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miR156 targets members of the SQUAMOSA PROMOTER BINDING PROTEIN LIKE ( SPL) family. In Arabidopsis and maize ( Zea mays), one of the microRNAs, miR156, is involved in plastochron regulation, and its overexpression causes a shortened plastochron ( Schwab et al., 2005 Wu and Poethig, 2006 Chuck et al., 2007). On the contrary, a cytokinin receptor triple mutant, cre1/ ahk4 ahk2 ahk3, showed a prolonged plastochron due to a reduced frequency of cell division in the SAM ( Higuchi et al., 2004 Nishimura et al., 2004). Furthermore, constitutive expression of cyclin D in tobacco ( Nicotinum tabacum) causes accelerated cell divisions and a shortened plastochron ( Cockcroft et al., 2000). In pla1, pla2 and amp1, cell divisions in the SAM occur more frequently than in wild type. In these studies, the rate of cell division is thought to be crucial for regulation of the plastochron.
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Several mutants with shortened or prolonged plastochrons have been reported ( Itoh et al., 2000 Ikeda et al., 2005, 2007)( Chaudhury et al., 1993 Reed et al., 1993 Prigge and Wagner, 2001 Kwon et al., 2005 Chuck et al., 2007). ABPH1 encodes an A-type response regulator, which is thought to act as a negative regulator of cytokinin signaling ( Giulini et al., 2004).Īlthough phyllotaxy has been extensively studied, less attention has been paid to the effect of the leaf initiation rate (the plastochron). Maize abphyl1 ( abph1) mutation drastically changes the spatial distribution of leaves (phyllotaxy) from 1/2 alternate to decussate phyllotaxy accompanying enlargement of the SAM ( Jackson and Hake, 1999). Cytokinin is also involved in the regulation of leaf initiation via SAM homeostasis. The newly initiated leaf primordium acts as a sink for auxin and interferes with initiation of new leaf primordia in its vicinity. Thus, auxin is regarded as an inducer of leaf initiation. Analysis of the auxin transport-deficient mutant pin-formed1 ( pin1) revealed that a new leaf primordium initiates at a position where the local auxin concentration becomes maximum, and a localized application of auxin to the SAM causes ectopic leaf initiation at that position ( Reinhardt et al., 2000, 2003). Plant hormones are thought to be important for the control of leaf initiation. Thus, genetic dissection of the pattern of leaf initiation is necessary for understanding how shoot architecture is established. In higher plants, leaves are initiated from the flank of the shoot apical meristem (SAM). Because branches are formed at the axils of leaves, the control of leaf production is of primary importance for the establishment of shoot architecture. Shoot architecture in higher plants is specified by the distribution of leaves and branches along the shoot axis.
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Our results suggest that PLA3 modulates various signaling pathways associated with a number of developmental processes. Double mutant analysis revealed that PLA1, PLA2 and PLA3 are regulated independently but function redundantly. Consistent with these pleiotropic phenotypes, PLA3 is expressed in the whole plant body, and is involved in plant hormone homeostasis. However, in contrast to pla1 and pla2, pla3 showed pleiotropic phenotypes including enlarged embryo, seed vivipary, defects in SAM maintenance and aberrant leaf morphology. pla3 exhibits similar phenotypes to pla1 and pla2– a shortened plastochron, precocious leaf maturation and rachis branch-to-shoot conversion in the reproductive phase. PLA3/ GO encodes a glutamate carboxypeptidase, which is thought to catabolize small acidic peptides and produce small signaling molecules. Here we report the identification of the rice gene PLASTOCHRON3 ( PLA3)/ GOLIATH ( GO) that regulates various developmental processes including the rate of leaf initiation (the plastochron). Leaves are the major component of the aerial plant body, and their temporal and spatial distribution mainly determines shoot architecture. Most aerial parts of the plant body are products of the continuous activity of the shoot apical meristem (SAM).