Auxin plays a fundamental role in organogenesis in plants. Multiple pathways for auxin biosynthesis have been proposed, but none of the predicted pathways are completely understood. Here, we report the positional cloning and characterization of the vanishing tassel2 (vt2) gene of maize (Zea mays). Phylogenetic analyses indicate that vt2 is a co-ortholog of TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS1 (TAA1), which converts Trp to indole-3-pyruvic acid in one of four hypothesized Trp-dependent auxin biosynthesis pathways. Unlike single mutations in TAA1, which cause subtle morphological phenotypes in Arabidopsis thaliana, vt2 mutants have dramatic effects on vegetative and reproductive development. vt2 mutants share many similarities with sparse inflorescence1 (spi1) mutants in maize. spi1 is proposed to encode an enzyme in the tryptamine pathway for Trp-dependent auxin biosynthesis, although this biochemical activity has recently been questioned. Surprisingly, spi1vt2 double mutants had only a slightly more severe phenotype than vt2 single mutants. Furthermore, both spi1 and vt2 single mutants exhibited a reduction in free auxin levels, but the spi1 vt2 double mutants did not have a further reduction compared with vt2 single mutants. Therefore, both spi1 and vt2 function in auxin biosynthesis in maize, possibly in the same pathway rather than independently as previously proposed.

Here, plant HAM proteins are shown to physically interact with the transcription factor WUSCHEL and the related WOX proteins, with this interaction driving downstream transcriptional programs and determining the activities of stem cells. Postembryonic development of above-ground tissues and of roots of plants depends on stem cells in the shoot and root apical meristems, respectively. The transcription factor WUSCHEL (WUS) and the related WOX proteins are known to be involved in the specification and maintenance of stem cells within all meristems throughout the plant kingdom. The HAIRY MERISTEM (HAM) family of transcription regulators is known to contribute to shoot stem cell function in certain plant species. The present paper shows that WOX and HAM family proteins act together to control the production of all types of stem cells in diverse niches in Arabidopsis, and that WUS and HAM share some common targets in vivo. Differences in the combined expression patterns of WOX and HAM family members appear to determine the formation of diverse stem cell niche locations. These findings demonstrate how co-operative transcriptional regulators drive common regulatory pathways and point to mechanisms underlying the evolution of stem cell regulation in plants. Plant stem cells in the shoot apical meristem (SAM) and root apical meristem are necessary for postembryonic development of aboveground tissues and roots, respectively, while secondary vascular stem cells sustain vascular development1,2,3,4. WUSCHEL (WUS), a homeodomain transcription factor expressed in the rib meristem of the Arabidopsis SAM, is a key regulatory factor controlling SAM stem cell populations5,6, and is thought to establish the shoot stem cell niche through a feedback circuit involving the CLAVATA3 (CLV3) peptide signalling pathway7. WUSCHEL-RELATED HOMEOBOX 5 (WOX5), which is specifically expressed in the root quiescent centre, defines quiescent centre identity and functions interchangeably with WUS in the control of shoot and root stem cell niches8. WOX4, expressed in Arabidopsis procambial cells, defines the vascular stem cell niche9,10,11. WUS/WOX family proteins are evolutionarily and functionally conserved throughout the plant kingdom12 and emerge as key actors in the specification and maintenance of stem cells within all meristems13. However, the nature of the genetic regime in stem cell niches that centre on WOX gene function has been elusive, and molecular links underlying conserved WUS/WOX function in stem cell niches remain unknown. Here we demonstrate that the Arabidopsis HAIRY MERISTEM (HAM) family of transcription regulators act as conserved interacting cofactors with WUS/WOX proteins. HAM and WUS share common targets in vivo and their physical interaction is important in driving downstream transcriptional programs and in promoting shoot stem cell proliferation. Differences in the overlapping expression patterns of WOX and HAM family members underlie the formation of diverse stem cell niche locations, and the HAM family is essential for all of these stem cell niches. These findings establish a new framework for the control of stem cell production during plant development.

In this study we investigated the role of ethylene in the formation of lateral and adventitious roots in tomato (Solanum lycopersicum) using mutants isolated for altered ethylene signaling and fruit ripening. Mutations that block ethylene responses and delay ripening –Nr (Never ripe), gr (green ripe), nor (non ripening), and rin (ripening inhibitor) – have enhanced lateral root formation. In contrast, the epi (epinastic) mutant, which has elevated ethylene and constitutive ethylene signaling in some tissues, or treatment with the ethylene precursor 1‐aminocyclopropane carboxylic acid (ACC), reduces lateral root formation. Treatment with ACC inhibits the initiation and elongation of lateral roots, except in the Nr genotype. Root basipetal and acropetal indole‐3‐acetic acid (IAA) transport increase with ACC treatments or in the epi mutant, while in the Nr mutant there is less auxin transport than in the wild type and transport is insensitive to ACC. In contrast, the process of adventitious root formation shows the opposite response to ethylene, with ACC treatment and the epi mutation increasing adventitious root formation and the Nr mutation reducing the number of adventitious roots. In hypocotyls, ACC treatment negatively regulated IAA transport while the Nr mutant showed increased IAA transport in hypocotyls. Ethylene significantly reduces free IAA content in roots, but only subtly changes free IAA content in tomato hypocotyls. These results indicate a negative role for ethylene in lateral root formation and a positive role in adventitious root formation with modulation of auxin transport as a central point of ethylene–auxin crosstalk.

Enough but not too many stem cells: In the shoot apical meristem of plants, just the right number of stem cells generates a steady supply of cells with which to build differentiated tissues. Too few stem cells, and the plant cannot grow. Too many, and growth runs amok. Zhou et al. analyzed the controls on stem cell proliferation. They found that the HAIRY MERISTEM proteins define a domain within which WUSCHEL (WUS) cannot work, but beyond which WUS is left free to promote stem cell proliferation. Science , this issue p. 502