DISTRIBUTION OF PECTINS IN CELL-WALLS OF MAIZE COLEOPTILES AND EVIDENCE AGAINST THEIR INVOLVEMENT IN AUXIN-INDUCED EXTENSION GROWTH

Aiming to elucidate the possible involvement of pectins in auxin-media ted elongation growth the distribution of pectins in cell walls of mai ze coleoptiles was investigated. Antibodies against defined epitopes o f pectin were used: JIM 5 recognizing pectin with a low degree of este rification, JIM 7 recognizing highly esterified pectin and 2F4 recogni zing a pectin epitope induced by Ca2+. JIM 5 weakly labeled the outer third of the outer epidermal wall and the center of filled cell comers in the parenchyma. A similar labeling pattern was obtained with 2F4. In contrast, JIM 7 densely labeled the whole outer epidermal wall exce pt the innermost layer, the middle lamellae, and the inner edges of op en cell corners in the parenchyma. Enzymatic de-esterification with pe ctin methylesterase increased the labeling by JIM 5 and 2F4 substantia lly. A further increase of the labeling density by JIM 5 and 2F4 and a n extension of the labeling over the whole outer epidermal wall could be observed after chemical de-esterification with alkali. This indicat es that both methyl- and other esters exist in maize outer epidermal w alls. Thus, in the growth-controlling outer epidermal wall a clear zon ation of pectin fractions was observed: the outermost layer (about one third to one half of wall thickness) contains whereas esterified pect in epitopes, presumably cross-linked by Ca2+, whereas esterified pecti ns are distributed evenly throughout the wall except the innermost lay er. Similar pectin patterns have been reported previously for dicot ce ll walls. The majority of uronic acid-containing material was solubili zed from isolated cell walls with CDTA and weak alkali. The CDTA- and Na2CO3-soluble fractions reacted with JIM 5, whereas JIM 7 showed high reactivity to a low-salt extract and a weak reactivity towards the CD TA-extractable fraction. 2F4 reacted strongly with both the CDTA extra ct and the Na2CO3 extract. Tracer experiments with H-3-myo-inositol sh owed rapid accumulation of tracer in all extractable pectin fractions and in a fraction tightly bound to the cell wall. A stimulatory effect of IAA on tracer incorporation could not be detected in any fraction. Summarizing the data a model of the pectin distribution in the cell w alls of maize coleoptiles was developed and its implications for the m echanism of auxin-induced wall loosening are discussed.