Abstract: 【Objective】 Phosphorus is one of the three essential nutrients for crops. Pores and channels in starch granules play an important role in starch biosynthesis and physicochemical characteristics of wheat (Triticum aestivum L.). This study was conducted to reveal the changes of micro-structural characteristics of starch granules and the mechanisms under different phosphorus application rates in wheat. It may provide more information to improve the pathway of starch biosynthesis. 【Method】 The wheat cultivar Xindong 23 was used in this study. On 160 d after sowing (about 5% plants returning green), three levels of phosphorus treatments, i.e., control (CK, P₂O₅, 0 kg·hm^-2), conventional phosphorus (CP, P₂O₅, 105 kg·hm^-2) and high phosphorus (HP, P₂O₅, 210 kg·hm^-2), were applied, and the samples were collected on the 7, 14, 21, 28 and 35 days post anthesis (DPA). The micro-structural morphology of mature starch granules and the morphological changes of starch granules digested by endogenous (germination) and exogenous (amyloglucosidase) enzymes were examined using scanning electron microscopy. The concentration of reducing sugars after amyloglucosidase digestion of starches was studied, and the channel structure within starch granules was observed using confocal laser scanning microscopy. The expression patterns of the genes involved in starch biosynthesis and degradation were investigated using real-time qPCR and in situ hybridization.【Result】The shape of the starch granules in different phosphorus treatments did not show a significant difference. However, in response to CP treatment, the “pinholes” on the granules were easier to find, and the fluorescence from the starch granules was stronger and clearly visible. At 6 days post germination (DPG), the number of “pinholes” and pits on the starch granules in CP treatment was most abundant. After exogenous amyloglucosidase digestion, the A-type starch granules developed under CP or HP conditions were more prone to be broken into halves than those formed under CK conditions; the concentration of reducing sugars was also higher after amyloglucosidase digestion of the starch granules developed under CP or HP conditions. These data suggest dissimilar micro-structural changes of starch granules induced by different phosphorus application rates. Under conventional phosphorus application rate, the expression levels of the genes involved in starch biosynthesis and degradation were significantly higher than those under the control and high phosphorus conditions during wheat grain filling. In addition, phosphorus application promoted the transcripts of amy4, bam1 and bam5 genes accumulated in the peripheral cells of the endosperm, and this effect was more pronounced in CP treatment. 【Conclusion】 Different phosphorus application rates did not significantly affect the shape of starch granules, but the micro-pores and channels in the starch granules and the relative expression levels of the genes involved in starch biosynthesis and degradation were significantly increased by CP application. Furthermore, CP treatment increased the transcript levels of amy4, bam1 and bam5 in the peripheral cells of the endosperm. These transcriptional changes may alter the balance between starch synthase and hydrolase activities, which may have an impact on the micro-structural characteristics of starch granules.

Key words: Triticum aestivum L., starch granule, phosphorus, micro-structure, gene expression, in situ hybridization