Development of basal endosperm transfer cells in Sorghum bicolor (L.) Moench and its relationship with caryopsis growth.

During sorghum caryopsis development, endosperm epidermal cells near the basal main vascular bundle are specialized by depositing wall ingrowths, differentiating into basal endosperm transfer cells (BETCs). All the BETCs together compose the basal endosperm transfer layer (BETL). BETCs are the first cell type to become histologically differentiated during endosperm development. The initiation and subsequent development of BETCs shows the pattern of temporal and spatial gradient. The developmental process of BETL can be divided into four stages: initiation, differentiation, functional, and apoptosis stage. A placental sac full of nutrient solutions would emerge, enlarge, and eventually disappear between the outmost layer of BETL and nucellar cells during caryopsis development. BETCs have dense cytoplasm rich in mitochondria, lamellar rough endoplasmic reticulum, Golgi bodies, and their secretory vesicles. They show a series of typical characteristics of senescence such as nuclei distortion and subcellular organelle deterioration during their specialization. BETCs probably play an active role in nutrient transfer into the starchy endosperm and embryo. The occurrence, development, and apoptosis of BETCs are in close relation to the caryopsis growth and maturation especially the enrichment of endosperm and the growth of embryo. The timing when BETL is fully developed, composed of three to four layers in radial direction and 70 to 80 rows in tangential direction, consists with the timing when average daily gain of caryopsis dry weight reaches its maximum. It is conceivable that measures that delay the senescence and death of BETCs would help to increase the crop yield.

[Changes in the activities of enzymes involved in starch synthesis and accumulation in caryopsis of transgenic rice with antisense Wx gene].

The amylose content of rice caryopsis is determined by Wx protein, one kind of granule-bound starch synthetase which is encoded by Wx gene. Different rice types and species have different levels of Wx gene expression and have different amylose contents in their caryopsis. Wuyunjing No.7 (2200), the japonica rice with an amylose content 17% and its transgenic rice lines with antisense Wx gene (2201 and 2203, with amylose contents 8.5% and 2% respectively), and Longtefu (LP03), the indica rice with a high amylose content (28%) and its transgenic rice with antisense Wx gene (A199, with an amylose content 9%) were used to investigate the effects of Wx protein content decrease on the activities of enzymes involved in starch synthesis and thereby starch accumulation. The results indicated that with the decrease in Wx protein, the amylose content in transgenic caryopsis was reduced accordingly, whereas the amylopectin content per caryopsis (mg/grain) was not affected, and made the total starch content in transgenic caryopsis markedly lower than their parents. With the development of caryopsis, the amylose/total starch ratio was not changed significantly in the two parent caryopses, LP03 and 2200, but it went down gradually in their transgenic caryopses. The amylose/total starch ratio in transgenic caryopses was very significantly lower than their parents in the same period. The activities of ADP-glucose pyrophosphorylase (ADPG-PPase), granule-bound starch synthase (GBSS), soluble starch synthase (SSS) and starch branching enzyme (SBE) rose rapidly in early periods of grain filling, and soon reached their maximum, then reduced quickly until the middle and later periods of grain filling. Compared with the parents, the GBSS activity in transgenic caryopsis was significantly lowered, and correlated with the amylose reduction. Besides, the maximum activity of GBSS appeared earlier, and the range of the activity was smaller. In transgenic caryopsis, the activities of ADPG-PPase and SSS were higher than their parents in early and middle periods of grain filling, while the SBE activity was higher than their parents in middle and late periods.