumulation between jujube and sour jujube, and that it may drive jujube domestication. This study provides valuable genetic information for jujube improvement, and offers new insights into fruit tree domestication related to sugar accumulation.

Changing Host Photosynthetic, Carbohydrate, and Energy Metabolisms Play Important Roles in Phytoplasma Infection.

Phytoplasmas parasitize plant phloem tissue and cause many economically important plant diseases. Jujube witches'-broom disease is a destructive phytoplasma disease of Chinese jujube (Ziziphus jujuba). To elucidate the influence of phytoplasma on host photosynthetic, carbohydrate and energy metabolisms, four types of jujube tissues showing disease symptoms with different severity were investigated at the structural, physiological, and molecular levels. Quantitative real-time PCR and high-performance liquid chromatography results showed that the down-regulation of genes related to photosynthesis and the lower contents of chlorophyll in diseased leaves. This clearly inhibited the light-harvesting and photosystem II activity of photosynthesis; however, overexpression of genes related to starch, sucrose and glucose synthesis led to higher contents of these carbohydrates. Meanwhile, transmission electron microscopy images revealed that dense amounts of phytoplasmas accumulated in the sieve elements of diseased petiole phloem, and the structure of the grana and stroma lamellae of chloroplasts in the diseased leaves was destroyed. Phytoplasma infection inhibited photosynthesis and led to abnormal carbohydrate accumulation in the diseased leaves. Furthermore, comparative metabolite analysis indicated that phytoplasma infection also stimulated amino acids and energy metabolisms of the diseased leaves. Continually inhibiting the photosynthetic process and stimulating carbohydrate and energy metabolisms of diseased trees may exhaust their nutrients. Our results highlight the importance of changing host metabolisms during the pathogenic process.

Floral biology of Ziziphus mauritiana (Rhamnaceae).

Floral development of the synchronous dichogamous species Ziziphus mauritiana, as followed by light and scanning electron microscopy (SEM), was divided into 11 stages using a series of landmark events. Main cellular events happen synchronously in the female and the male structures, such as meiosis in micro- and macrosporocyte cells, tetrad microspore formation and appearance of the functional megaspore cell, and onset of embryo sac differentiation coinciding with mitosis in the microspores. The last stage was characterized by anthesis and continued development of the flower, beginning with anther dehiscence (male phase) and proceeding to the female phase, which was characterized by style elongation. Flowers exhibit synchronous protandrous dichogamy; anthesis takes place in the morning (group A, e.g., clone Q-29) and afternoon (group B, e.g., clone B5/4). Stigma receptivity started after the male phase and occurred synchronously and complementarily with pollen dispersal in the two clones. Pollen viability and production were similar in the two clones, but the pollen diameter of Q-29 was significantly larger than that of B5/4. This study provides the basis for understanding the biological mechanisms regulating floral development, thus expanding the prospects for Z. mauritiana breeding programs and for further molecular and genetic studies of this species.