Rapid auxin-induced stimulation of cell wall synthesis in pea internodes

Abstract

The effect of auxin (indole-3-acetic acid; IAA) on growth and incorporation of myo-[2-(3)H(N)]inositol ([(3)H]Ins) into noncellulosic polysaccharides in the cell walls of third internode sections from red light-grown pea seedlings (Pisum sativum L. cv. Alaska) was investigated. Intact sections were incubated on [(3)H]Ins for 4 hr to permit uptake of the tracer and then IAA was added. Growth started after a lag phase of 15 min under these conditions. The sections were removed from the tracer and separated into epidermis and cortical cylinder (cortex plus vascular tissue). In the epidermis, IAA-induced stimulation of [(3)H]Ins incorporation started after a lag of 15 min. The amount of incorporation was 15% higher after 30 min and 24% higher after 2 hr than in the control. In the cortical cylinder, IAA-induced stimulation of [(3)H]Ins incorporation started only approximately 1 hr after adding IAA. The ionophore monensin (20 muM) inhibited the IAA-induced growth by 95%. Under these conditions, the IAA-induced stimulation of [(3)H]Ins incorporation and the IAA-induced increase in in vivo extensibility of the sections was almost completely inhibited, although oxygen uptake was unaffected. We suggest that wall synthesis (as represented by [(3)H]Ins incorporation) and wall loosening (increase in in vivo extensibility) are related processes. The results support the hypothesis that IAA induces growth by rapid stimulation of cell wall synthesis in the growth-limiting epidermal cell layer.