PLASTOCHRON2Regulates Leaf Initiation and Maturation in Rice

Abstract

In higher plants, leaves initiate in constant spatial and temporal patterns. Although the pattern of leaf initiation is a key element of plant shoot architecture, little is known about how the time interval between initiation events, termed plastochron, is regulated. Here, we present a detailed analysis of plastochron2 (pla2), a rice (Oryza sativa) mutant that exhibits shortened plastochron and precocious maturation of leaves during the vegetative phase and ectopic shoot formation during the reproductive phase. The corresponding PLA2 gene is revealed to be an orthologue of terminal ear1, a maize (Zea mays) gene that encodes a MEI2-like RNA binding protein. PLA2 is expressed predominantly in young leaf primordia. We show that PLA2 normally acts to retard the rate of leaf maturation but does so independently of PLA1, which encodes a member of the P450 family. Based on these analyses, we propose a model in which plastochron is determined by signals from immature leaves that act non-cell-autonomously in the shoot apical meristem to inhibit the initiation of new leaves.

Dual Effects of miR156-TargetedSPLGenes andCYP78A5/KLUHon Plastochron Length and Organ Size inArabidopsis thaliana

“…Both are preferentially expressed in young leaf primordia, and they could potentially act in a pathway dependent on miR156 and its SPL targets, which are conserved in monocotyledonous plants (Xie et al, 2006;Chuck et al, 2007). Consistent with this hypothesis is the finding that PLA2 appears to function independently of PLA1 in rice (Kawakatsu et al, 2006), similar to the relationship between miR156/SPLs and CYP78A5 in Arabidopsis. Unfortunately, it is unclear whether TE1/PLA2 homologs fulfill related roles in Arabidopsis, especially since the most similar genes, TEL1 and TEL2, are expressed in the shoot apical meristem only, different from TE1 and PLA2 (Anderson et al, 2004).…”

“…A similar correlation between reduced plastochron length and delayed phase change is seen in rice pla1 and pla2 mutants (Itoh et al, 1998;Kawakatsu et al, 2006). A net outcome of this would be that phase change is more normal in these mutants when measured in absolute time.…”

“…In these plants, it appears that the increase in plastochron length is mostly due to increased cell division rates, which is also the case in several rice mutants, including pla1 and pla2 (Itoh et al, 1998;Ikeda et al, 2005;Kawakatsu et al, 2006). A direct link between cell division and plastochron length has been made with plants that overexpress Cyclin D2, which leads to faster leaf initiation correlated with higher cell division rates.…”

“…A final phenotype that is shared between several mutants is the reduced size of leaves and floral organs (Itoh et al, 1998;Veit et al, 1998;Kawakatsu et al, 2006;Anastasiou et al, 2007;this work). An increase in plastochron length, at least during inflorescence development, has also been suggested for the big brother mutant, which has enlarged organs as well (Disch et al, 2006).…”

“…Inactivation of several genes, including rice (Oryza sativa) PLASTOCHRON1 (PLA1), maize (Zea mays) TERMINAL EAR1 (TE1) and its rice ortholog PLA2, and ALTERED MERISTEM PROGRAM1 (AMP1) in Arabidopsis thaliana, causes a shortening of the plastochron (Conway and Poethig, 1997;Itoh et al, 1998;Veit et al, 1998;Helliwell et al, 2001;Miyoshi et al, 2004;Kawakatsu et al, 2006). By contrast, phytochrome b (phyb) and serrate (se) mutants in Arabidopsis suffer from delayed initiation of new leaves (Reed et al, 1993;Clarke et al, 1999;Prigge and Wagner, 2001).…”