LAX and SPA : Major regulators of shoot branching in rice

Abstract

The aerial architecture of plants is determined primarily by the pattern of shoot branching. Although shoot apical meristem initiation during embryogenesis has been extensively studied by molecular genetic approaches using Arabidopsis, little is known about the genetic mechanisms controlling axillary meristem initiation, mainly because of the insufficient number of mutants that specifically alter it. We identified the LAX PANICLE (LAX) and SMALL PANICLE (SPA) genes as the main regulators of axillary meristem formation in rice. LAX encodes a basic helix-loop-helix transcription factor and is expressed in the boundary between the shoot apical meristem and the region of new meristem formation. This pattern of LAX expression was repeatedly observed in every axillary meristem, consistent with our observation that LAX is involved in the formation of all types of axillary meristems throughout the ontogeny of a rice plant. Ectopic LAX expression in rice caused pleiotropic effects, including dwarfing, an altered pattern of stem elongation, darker color, bending of the lamina joint, absence of the midribs of leaves, and severe sterility. O rganogenesis occurs in plants throughout their lifetimes. The main axis of growth is determined by the production of two meristems: a primary shoot apical meristem (SAM) and a root meristem at embryogenesis. During postembryonic development, plants initiate a multitude of growth axes by forming new meristems called axillary meristems, which are generated in the axils of leaves and give rise to branch shoots and flowers (1). Therefore, the pattern of axillary meristem initiation and development is a key factor in determining plant architecture. Significant progress has been made on the molecular genetic analysis of SAM initiation during embryo development; however, little is known about the initiation of axillary meristems.The development of an axillary meristem is controlled by two distinctive steps, namely, the initiation of a new meristem in the axil of a leaf and its outgrowth. Mutations that exhibit an altered pattern of axillary bud outgrowth have been described for various plant species (1), e.g., Arabidopsis auxin resistant1 (2), supershoot (3), max1, and max2 (4) and maize teosinte branched (5). On the other hand, there are only a few mutants in which the axillary meristem initiation is specifically altered. Maize barren inflorescence 2 (bif2) (6) and barren stalk1 (ba1) (7), Arabidopsis revoluta (rev) (8, 9), tomato lateral suppressor (ls) (10) and lateral suppressor of Arabidopsis (las), its cognate ortholog in Arabidopsis (11), tomato blind (bl) (12), and rice lax panicle (lax) (13) and monoculm 1(moc1) (14) are categorized in this class of mutants. The bif2 and ba1 exhibit severe suppression of all types of axillary meristems, implying that they are involved in genetic pathways controlling the general steps of axillary meristem initiation. Similarly, defects are observed in all types of lateral branches in tomato bl and Arabidopsis rev; however, the expressivity of their mut...

Hormonal Regulation of Branching in Grasses

“…There is an ortholog of BA1 in rice, LAX PANICLE (LAX; Komatsu et al, 2003), but an ortholog of BA1 has not been reported in Arabidopsis.…”

Rice leaf inclination2, a VIN3-like protein, regulates leaf angle through modulating cell division of the collar

“…Ectopic expression of LAX PANICLE (LAX), a basic helix-loop-helix transcription factor, increased the rice lamina joint bending [19]. Overexpression of BRASSINOSTEROID UPREGULATED 1 (BU1), a helix-loop-helix transcription factor involved in BR signaling in rice, results in the enhanced lamina joint bending whereas the RNAi plants with suppressed BU1 expression showed the erected leaf phenotype [20].…”

Rice DENSE AND ERECT PANICLE 2 is essential for determining panicle outgrowth and elongation

“…Interestingly, rapid elongation of the panicle does not start until stage In 8 [17], when inflorescence meristems abort and all floral organs, like primary branches, secondary branches, and spikelets, are differentiated. Currently, several genes involved in the formation or initiation of floral meristems have been isolated [18][19][20][21][22][23][24]. MOC1 (MONOCULM 1) is required for both vegetative and reproductive axillary meristem formation [23].…”

“…MOC1 (MONOCULM 1) is required for both vegetative and reproductive axillary meristem formation [23]. The b-HLH transcriptional regulator, encoded by LAX (LAX PANICLE), is involved in the initiation/maintenance of the floral branch meristem [21]. FZP (FRIZZY PANI-CLE) is a positive regulator of floral meristem identity, suppressing the formation of axillary meristems of rice spikelets [22].…”