Maintenance of indeterminacy is fundamental to the generation of plant architecture and a central component of the plant life strategy. Indeterminacy in plants is a characteristic of shoot and root meristems, which must balance maintenance of indeterminacy with organogenesis. The Petunia hybrida HAIRY MERISTEM (HAM) gene, a member of the GRAS family of transcriptional regulators, promotes shoot indeterminacy by an undefined non-cell-autonomous signaling mechanism(s). Here, we report that Arabidopsis (Arabidopsis thaliana) mutants triply homozygous for knockout alleles in three Arabidopsis HAM orthologs (Atham1,2,3 mutants) exhibit loss of indeterminacy in both the shoot and root. In the shoot, the degree of penetrance of the loss-of-indeterminacy phenotype of Atham1,2,3 mutants varies among shoot systems, with arrest of the primary vegetative shoot meristem occurring rarely or never, secondary shoot meristems typically arresting prior to initiating organogenesis, and inflorescence and flower meristems exhibiting a phenotypic range extending from wild type (flowers) to meristem arrest preempting organogenesis (flowers and inflorescence). Atham1,2,3 mutants also exhibit aberrant shoot phyllotaxis, lateral organ abnormalities, and altered meristem morphology in functioning meristems of both rosette and inflorescence. Root meristems of Atham1,2,3 mutants are significantly smaller than in the wild type in both longitudinal and radial axes, a consequence of reduced rates of meristem cell division that culminate in root meristem arrest. Atham1,2,3 phenotypes are unlikely to reflect complete loss of HAM function, as a fourth, more distantly related Arabidopsis HAM homolog, AtHAM4, exhibits overlapping function with AtHAM1 and AtHAM2 in promoting shoot indeterminacy.Indeterminate growth, the continuing generation and growth of organs and tissues throughout the life cycle of an organism, is a fundamental component of postembryonic plant development. Vascular plants grow discontinuously throughout their life spans, repeatedly initiating new shoot and root systems. This capacity for growth throughout the life span permits plants to adaptively regulate their growth patterns in response to dynamic environments, since, as sessile organisms, they cannot relocate in response to environmental stressors. Indeterminate growth is also a fundamental aspect of the "life strategy" of vascular plants, endowing woody perennials with the capacity for individuals to achieve life spans of thousands of years.In plants, indeterminate growth is the function of plant meristems. Lateral organs (leaves and floral organs) and stems are derived from shoot meristems, located at shoot apices. Root meristems, internal meristems located immediately above the columella of root apices, generate the radially organized tissues of the root. The primary shoot apical and root apical meristems arise during embryogenesis, while secondary meristems arise de novo during postembryonic development (McConnell and Barton, 1998;Laskowski et al., 2008). Meristems must ...

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