Inhibitor of striate conditionally suppresses cell proliferation in variegated maize

Since the work done by R.A. Emerson in the 1930s, Inhibitor of striate (Isr ) has been recognized as a dose-dependent genetic modifier of variegation i n chlorotic leaf striping mutants of maize such as striate2 (sr2). We have shown that Isr specifically inhibits proliferation and differentiation of p lastid defective cells in sr2 mutants. Leaf narrowing is due to loss of int ermediate veins and ground tissue located at leaf margins, and the few rema ining plastid defective tells are of irregular size and aberrant organizati on. The Isr gene has been cloned by targeted transposon tagging. Isr mRNA i s expressed throughout young leaves, but Isr chimeras indicate that the exp ression of Isr at leaf margins is sufficient to suppress both the lateral e xpansion of sr2 leaves and the extent of striping. Isr protein appears to e ncode a chloroplast protein with sequence similarity to a family of bacteri al phosphatases involved in carbon catabolite repression or in carbon metab olism. We propose that the action of Isr in nuclear and plastid communicati on could be triggered by carbon stress.