Cell wall proteins at low water potentials

We investigated the proteins extractable from cell walls of stem tissues when plants were subjected to low water potentials (low $\psi _{w}$). Dark-grown soybean seedlings (Glycine max [L.] Merr.) showed decreased stem growth when the roots were exposed to vermiculite having low water content ($\psi _{w}$ = -3 bar). After a time, growth resumed but at a reduced rate relative to the controls. The extractable protein increased in the cell walls as $\psi _{w}$ decreased, especially a 28-kilodalton protein in the young tissue. In contrast, a 70 kilodalton protein, mainly extractable from mature cell walls, appeared to decrease slightly at low $\psi _{w}$. No hydroxyproline was present in either protein, which shows that neither protein is related to extensin. The level of the 28 kilodalton protein increased in the cell wall of the dividing region soon after the initial growth inhibition, and it appeared in the elongating tissue at about the time growth resumed. The correlation between growth and these protein changes suggests that the two events could be related.