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PLANT PHYSIOLOGY , Vol 105, Issue 2 699-706, Copyright © 1994 by American Society of Plant Biologists
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DEVELOPMENT AND GROWTH REGULATION |
Cell Expansion and Tracheary Element Differentiation Are Regulated by Extracellular pH in Mesophyll Cultures of Zinnia elegans L
A. W. Roberts and C. H. Haigler
Botany Department, University of Rhode Island, Kingston, Rhode Island 02881 (A.W.R.)
The effects of medium pH on cell expansion and tracheary element (TE)
differentiation were investigated in differentiating mesophyll suspension
cultures of Zinnia elegans L. In unbuffered cultures initially adjusted to
pH 5.5, the medium pH fluctuated reproducibly, decreasing about 1 unit
prior to the onset of TE differentiation and then increasing when the
initiation of new Tes was complete. Elimination of large pH fluctuations by
buffering the culture medium with 20 mM 2-(N-morpholino)ethanesulfonic acid
altered both cell expansion and TE differentiation, whereas altering the
starting pH of unbuffered culture medium had no effect on either process.
Cell expansion in buffered cultures was pH dependent with an optimum of 5.5
to 6.0. The direction of cell expansion was also pH dependent in buffered
cultures. Cells elongated at pH 5.5 to 6.0, whereas isodiametric cell
expansion was predominant at pH 6.5 to 7.0. The onset of TE differentiation
was delayed when the pH was buffered higher or lower than 5.0. However, TEs
eventually appeared in cultures buffered at pH 6.5 to 7.0, indicating that
a decrease in pH to 5.0 is not necessary for differentiation. Very large
TEs with secondary cell wall thickenings resembling metaxylem
differentiated in cultures buffered at pH 5.5 to 6.0, which also showed the
greatest cell expansion. The correlation between cell expansion and delayed
differentiation of large, metaxylem-like TEs may indicate a link between
the regulatory mechanisms controlling cell expansion and TE
differentiation.
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