Regulation of SHOOT MERISTEMLESS genes via an upstream-conserved noncoding sequence coordinates leaf development

Abstract

The indeterminate shoot apical meristem of plants is characterized by the expression of the Class 1 KNOTTED1-LIKE HOMEOBOX (KNOX1) genes. KNOX1 genes have been implicated in the acquisition and/or maintenance of meristematic fate. One of the earliest indicators of a switch in fate from indeterminate meristem to determinate leaf primordium is the down-regulation of KNOX1 genes orthologous to SHOOT MERISTEMLESS (STM) in Arabidopsis (hereafter called STM genes) in the initiating primordia. In simple leafed plants, this down-regulation persists during leaf formation. In compound leafed plants, however, KNOX1 gene expression is reestablished later in the developing primordia, creating an indeterminate environment for leaflet formation. Despite this knowledge, most aspects of how STM gene expression is regulated remain largely unknown. Here, we identify two evolutionarily conserved noncoding sequences within the 5 upstream region of STM genes in both simple and compound leafed species across monocots and dicots. We show that one of these elements is involved in the regulation of the persistent repression and/or the reestablishment of STM expression in the developing leaves but is not involved in the initial down-regulation in the initiating primordia. We also show evidence that this regulation is developmentally significant for leaf formation in the pathway involving ASYMMETRIC LEAVES1/2 (AS1/2) gene expression; these genes are known to function in leaf development. Together, these findings reveal a regulatory point of leaf development mediated through a conserved, noncoding sequence in STM genes.evolution ͉ KNOX1 ͉ shoot apical meristem T he shoot apical meristem (SAM) of plants is an indeterminate structure and the source of stem cells from which all aerial organs are derived. Indeterminacy during development is regulated by a suite of genes that function in the SAM. The process of organ initiation begins when cells in the incipient organ primordium change identity from indeterminate to determinate. The indeterminate SAM is characterized by the expression of the Class 1 KNOTTED1-LIKE HOMEOBOX (KNOX1) genes. KNOX1 genes have been implicated in the acquisition and/or maintenance of meristematic fate (1, 2). One of the earliest indicators of a switch in fate from indeterminate meristem to determinate leaf primordium is the down-regulation of KNOX1 genes orthologous to SHOOT MERISTEMLESS (STM) in Arabidopsis (hereafter called STM genes) in the incipient primordia (1).The SAM can give rise to two different leaf forms, simple or compound (3), and KNOX1 genes are down-regulated in the incipient primordia in both compound and simple leafed species. In simple leafed plants, this down-regulation persists during leaf formation (4-7), and overexpression of these genes is sufficient to cause leaf lobing and ectopic meristem formation (8-11). However, KNOX1 expression is reestablished later in the developing primordia of compound leafed plants (12-15). Additionally, overexpression of KNOX1 genes in transgenic tomato plants or spo...

KNOX genes: versatile regulators of plant development and diversity

“…It will, therefore, be interesting to identify the precise KNOX gene regulatory elements responsible for leaf expression in C. hirsuta and other species. Phylogenetic foot-printing of STM regulatory sequences, sampled from species with both simple and dissected leaves, identified the K-box as a conserved regulatory element required for repression of STM in the simple leaves of A. thaliana and tobacco (Uchida et al, 2007). Notably, this defined the first cisregulatory element to mediate KNOX repression.…”

Leaf Development

“…brief, all leaf primordia initially down-regulate class I KNOX. There is subsequent persistent repression in simple-leaf primordia through functioning of the K-box region in the promoter, but there is a class I KNOX reactivation in compound leaf primordia (Bharathan et al 2002;Uchida et al 2007). When expression is ectopic in the primordia, leaves become deformed, developing irregular lobes or deep serrations due to an abnormally increased potential for organogenesis.…”

LATERAL ORGAN BOUNDARIES DOMAIN transcription factors direct callus formation in Arabidopsis regeneration

“…Functional characterization of several LBD members revealed that LBD genes play critical roles in defining lateral organ boundaries and regulating many aspects of plant development, including root, leaf, inflorescence and embryo development. For example, the founding member of this family, Arabidopsis LBD6/AS2, is involved not only in a regulatory loop that maintains shoot meristem and defines lateral organ boundary antagonistically with SHOOT MER-ISTEMLESS [22], but also in the control of leaf polarity and flower development by interacting with AS1, a MYB transcription factor [23,24]. Arabidopsis LBD30 and LBD18 positively regulate xylem differentiation in leaf and root [25], and poplar LBD1 is involved in the regulation of secondary growth [26].…”