EFFECT OF WATER-STRESS ON CELL-DIVISION AND CELL-DIVISION-CYCLE 2-LIKE CELL-CYCLE KINASE-ACTIVITY IN WHEAT LEAVES

In wheat (Triticum aestivum) seedlings subjected to a mild water stres s (water potential of -0.3 MPa), the leaf-elongation rate was reduced by one-half and the mitotic activity of mesophyll cells was reduced to 42% of well-watered controls within 1 d. There was also a reduction i n the length of the zone of mesophyll cell division to within 4 mm fro m the base compared with 8 mm in control leaves. However, the period o f division continued longer in the stressed than in the control leaves , and the final cell number in the stressed leaves reached 85% of cont rols. Cyclin-dependent protein kinase enzymes that are required in viv o for DNA replication and mitosis were recovered from the meristematic zone of leaves by affinity for p13(suc1). Water stress caused a reduc tion in H1 histone kinase activity to one-half of the control level, a lthough amounts of the enzyme were unaffected. Reduced activity was co rrelated with an increased proportion of the 34-kD Cdca-like kinase (a n enzyme sharing with the Cdc2 protein of other eukaryotes the same si ze, antigenic sites, affinity for p13(suc1) and H1 histone kinase cata lytic activity) deactivated by tyrosine phosphorylation. Deactivation to 50% occurred within 3 h of stress imposition in cells at the base o f the meristematic zone and was therefore too fast to be explained by a reduction in the rate at which cells reached mitosis because of slow ing of growth; rather, stress must have acted more immediately on the enzyme. The operation of controls slowing the exit from the G1 and G2 phases is discussed. We suggest that a water-stress signal acts on Cdc 2 kinase by increasing phosphorylation of tyrosine, causing a shift to the inhibited form and slowing cell production.