Abstract
Hybrids lose heterotic yield advantage when multiplied sexually via meiosis. A potential alternative breeding system for hybrids is apospory, where female gametes develop without meiosis. Common among grasses, apospory begins in the nucellus, where aposporous initials (AIs) appear near the sexual megaspore mother cell (MeMC). The cellular origin of AIs is obscure, but one possibility, suggested by the mac1 and msp1 mutants of maize and rice, is that AIs are apomeiotic derivatives of the additional MeMCs that appear when genetic control over sporocyte numbers is relaxed. MULTIPLE SPOROCYTES1 (MSP1) encodes a leucine-rich-repeat receptor kinase, which is orthologous to EXS/EMS1 in Arabidopsis. Like mac1 and msp1, exs/ems1 mutants produce extra sporocytes in the anther instead of a tapetum, causing male sterility. This phenotype is copied in mutants of TAPETUM DETERMINANT1 (TPD1), which encodes a small protein hypothesized to be an extracellular ligand of EXS/EMS1. Here we show that rice contains two TPD1-like genes, OsTDL1A and OsTDL1B. Both are co-expressed with MSP1 in anthers during meiosis, but only OsTDL1A and MSP1 are co-expressed in the ovule. OsTDL1A binds to the leucine-rich-repeat domain of MSP1 in yeast two-hybrid assays and bimolecular fluorescence complementation in onion cells; OsTDL1B lacks this capacity. When driven by the maize Ubiquitin1 promoter, RNA interference against OsTDL1A phenocopies msp1 in the ovule but not in the anther. Thus, RNAi produces multiple MeMCs without causing male sterility. We conclude that OsTDL1A binds MSP1 in order to limit sporocyte numbers. OsTDL1A-RNAi lines may be suitable starting points for achieving synthetic apospory in rice.
Highlights
Hybrids out-perform inbreds under favorable and unfavorable conditions, but they lose heterotic yield advantage when multiplied sexually because of segregation and recombination occurring during meiosis (Bi et al, 2003)
We examine the sites of expression of these genes relative to MULTIPLE SPOROCYTES1 (MSP1), examine the affinity of OsTDL1A and OsTDL1B proteins for the LRR domain of MSP1, and study the effect of RNA interference of OsTDL1A on anther and ovule
We detected two TPD1like genes and named them OsTDL1A (BLAST e-value: 5 · 10)27) and OsTDL1B (BLAST e-value: 1 · 10)18). They are located on chromosomes 12 and 10, respectively, and the corresponding full-length cDNAs are AK108523 and AK121594. When their predicted protein sequences (NP_001066753, 226 aa; NP_001064316, 169 aa) were used as TBLASTN queries of the Arabidopsis genome, the best hits were to TAPETUM DETERMINANT1 (TPD1) and another Arabidopsis protein (ABF59206, 179 aa), which we have named AtTDL1
Summary
Hybrids out-perform inbreds under favorable and unfavorable conditions, but they lose heterotic yield advantage when multiplied sexually because of segregation and recombination occurring during meiosis (Bi et al, 2003). A possible alternative breeding system for hybrids is apospory, a form of seed production in which the embryo arises from a nucellar cell without meiosis Apospory is common among grasses, including relatives of several major cereal crops (Carman, 1997), but it is unknown in rice and its relatives (Brar et al, 1995). We have initiated a project to develop synthetic apospory for hybrid rice (Bi et al, 2003). Occurring apospory begins with the appearance of aposporous initials (AIs) alongside the megapore mother cell (MeMC) of the ovule (Bicknell and Koltunow, 2004). Like the MeMC, AIs are enlarged nucellar cells that have the capacity to develop into embryo sacs, but because they do so without undergoing meiosis, the resulting embryo sacs are diploid rather than haploid. Diploid egg cells can develop into embryos by parthenogenesis, i.e. without fertilization, producing progeny that are genetically identical to the maternal tissue (Matzk et al, 2005)
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