Pre-formed xyloglucans and xylans increase in molecular weight in three distinct compartments of a maize cell-suspension culture

Abstract

Cultured cells of maize ( Zea mays L.) were pulse-labelled with l-[1-(3)H]arabinose (Ara) and then monitored for 7 days. The (3)H-hemicelluloses present in three compartments (protoplasm, cell wall and culture medium) were size-fractionated and the fractions assayed for [(3)H]xyloglucans and [(3)H]xylans. Protoplasmic [(3)H]xylans and [(3)H]xyloglucans initially (15 min after [(3)H]Ara-feeding) had weight-average relative molecular masses ( M(w)) approximately 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 (3)H-hemicelluloses very rapidly bound to the cell wall; however, after 1 day, some [(3)H]xyloglucan and [(3)H]xylan was sloughed from the wall into the medium. The wall-bound [(3)H]xyloglucans were present in the form of extremely large complexes, of M(w)>17x10(6), even as early as 15 min after [(3)H]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 [(3)H]xyloglucan occurred later. The earliest wall-bound [(3)H]xylan had M(w) approximately 2x10(6), similar to the protoplasmic [(3)H]xylan; this increased to approximately 4x10(6) by 6 h. For the first 2 days after [(3)H]Ara-feeding, the soluble extracellular (3)H-hemicelluloses present in the culture medium had M(w) approximately 1x10(6)-2x10(6), comparable to the protoplasmic hemicelluloses. However, between 2 and 3 days after [(3)H]Ara-feeding, the M(w) of the soluble extracellular [(3)H]xylans increased abruptly to approximately 10x10(6); the soluble extracellular [(3)H]xyloglucans underwent a similar but more gradual increase in M(w). Maize (3)H-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.