Inhibition of IAA‐induced cell elongation in Avena coleoptile segments by galactose: Its effect on UDP‐glucose formation

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Abstract
The effect of exogenously applied galactose on the cell wall polysaccharide synthesis and UDP‐sugar levels in oat (Avena sativa L. cv. Victory I) coleoptile segments was studied to clarify the mechanism of inhibition of IAA‐induced cell elongation by galactose, and the following results were obtained: (1) The inhibition of IAA‐induced cell elongation by galactose became apparent after a 2 h‐lag, while the lag was shortened to 1 h when galactose was added to the segments after more than 1 h of IAA application. (2) Galactose inhibited the [14C]‐glucose incorporation into cellulosic and non‐cellulosic fractions of the cell wall and the increase in net polysaccharide content in the fractions during long‐term incubation. (3) The dominant sugar nucleotide in oat coleoptiles was UDP‐glucose (2.1 nmol segment−1). Galactose application caused a remarkable decrease in the UDP‐glucose level, accompanying a strong accumulation of galactose‐1‐phosphate and UDP‐galactose. (4) Galactose‐1‐phosphate competitively inhibited the UTP: a‐d‐glucose‐1‐phosphate uridylyltransferase (EC 2.7.7.9) activity of the crude enzyme preparation from oat coleoptiles. From these results we conclude that galactose inhibits the IAA‐induced cell elongation by inhibiting the formation of UDP‐glucose, which is a key intermediate of cell wall polysaccharide synthesis.