Inhibitor of striateconditionally suppresses cell proliferation in variegated maize

Abstract

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 in chlorotic leaf striping mutants of maize such as striate2 (sr2). We have shown thatIsr specifically inhibits proliferation and differentiation of plastid defective cells in sr2 mutants. Leaf narrowing is due to loss of intermediate veins and ground tissue located at leaf margins, and the few remaining plastid defective cells are of irregular size and aberrant organization. The Isr gene has been cloned by targeted transposon tagging. Isr mRNA is expressed throughout young leaves, but Isr chimeras indicate that the expression ofIsr at leaf margins is sufficient to suppress both the lateral expansion of sr2 leaves and the extent of striping. Isr protein appears to encode a chloroplast protein with sequence similarity to a family of bacterial phosphatases involved in carbon catabolite repression or in carbon metabolism. We propose that the action ofIsr in nuclear and plastid communication could be triggered by carbon stress.

The maizestriate leaves2(sr2) gene encodes a conserved DUF3732 domain homologous to the riceyss1gene

“…All UniformMu lines carry an allele of the colored1 gene known as R1-r:standard (Settles et al, 2007; McCarty et al, 2013). R1-r:standard is tightly linked to a dominant allele of inhibitor of striate1 ( Isr1) that inhibits the sr2 mutant phenotype (Kermicle and Axtell, 1981; Park et al, 2000). Although it is theoretically possible that Isr1 was recombined from R1-r:standard during the preparation of the UniformMu lines (McCarty et al, 2005), it seems highly unlikely, as Isr1 (Zm00001eb429350 on chr10:141210513-141213010) is only ∼20 kb from the P component of the complex r1 locus (Zm00001eb429330 on chr10:141187279-141196584) (Walker et al, 1995; Park et al, 2000).…”

“…One copy of Isr1 reduces the striping in a homozygous sr2 line, while two copies nearly eliminates the sr2 phenotype. In plants homozygous for both Isr1 and sr2 , leaves are thinner, and white stripes are only observed at the edges of leaf margins (Park et al, 2000).…”

“…The degree of striping is also environmentally sensitive, with full sibling sr2-ref plants grown in the field having more severe leaf margin striping than those grown concurrently in the greenhouse (Supplementary Figure 5). The powerful effects of Isr1 in reducing the sr2 phenotype illustrates that there are also genetic modifiers of sr2 (Park et al, 2000). The Isr1 gene suppresses the growth of white tissue in sr2 and other white striped mutants such as ij1 .…”

“…Striping is likely caused by the sorting out of mixed populations of functional and non-functional chloroplasts in a way that some lineages inherit no functional chloroplasts and appear as white sectors (Birky, 1983; Coe et al, 1988). White stripes tend to be wider and more common at the margins of leaves, because cells at the margin undergo more division to expand the width of the leaf than cells in the center of the leaf (Walbot and Coe, 1979; Han et al, 1992; Park et al, 2000).…”

The maize striate leaves2 (sr2) gene encodes a conserved DUF3732 domain and is homologous to the rice yss1 gene

“…Similar to ij1, sr2 appears to promote faster and/or more accurate chloroplast biogenesis (Han et al, 1992). sr2 phenotype illustrates that there are also genetic modifiers of sr2 (Park et al, 2000). The Isr1 gene suppresses the growth of white tissue in sr2 and other white striped mutants such as ij1.…”