BackgroundThe Arabidopsis thaliana gene SPATULA (SPT), encoding a bHLH transcription factor, was originally identified for its role in pistil development. SPT is necessary for the growth and development of all carpel margin tissues including the style, stigma, septum and transmitting tract. Since then, it has been shown to have pleiotropic roles during development, including restricting the meristematic region of the leaf primordia and cotyledon expansion. Although SPT is expressed in roots, its role in this organ has not been investigated.ResultsAn analysis of embryo and root development showed that loss of SPT function causes an increase in quiescent center size in both the embryonic and postembryonic stem cell niches. In addition, root meristem size is larger due to increased division, which leads to a longer primary root. spt mutants exhibit other pleiotropic developmental phenotypes, including more flowers, shorter internodes and an extended flowering period. Genetic and molecular analysis suggests that SPT regulates cell proliferation in parallel to gibberellic acid as well as affecting auxin accumulation or transport.ConclusionsOur data suggest that SPT functions in growth control throughout sporophytic growth of Arabidopsis, but is not necessary for cell fate decisions except during carpel development. SPT functions independently of gibberellic acid during root development, but may play a role in regulating auxin transport or accumulation. Our data suggests that SPT plays a role in control of root growth, similar to its roles in above ground tissues.

Gamete formation is an important step in the life cycle of sexually reproducing organisms. In flowering plants, haploid spores are formed after the meiotic division of spore mother cells. These spores develop into male and female gametophytes containing gametes after undergoing mitotic divisions. In the female, the megaspore mother cell undergoes meiosis forming four megaspores, of which one is functional and three degenerate. The megaspore then undergoes three mitotic cycles thus generating an embryo sac with eight nuclei. The embryo sac undergoes cellularization to form the mature seven-celled female gametophyte. Entry into and progression through meiosis is essential for megasporogenesis and subsequent megagametogenesis, but control of this process is not well understood. FOUR LIPS (FLP) and its paralogue MYB88, encoding R2R3 MYB transcription factors, have been extensively studied for their role in limiting the terminal division in stomatal development by direct regulation of the expression of cell cycle genes. Here it is demonstrated that FLP and MYB88 also regulate female reproduction. Both FLP and MYB88 are expressed during ovule development and their loss significantly increases the number of ovules produced by the placenta. Despite the presence of excess ovules, single and double mutants exhibit reduced seed set due to reduced female fertility. The sterility results at least in part from defective meiotic entry and progression. Therefore, FLP and MYB88 are important regulators of entry into megasporogenesis, and probably act via the regulation of cell cycle genes.

Insights into the molecular control of cross-incompatibility in Zea mays Insights into the molecular control of cross incompatibility in Zea mays

Plant organ size and thus plant size is determined by both cell proliferation and cell expansion. The bHLH transcription factor SPATULA (SPT) was originally identified as a regulator of carpel patterning. It has subsequently been found to control growth of the organs of the shoot. It does this at least in part by controlling the size of meristematic regions of organs in parallel to gibberellic acid (GA). It also acts downstream of several environmental signals, influencing growth in response to light and temperature. We have recently demonstrated that SPT functions to repress the size of the root meristem and thus root growth and size. It appears to do this using a similar mechanism to its control of leaf size. Based on the recent work on SPT, we propose that it is a growth repressor that acts to limit the size of meristems in response to environmental signals, perhaps by regulating auxin transport.