Gibberellic acid promotes growth and cell wall synthesis in Avena internodes regardless of the orientation of cell expansion

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Abstract
Segments cut from the next‐to‐last (peduncular‐1) internode of Avena sativa L. cv. Victory (oat) shoots elongate as much as 10‐fold in response to gibberellic acid (GA3). The objective of the present investigation was to differentiate the effects of GA3 on growth from its effects on wall synthesis (measured gravimetrically and through the incorporation of [14C]‐glucose) by using several cell wall synthesis inhibitors with widely varying mechanisms of action. Four compounds, viz. monensin, cycloheximide, lanthanum, and galactose. caused (1) relatively little inhibition of either cell wall synthesis or elongation in segments without GA3, (2) roughly proportionate, dose‐dependent inhibition of elongation and wall synthesis in GA3‐treated segments and (3) generally greater inhibition of GA3‐promoted uptake of radioactivity than of wall incorporation or elongation. Two other compounds, colchicine and 2,6‐dichlorobenzonitrile (DCB). (1) inhibited GA3‐induced elongation considerably more than cell wall synthesis and (2) caused swelling (radial expansion). especially of GA3‐treated segments. DCB‐treated internodal cells apparently compensated for inhibited cellulose synthesis by greater synthesis of matrix polysaccharide (beginning between 3 and 6 h). While normal cellulose synthesis was not required for short‐term (up to 6 h) GA3‐induced elongation or for long‐term hormone‐promoted radial expansion, it was required for sustained GA3‐induced elongation. These results indicate that GA3‐promoted cell wall loosening (manifested as radial expansion) and cell wall synthesis in Avena internodes occur at least partially independently of any hormonal effect on the orientation of microtubules and microfibrils.