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

摘要

Aiming to elucidate the possible involvement of pectins in auxin-mediated elongation growth the distribution of pectins in cell walls of maize coleoptiles was investigated. Antibodies against defined epitopes of pectin were used: JIM 5 recognizing pectin with a low degree of esterification, JIM 7 recognizing highly esterified pectin and 2F4 recognizing 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 except the innermost layer, the middle lamellae, and the inner edges of open cell corners in the parenchyma. Enzymatic de-esterification with pectin methylesterase increased the labeling by JIM 5 and 2F4 substantially. A further increase of the labeling density by JIM 5 and 2F4 and an extension of the labeling over the whole outer epidermal wall could be observed after chemical de-esterification with alkali. This indicates that both methyl- and other esters exist in maize outer epidermal walls. Thus, in the growth-controlling outer epidermal wall a clear zonation of pectin fractions was observed: the outermost layer (about one third to one half of wall thickness) contains whereas esterified pectin epitopes, presumably cross-linked by Ca2+, whereas esterified pectins are distributed evenly throughout the wall except the innermost layer. Similar pectin patterns have been reported previously for dicot cell walls. The majority of uronic acid-containing material was solubilized 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 CDTA-extractable fraction. 2F4 reacted strongly with both the CDTA extract and the Na2CO3 extract. Tracer experiments with H-3-myo-inositol showed 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 walls of maize coleoptiles was developed and its implications for the mechanism of auxin-induced wall loosening are discussed. [References: 42]