摘要:

The roots of many angiosperm species possess two Casparian bands, one in the endodermis and one in the outermost layer of the cortex. Over most of the root surface in these species, ions are taken up by the epidermis and may be transported symplastically to the xylem.

ISSN:0031-9317    

DOI:10.1111/j.1399-3054.1988.tb06644.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Until recently nearly all available experimental evidence seemed to indicate that the largest subunit of about 50 kDa in the photosystem II core complex (psbB gene product) is the site of primary photochemistry, and thus the name “P‐680 apoprotcin” has been given to this subunit. The notion, however, has also been challenged on the basis of deduced amino acid sequence homology between the proteins in the photosystem II and those of the purple bacterial reaction center. The actual preparation of a functionally active photosystem II reaction center completely devoid of thepsbB gene product, but consisting of D‐1 and D‐2 proteins and cytochromeb‐559, has now be

ISSN:0031-9317    

DOI:10.1111/j.1399-3054.1988.tb06645.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

The role of calcium in the individual cellular events leading to exocytosis is considered. Both vesicle movement processes and vesicle fusion at the cell surface require calcium for completion of specific events in this pathway. Our knowledge of these events is incomplete. In particular the movement of secretory vesicles by the cytoskeleton in response to added calcium is a key event that is beyond our comprehension at present. At the whole cell level, it is shown that external calcium, at the appropriate concentration, is required to elicit secretion at optimal rates. In both plant and animal cells secretion appears to be dependent on, or is triggered by, a rise in the level of internal free calcium ions from about 10‐7to 10‐6Mor even higher. In these eukaryotes internal organelles take up calcium and maintain a low level of calcium in the cell, offsetting the inflow of calcium from the plasma membrane. In some systems the inflow is restricted to a certain part of the plasma membrane, which then acts as a focus for exocytosis and, thereby, establishes a cellular polarity. In plant tissues there appears to be a requirement for some circulation of calcium within the apoplast, to sustain secretion.Recent papers on endocytosis have confirmed its occurrence in plant cells and made significant advances in isolating and characterising the clathrin coats of the coated vesicles involved in the uptake. There is no evidence, at present, for a direct role for calcium in these events. Indirectly, calcium stimulates exocytosis, and hence the delivery of excess membrane to the cell surface, which may be retrieved by an increase in the rate of endocytosis. Quantitative comparisons of the membrane flow occurring in these pathways are not available.Several plant cellular systems have been employed to study secretion and some of these may prove to be superior model systems for the investigation of certain aspects of the control of exocytosis and endocytosis by calcium i

ISSN:0031-9317    

DOI:10.1111/j.1399-3054.1988.tb06646.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY