Arabinogalactan proteins (AGPs) comprise a large family of hydroxyproline- containing glycoproteins associated with plant cell surfaces, both the plasmalemma and the extra-cellular matrix. Evidence for their having important roles in plant growth and development is beginning to accumulate at an accelerating rate. Among other things, they have been implicated as determinants of identity and as morphoregulatory molecules. We have obtained experimental evidence that a morphoregulatory role for certain AGPs may derive from their capacity to bring about and/or maintain potentially proliferating cells in a state of suppression. AGPs exhibit a range in buoyant densities from 1.55-1.73 in the plants we are working with. We found that surface-bound AGP fractions with buoyant densities about l.62 in Gymnocolea inflata gametophytes and l.70 in Lactuca sativa v Romana stem pith in which cell proliferation is normally suppressed, are absent from plants/tissues in which cell proliferation is experimentally """"""""desuppressed"""""""". We characterize the AGP fractions that become """"""""released"""""""" as HBD- (high buoyant density) AGPs and those that remain bound as LBD- (low buoyant density) AGPs. The different density classes are readily separated in total AGP extracts using CsCl gradient analysis. We propose to separate and compare the chemical compositions of the HBD- with LBD- AGPs of G. inflata as well as of L. sativa to see if there is something other than the degree of glycosylation that distinguishes the density classes. We will also compare HBD-AGPs of G. inflata with those of L. sativa to see if we can find some common, conserved feature(s) in this class in which binding properties change correlated with suppressed vs. desuppressed cell proliferation. The combined, parallel comparative chemical analysis of the liverwort gametophytes and the lettuce stem tissue are expected to provide a clue as to how the presence of certain HBD-AGPs at the cell surface helps maintain potentially proliferating cells in a suppressed state.
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