Pre-formed xyloglucans and xylans increase in molecular weight in three distinct compartments of a maize cell-suspension culture
2003
Kerr, E.M. | Fry, S.C.
Cultured cells of maize (Zea mays L.) were pulse-labelled with L-[1-3H]arabinose (Ara) and then monitored for 7 days. The 3H-hemicelluloses present in three compartments (protoplasm, cell wall and culture medium) were size-fractionated and the fractions assayed for [3H]xyloglucans and [3H]xylans. Protoplasmic [3H]xylans and [3H]xyloglucans initially (15 min after [3H]Ara-feeding) had weight-average relative molecular masses (M(w)) approximately equal to 0.5x10(6) and 0.3x10(6), respectively, both rising to 2x10(6) by 30 min. Thus, newly formed hemicellulose molecules were joined to other polymers, or to each other, presumably within Golgi vesicles. New 3H-hemicelluloses very rapidly bound to the cell wall; however, after 1 day, some [3H]xyloglucan and [3H]xylan was sloughed from the wall into the medium. The wall-bound [3H]xyloglucans were present in the form of extremely large complexes, of M(w)>17x10(6), even as early as 15 min after [3H]Ara-feeding. This M(w) is > 70-fold greater than that observed by similar methods in cultures of a dicotyledon (Rosa sp.). Thus, during wall-binding, newly secreted xyloglucans greatly increased in size, possibly by transglucosylation. Some modest degradation (trimming) of wall-bound [3H]xyloglucan occurred later. The earliest wall-bound [3H]xylan had M(w) approximately equal to 2x10(6), similar to the protoplasmic [3H]xylan; this increased to approximately equal to 4x10(6) by 6 h. For the first 2 days after [3H]Ara-feeding, the soluble extracellular 3H-hemicelluloses present in the culture medium had M(w) approximately equal to 1x10(6)-2x10(6), comparable to the protoplasmic hemicelluloses. However, between 2 and 3 days after [3H]Ara-feeding, the M(w) of the soluble extracellular [3H]xylans increased abruptly to approximately equal to 10x10(6); the soluble extracellular [3H]xyloglucans underwent a similar but more gradual increase in M(w). Maize 3H-hemicelluloses thus underwent increases in M(w) in three episodes: (i) intra-protoplasmically, (ii) during wall-binding (especially xyloglucans), and (iii) after sloughing into the medium. Possible mechanisms and roles of these increases are discussed.
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