Fostering in situ conservation of wild relatives of forage crops.

Most plant conservation strategies generally overlook the intra-specific genetic diversity of crop gene pools. Focusing on forage crops and their wild relatives, we present a novel approach to address the conservation of these species on meadows. Two-thirds of Swiss agricultural land is green land, mostly used for forage purposes, and their genetic diversity is being threatened. We focused here on eight plant associations gathering at least 18 taxa considered priority crop wild relatives of forage crops. Since 2020, about 1,217 high-quality surfaces (representing 1,566 hectares) nationwide have been integrated into an innovative auction-based policy instrument dedicated to conserving these populations. Here, we report the benefits and hurdles of implementing this bottom-up approach and try to estimate the quality of conservation of the forage plants' CWR gene pool. Although we focus on the Swiss case, our approach to in situ conservation offers opportunities to effectively guide conservation in other contexts. We also discuss possible ways to improve CWR conservation policy, particularly the need to better consider the populations and habitat levels.

Towards a Joint International Database: Alignment of SSR Marker Data for European Collections of Cherry Germplasm.

The objective of our study was the alignment of microsatellite or simple sequence repeat (SSR) marker data across germplasm collections of cherry within Europe. Through the European Cooperative program for Plant Genetic Resources ECPGR, a number of European germplasm collections had previously been analysed using standard sets of SSR loci. However, until now these datasets remained unaligned. We used a combination of standard reference genotypes and ad-hoc selections to compile a central dataset representing as many alleles as possible from national datasets produced in France, Great Britain, Germany, Italy, Sweden and Switzerland. Through the comparison of alleles called in data from replicated samples we were able to create a series of alignment factors, supported across 448 different allele calls, that allowed us to align a dataset of 2241 SSR profiles from six countries. The proportion of allele comparisons that were either in agreement with the alignment factor or confounded by null alleles ranged from 67% to 100% and this was further improved by the inclusion of a series of allele-specific adjustments. The aligned dataset allowed us to identify groups of previously unknown matching accessions and to identify and resolve a number of errors in the prior datasets. The combined and aligned dataset represents a significant step forward in the co-ordinated management of field collections of cherry in Europe.

Polyspermy barriers: a plant perspective.

A common denominator of sexual reproduction in many eukaryotic species is the exposure of an egg to excess sperm to maximize the chances of reproductive success. To avoid potential harmful or deleterious consequences of supernumerary sperm fusion to a single female gamete (polyspermy), many eukaryotes, including plants, have evolved barriers preventing polyspermy. Typically, these checkpoints are implemented at different stages in the reproduction process. The virtual absence of unambiguous reports of naturally occurring egg cell polyspermy in flowering plants is likely reflecting the success of this multiphasic strategy and highlights the difficulty to trace this presumably rare event. We here focus on potential polyspermy avoidance mechanisms in plants and discuss them in light of analogous processes in animals.

Female gametophytic cell specification and seed development require the function of the putative Arabidopsis INCENP ortholog WYRD.

In plants, gametes, along with accessory cells, are formed by the haploid gametophytes through a series of mitotic divisions, cell specification and differentiation events. How the cells in the female gametophyte of flowering plants differentiate into gametes (the egg and central cell) and accessory cells remains largely unknown. In a screen for mutations that affect egg cell differentiation in Arabidopsis, we identified the wyrd (wyr) mutant, which produces additional egg cells at the expense of the accessory synergids. WYR not only restricts gametic fate in the egg apparatus, but is also necessary for central cell differentiation. In addition, wyr mutants impair mitotic divisions in the male gametophyte and endosperm, and have a parental effect on embryo cytokinesis, consistent with a function of WYR in cell cycle regulation. WYR is upregulated in gametic cells and encodes a putative plant ortholog of the inner centromere protein (INCENP), which is implicated in the control of chromosome segregation and cytokinesis in yeast and animals. Our data reveal a novel developmental function of the conserved cell cycle-associated INCENP protein in plant reproduction, in particular in the regulation of egg and central cell fate and differentiation.

The gametic central cell of Arabidopsis determines the lifespan of adjacent accessory cells.

Plant germ cells develop in specialized haploid structures, termed gametophytes. The female gametophyte patterns of flowering plants are diverse, with often unknown adaptive value. Here we present the Arabidopsis fiona mutant, which forms a female gametophyte that is structurally and functionally reminiscent of a phylogenetic distant female gametophyte. The respective changes include a modified reproductive behavior of one of the female germ cells (central cell) and an extended lifespan of three adjacent accessory cells (antipodals). FIONA encodes the cysteinyl t-RNA synthetase SYCO ARATH (SYCO), which is expressed and required in the central cell but not in the antipodals, suggesting that antipodal lifespan is controlled by the adjacent gamete. SYCO localizes to the mitochondria, and ultrastructural analysis of mutant central cells revealed that the protein is necessary for mitochondrial cristae integrity. Furthermore, a dominant ATP/ADP translocator caused mitochondrial cristae degeneration and extended antipodal lifespan when expressed in the central cell of wild-type plants. Notably, this construct did not affect antipodal lifespan when expressed in antipodals. Our results thus identify an unexpected noncell autonomous role for mitochondria in the regulation of cellular lifespan and provide a basis for the coordinated development of gametic and nongametic cells.

Analyzing female gametophyte development and function: There is more than one way to crack an egg.

In flowering plants, gametes are formed in specialized haploid structures, termed gametophytes. The female gametophyte is a few-celled structure that integrates such diverse functions as pollen tube attraction, sperm cell release, gamete fusion and seed initiation. These processes are realized by distinct cell types, which ensure reproductive success in a coordinated manner. In the past decade, much progress has been made concerning the molecular nature of the functions carried out by the different cell types. Here, we review recent work that has shed light on female gametophyte development and function with a particular focus on approaches that have led to the isolation of genes involved in these processes.

CLO/GFA1 and ATO are novel regulators of gametic cell fate in plants.

The formation of gametes is a key step in the life cycle of any sexually reproducing organism. In flowering plants, gametes develop in haploid structures termed gametophytes that comprise a few cells. The female gametophyte forms gametic cells and flanking accessory cells. During a screen for regulators of egg-cell fate, we isolated three mutants, lachesis (lis), clotho (clo) and atropos (ato), that show deregulated expression of an egg-cell marker. We have previously shown that, in lis mutants, which are defective for the splicing factor PRP4, accessory cells can differentiate gametic cell fate. Here, we show that CLOTHO/GAMETOPHYTIC FACTOR 1 (CLO/GFA1) is necessary for the restricted expression of egg- and central-cell fate and hence reproductive success. Surprisingly, infertile gametophytes can be expelled from the maternal ovule tissue, thereby preventing the needless allocation of maternal resources to sterile tissue. CLO/GFA1 encodes the Arabidopsis homologue of Snu114, a protein that is considered to be an essential component of the spliceosome. In agreement with their proposed role in pre-mRNA splicing, CLO/GFA1 and LIS co-localize to nuclear speckles. Our data also suggest that CLO/GFA1 is necessary for the tissue-specific expression of LIS. Furthermore, we demonstrate that ATO encodes the Arabidopsis homologue of SF3a60, a protein that has been implicated in pre-spliceosome formation. Our results thus establish that the restriction of gametic cell fate is specifically coupled to the function of various core spliceosomal components.

How females become complex: cell differentiation in the gametophyte.

In contrast to animals, gametes in plants form a separate haploid generation, the gametophyte. The female gametophyte of flowering plants consists of just four different cell types that play distinct roles in the reproductive process. Differentiation of the distinct cell fates is tightly controlled and appears to follow regional cues that are arranged along a polar axis. Mutant analysis suggests that important aspects of gametophyte patterning are gametophytically regulated. Additionally, structural and molecular changes following misspecification indicate that the female gametophyte is a remarkably versatile structure with enormous respecification potential. Recently, new tools have been developed that open fascinating possibilities to access and analyze those processes that ultimately ensure successful fertilization.

LACHESIS restricts gametic cell fate in the female gametophyte of Arabidopsis.

In flowering plants, the egg and sperm cells form within haploid gametophytes. The female gametophyte of Arabidopsis consists of two gametic cells, the egg cell and the central cell, which are flanked by five accessory cells. Both gametic and accessory cells are vital for fertilization; however, the mechanisms that underlie the formation of accessory versus gametic cell fate are unknown. In a screen for regulators of egg cell fate, we isolated the lachesis (lis) mutant which forms supernumerary egg cells. In lis mutants, accessory cells differentiate gametic cell fate, indicating that LIS is involved in a mechanism that prevents accessory cells from adopting gametic cell fate. The temporal and spatial pattern of LIS expression suggests that this mechanism is generated in gametic cells. LIS is homologous to the yeast splicing factor PRP4, indicating that components of the splice apparatus participate in cell fate decisions.