Incorporation of Proline and Aromatic Amino Acids into Cell Walls of Maize Coleoptiles

Abstract

ABSTRACI7Sections excised from maize coleoptiles incorporated radioactivity from proline, tyrosine, and phenylalanine into structurl components of the cell wall. Only about 2% of radioactivity from proline taken up by sections was incorporated into cell wall; about 24% of that incorporated was in hydroxyproline and the rest remained in proline. In contrast, as much as 40% of the radioactivity from phenylalanine and 30% from tyrosine was incorporated into cell wall material. Most of this radioactivity was in saponifible ferulic acid. Small amounts of p-coumaric and diferulic acid were found, but only a small fraction of the hemicellulose can possibly be immobilized directly through cross-linking of diferulic esters.

Cell Wall Architecture of the Elongating Maize Coleoptile

“…In maize coleoptiles, the content of aromatic substances is lowest during the highest rates of growth, and the content increases markedly, as much as 10-fold in some fractions, as growth wanes. Coincident with this, Pro and Hyp, present in vanishingly small amounts during growth, also increase as growth stops (Carpita, 1986). Mü sel et al (1997) confirmed these findings in a study of 5-d-old coleoptiles, and showed further that, in addition to the hydroxycinnamic acids, lignin-like substances (thioacidolysis positive) also constitute part of the aromatic composition.…”

“…However, when the polyphenolic network is broken by oxidation in acidic chlorite, the dilute NaOH extracts a majority of the GAX, indicating that a significant amount of the wall matrix polymers are held in a matrix by the aromatic substances (Carpita, 1983b(Carpita, , 1986. Despite the marked differences in wall thickness, and the proportions of cellulose, ␤-glucan and glucomannan, this extraction behavior was observed for both mesophyll and epidermal cells.…”

“…These data indicate that the aromatic residues and polysaccharides (GAX) form coextensive matrices with the highly substituted, GAX-rich interstitial matrix and the relatively unsubstituted GAX of the ␤-glucan and glucomannan-enriched microfibril coatings. Based on observations of a marked inverse relationship between either growth rate or auxin responsiveness and the accumulation of these substances in the maize coleoptile (Carpita, 1986), it is doubtful that polysaccharide-polyphenol dynamics play a major role in cell growth.…”

“…The aromatic residues are largely ferulate and diferulate esterified to GAX, which are extracted completely by less than 0.5 m NaOH (Carpita 1986). Additional aromatic substances remain tightly bound and require much more concentrated NaOH to remove them.…”

“…As the elongation rate slows, the ␤-glucan is hydrolyzed by exo-and endo-␤-d-glucanases located in the wall. Concomitant with the decrease in ␤-glucan, there is an increase in the content of etherified and esterified hydroxycinnamic acids and other related lignin-like aromatic substances associated with the cessation of growth (Carpita, 1986;Mü sel et al, 1997). The coleoptile is a convenient model to examine the dynamic changes in cellular architecture during cell elongation in grasses; the polymer composition and architectural changes that occur in this organ reflect those of the primary wall of all developing cells (for review, see Carpita, 1996).…”