PHASE-SEPARATION OF PLANT-CELL WALL POLYSACCHARIDES AND ITS IMPLICATIONS FOR CELL-WALL ASSEMBLY

Concentrated binary mixtures of polymers in solution commonly exhibit immiscibility, resolving into two separate phases each of which is enr iched in one polymer. The plant cell wall is a concentrated polymer as sembly, and phase separation of the constituent polymers could make an important contribution to its structural organization and functional properties. However, to our knowledge,there have been no published rep orts of the phase behavior of cell wall polymers, and this phenomenon is not included in current cell wall models. We fractionated cell wall s purified from the pericarp of unripe tomatoes (Lycopersicon esculent um) by extraction with cyclohexane diamine tetraacetic acid (CDTA), Na 2CO3, and KOH and examined the behavior of concentrated mixtures. Seve ral different combinations of fractions exhibited phase separation. An alysis of coexisting phases demonstrated the immiscibility of the este rified, relatively unbranched pectic polysaccharide extracted by CDTA and a highly branched, de-esterified pectic polysaccharide present in the 0.5 N KOH extract. Some evidence for phase separation of the CDTA extract and hemicellulosic polymers was also found. We believe that ph ase separation is likely to be a factor in the assembly of pectic poly saccharides in the cell wall and could, for example, provide the basis for explaining the formation of the middle lamella.