Shedding light on flower development: phytochrome B regulates gynoecium formation in association with the transcription factor SPATULA

A high-resolution gene expression map of the medial and lateral domains of the gynoecium of Arabidopsis

Angiosperms are characterized by the formation of flowers, and in their inner floral whorl, one or various gynoecia are produced. These female reproductive structures are responsible for fruit and seed production, thus ensuring the reproductive competence of angiosperms. In Arabidopsis (Arabidopsis thaliana), the gynoecium is composed of two fused carpels with different tissues that need to develop and differentiate to form a mature gynoecium and thus the reproductive competence of Arabidopsis. For these reasons, they have become the object of study for floral and fruit development. However, due to the complexity of the gynoecium, specific spatio-temporal tissue expression patterns are still scarce. In this study, we used precise laser-assisted microdissection and high-throughput RNA sequencing to describe the transcriptional profiles of the medial and lateral domain tissues of the Arabidopsis gynoecium. We provide evidence that the method used is reliable and that, in addition to corroborating gene expression patterns of previously reported regulators of these tissues, we found genes whose expression dynamics point to being involved in cytokinin and auxin homeostasis and in cell cycle progression. Furthermore, based on differential gene expression analyses, we functionally characterized several genes and found that they are involved in gynoecium development. This resource is available via the Arabidopsis eFP browser and will serve the community in future studies on developmental and reproductive biology.

A mutation in CsHY2 encoding a phytochromobilin (PΦB) synthase leads to an elongated hypocotyl 1(elh1) phenotype in cucumber (Cucumis sativus L.)

The elongated hypocotyl1 (elh1) mutant in cucumber is due to a mutation in CsHY2, which is a homolog of the Arabidopsis HY2 encoding the phytochromobilin (PΦB) synthase for phytochrome biosynthesis Hypocotyl length is a critical determinant in establishing high quality seedlings for successful cucumber production, but knowledge on the molecular regulation of hypocotyl growth in cucumber is very limited. Here, we reported identification and characterization of a cucumber elongated hypocotyl 1 (elh1) mutant. We found that the longer hypocotyl in elh1 was due to longitudinal growth of hypocotyl cells. With fine mapping, the elh1 locus was delimited to a 20.9-kb region containing three annotated genes; only one polymorphism was identified in this region between two parental lines, which was a non-synonymous SNP (G28153633A) in the third exon of CsHY2 (CsGy1G030000) that encodes a phytochromobilin (PΦB) synthase. Uniqueness of the mutant allele at CsHY2 was verified in natural cucumber populations. Ectopic expression of CsHY2 in Arabidopsis hy2-1 long-hypocotyl mutant led to reduced hypocotyl length. The PΦB protein was targeted to the chloroplast. The expression levels of CsHY2 and five phytochrome genes CsPHYA1, CsPHYA2, CsPHYB, CsPHYC and CsPHYE were all significantly down-regulated while several cell elongation related genes were up-regulated in elh1 mutant compared to wild-type cucumber, which are correlated with dynamic hypocotyl elongation in the mutant. RNA-seq analysis in the WT and mutant revealed differentially expressed genes involved in porphyrin and chlorophyll metabolisms, cell elongation and plant hormone signal transduction pathways. This is the first report to characterize and clone the CsHY2 gene in cucumber. This work reveals the important of CsHY2 in regulating hypocotyl length and extends our understanding of the roles of CsHY2 in cucumber.

SPATULA regulates floral transition and photomorphogenesis in a PHYTOCHROME B-dependent manner in Arabidopsis

Light is the most important exogenous stimulus regulating plant growth and various developmental processes. Phytochromes, especially PHYTOCHROME B (PHYB) mediates the various light-mediated processes in Arabidopsis. SPATULA (SPT) is an important transcription factor, which has been reported previously to participate in temperature-mediated transition from seed dormancy to germination. Here we investigate the function of SPT in the floral transition under long day conditions and photomorphogenesis in Arabidopsis. In this study, spt-2 shows significantly delayed flowering time. But mutation of SPT in the background of phyb-1 rescues the phenotype of spt-2. The flowering time of double mutant of spt-2/phyb-1 is similar with the wild type. These results indicate that SPT promotes the transition from vegetative stage to floral stage and it regulates this transition in a PHYB-dependent manner. With qRT-PCR analysis, it is found that SPT regulates flowering time via FLC, SVP, FT and SOC1. Furthermore, SPT also controls photomorphogenesis. spt-2 displays shortened hypocotyls and increased chlorophylls contents compared with the wild type. These phenotypes are also rescued in the double mutant of spt-2/phyb-1. These results indicate that SPT is also involved in photomorphogenic development in Arabidopsis and SPT regulates photomorphogenesis in a PHYB-dependent manner. Collectively, SPT is not only a temperature responder but it is also an important light regulator during plant growth and development.

Shade avoidance components and pathways in adult plants revealed by phenotypic profiling

Shade from neighboring plants limits light for photosynthesis; as a consequence, plants have a variety of strategies to avoid canopy shade and compete with their neighbors for light. Collectively the response to foliar shade is called the shade avoidance syndrome (SAS). The SAS includes elongation of a variety of organs, acceleration of flowering time, and additional physiological responses, which are seen throughout the plant life cycle. However, current mechanistic knowledge is mainly limited to shade-induced elongation of seedlings. Here we use phenotypic profiling of seedling, leaf, and flowering time traits to untangle complex SAS networks. We used over-representation analysis (ORA) of shade-responsive genes, combined with previous annotation, to logically select 59 known and candidate novel mutants for phenotyping. Our analysis reveals shared and separate pathways for each shade avoidance response. In particular, auxin pathway components were required for shade avoidance responses in hypocotyl, petiole, and flowering time, whereas jasmonic acid pathway components were only required for petiole and flowering time responses. Our phenotypic profiling allowed discovery of seventeen novel shade avoidance mutants. Our results demonstrate that logical selection of mutants increased success of phenotypic profiling to dissect complex traits and discover novel components.

The bHLH transcription factor SPATULA is a key regulator of organ size in Arabidopsis thaliana

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.

The bHLH transcription factor SPATULA regulates root growth by controlling the size of the root meristem

BACKGROUND

The 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.

RESULTS

An 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.

CONCLUSIONS

Our 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.

A light-regulated genetic module was recruited to carpel development in Arabidopsis following a structural change to SPATULA

A key innovation of flowering plants is the female reproductive organ, the carpel. Here, we show that a mechanism that regulates carpel margin development in the model flowering plant Arabidopsis thaliana was recruited from light-regulated processes. This recruitment followed the loss from the basic helix-loop-helix transcription factor SPATULA (SPT) of a domain previously responsible for its negative regulation by phytochrome. We propose that the loss of this domain was a prerequisite for the light-independent expression in female reproductive tissues of a genetic module that also promotes shade avoidance responses in vegetative organs. Striking evidence for this proposition is provided by the restoration of wild-type carpel development to spt mutants by low red/far-red light ratios, simulating vegetation shade, which we show to occur via phytochrome B, PHYTOCHROME INTERACTING FACTOR4 (PIF4), and PIF5. Our data illustrate the potential of modular evolutionary events to generate rapid morphological change and thereby provide a molecular basis for neo-Darwinian theories that describe this nongradualist phenomenon. Furthermore, the effects shown here of light quality perception on carpel development lead us to speculate on the potential role of light-regulated mechanisms in plant organs that, like the carpel, form within the shade of surrounding tissues.