Conservation of Arabidopsis Flowering Genes in Model Legumes^{<a href="#fn1" rid="fn1" class=" fn">1</a>,}^{<a href="#fn4" rid="fn4" class=" fn">[w]</a>}

Abstract

The change from vegetative to reproductive growth is a critical developmental transition in the life of a plant, and the induction, expression, and maintenance of the flowering state are regulated by many external and endogenous factors. A vast number of applied and fundamental studies have demonstrated the importance of light (through daylength and light-quality effects) and temperature (through vernalization and ambient temperature effects) as the main environmental regulators of flowering. However, other factors, including nutrient status, endogenous hormones, stress, and the developmental state of the plant, can also be important. Even with respect to light and temperature, great diversity in responsiveness exists within and between different plant species. These differences are important in the adaptation of species to particular latitudinal and climatic regions, and have also been extremely important for determining the environments and agronomic regimes under which crop species can be most effectively grown.

The flowering process has been subject to detailed genetic analysis in Arabidopsis (Arabidopsis thaliana). As a small, weedy annual, Arabidopsis is responsive to a wide range of factors and has been invaluable in outlining the major genetic pathways that are likely to function in the control of flowering responses to photoperiod, vernalization, and hormone responses (Amasino, 2004; Boss et al., 2004; Putterill et al., 2004). It is likely that many of the genetic mechanisms discovered in Arabidopsis identify general themes that have been elaborated in different ways across the plant kingdom. However, plants show incredible diversity in growth habit and phenology, and it is clear that we have only scratched the surface in understanding how this diversity might be generated.

Several reports provide an illustration of the ways in which similar basic mechanisms might be adapted to produce quite different patterns of environmental response. Recent comparative studies have shown that the function of several genes involved in photoperiod responsiveness is conserved between Arabidopsis and rice (Oryza sativa), and suggest that the difference between long-day (LD)- and short-day (SD)-responsive plants results from a different regulatory interaction between two genes, CONSTANS (CO) and FLOWERING LOCUS T (FT; Hayama et al., 2003). In other cases, different genes can achieve similar patterns of environmental response. For example, in both Arabidopsis and wheat (Triticum aestivum), vernalization acts to promote flowering by repressing the expression of an important floral regulator. In Arabidopsis, this repressor is the MADS-domain protein FLOWERING LOCUS C (FLC), whereas in cereals, which do not appear to possess FLC-like genes, the corresponding role is played by an unrelated zinc-finger transcription factor (Yan et al., 2004).

With the advent of genomic approaches in a range of model plant systems, the information gained from Arabidopsis is rapidly being extended into other species. The complete sequence of the rice genome has allowed a global comparison of flowering pathways between rice and Arabidopsis (Izawa et al., 2003), and the same kind of analysis is also now possible in poplar. A number of studies have already provided detailed phylogenetic descriptions of particular flowering-related gene families and/or functional analysis of individual genes in species such as rice, barley (Hordeum vulgare), tomato (Lycopersicon esculentum), petunia (Petunia hybrida), and Antirrhinum (e.g. Carmel-Goren et al., 2003; Griffiths et al., 2003; Hayama et al., 2003; Vandenbussche et al., 2003). However, there has been only limited molecular analysis of flowering in legumes, despite the importance of flowering time in legume production systems and the availability of extensive expressed sequence tag (EST) collections for model legumes such as soybean (Glycine max) and Medicago truncatula. In soybean, precise genetic control of flowering time has been achieved using classical breeding and is essential for efficient cropping in different latitudinal and climatic regions (e.g. Curtis et al., 2000), but molecular information about the genes involved has not yet emerged into the public domain. The timing of flowering is also an agronomically important trait in many other legume species, including pea (Pisum sativum), bean (Phaseolus spp.), lentil (Lens culinaris), chickpea (Cicer arietinum), and lupin (Lupinus spp.; Huyghe, 1998), and genetic variation for flowering is being utilized in all of these (e.g. Wallace et al., 1993; Sarker et al., 1999; Kumar and van Rheenen, 2000). A better understanding of flowering control in legumes will also benefit general understanding of the flowering process. Crop and model legumes exhibit great diversity in phenology with respect to photoperiod and temperature responses, lifespan, mono/polycarpy, and the determinacy and architecture of inflorescences. For example, soybean is a vernalization-unresponsive SD species (Summerfield and Roberts, 1985), whereas both M. truncatula and Lotus japonicus are vernalization-responsive LD species (Clarkson and Russell, 1975). Both soybean and M. truncatula are annual, but another closely related Medicago species, alfalfa (Medicago sativa), and L. japonicus are perennial (Handberg and Stougaard, 1992).

It is only in the garden pea, an annual, vernalization-responsive LD species, that genetic, physiological, and molecular approaches to flowering have converged to any appreciable extent. Pioneering physiological-genetic studies through the 1970s identified a number of major flowering genes in pea and provided a model for the flowering process that incorporated vernalization, photoperiod, and mobile flowering signals (Murfet, 1985; Weller et al., 1997). The lack of subsequent progress in identifying these genes at the molecular level has meant that, until recently, it has not been possible to relate this model to those in other systems. However, several recent reports have presented mutant-based functional analyses of flowering-related genes in pea, including the photoreceptor genes PHYTOCHROME A (PHYA) and PHYB, and homologs of Arabidopsis inflorescence identity genes LEAFY (LFY), UNUSUAL FLOWER ORGAN (UFO), and APETALA1 (AP1; Hofer et al., 1997; Berbel et al., 2001; Taylor et al., 2001, 2002; Weller et al., 2001, 2004). Interestingly, the LFY ortholog in pea is not only involved in floral initiation, as it is in Arabidopsis, but also in leaf development, a function not described in Arabidopsis (Hofer et al., 1997). In other studies, the pea homologs of AP1 and PISTILLATA (PI) have been shown to fully complement the corresponding Arabidopsis mutants, despite lacking C-terminal motifs suggested to be essential for the function of the Arabidopsis genes (Yalovsky et al., 2000; Berbel et al., 2001; A. Berbel, C. Navarro, C. Ferrándiz, L. Cañas, J.-P. Beltrán, and F. Madueño, unpublished data). Other pea flowering loci, DETERMINATE (DET) and LATE FLOWERING (LF), have recently been shown to be homologs of the Arabidopsis gene TERMINAL FLOWER1 (TFL1; Foucher et al., 2003). The DET gene maintains indeterminacy of primary shoot apex, which in det mutants is converted into a determinate, secondary inflorescence. The LF gene delays flowering in a photoperiod-independent manner, and loss-of-function mutants are early flowering but retain photoperiod responsiveness. It thus appears that multiple roles of Arabidopsis TFL1 may have been differentially apportioned in different pea homologs. Overall, these reports suggest that basic flowering pathways are likely to be relatively well conserved in pea and other legumes and support the use of a candidate gene approach as a first step in identifying the molecular nature of other pea flowering genes.

We set out to define on a broad scale the extent to which genes important for the flowering process in Arabidopsis are conserved in model legumes. We found that a large proportion of Arabidopsis flowering genes are represented in legumes, and have isolated partial sequences for many of these genes from pea. Preliminary mapping analyses emphasize the close synteny between pea and Medicago and suggest some potential candidate genes for known pea mutants.

Acknowledgments

Notes

^{This work was supported by the Australian Research Council Discovery Project (grant no. DP0210947 to J.L.W.), Génoplante (project PEA-A; C.R.), the Secretaría General del Plan Nacional de Investigación Científica y Desarrollo Tecnológico (grant no. BIO2000–0940 to J.P.B.), the Ministerio de Educación y Ciencia (fellowships to C.F. and C.N.), the European Union Grain Legumes Integrated Project (grant no. FP6–2002–FOOD–1–506223 to N.E. and C.R.), and a New Zealand FRS&amp;T Fellowship and Marsden Fund grant (R.M.).}

^{The online version of this article contains Web-only data.}

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.104.057018.