Genetic Dissection of Apomixis in Dandelions Identifies a Dominant Parthenogenesis Locus and Highlights the Complexity of Autonomous Endosperm Formation

Apomixis in the common dandelion (Taraxacum officinale) consists of three developmental components: diplospory (apomeiosis), parthenogenesis, and autonomous endosperm development. The genetic basis of diplospory, which is inherited as a single dominant factor, has been previously elucidated. To uncover the genetic basis of the remaining components, a cross between a diploid sexual seed parent and a triploid apomictic pollen donor was made. The resulting 95 triploid progeny plants were genotyped with co-dominant simple-sequence repeat (SSR) markers and phenotyped for apomixis as a whole and for the individual apomixis components using Nomarski Differential Interference Contrast (DIC) microscopy of cleared ovules and seed flow cytometry. From this, a new SSR marker allele was discovered that was closely linked to parthenogenesis and unlinked to diplospory. The segregation of apomixis as a whole does not differ significantly from a three-locus model, with diplospory and parthenogenesis segregating as unlinked dominant loci. Autonomous endosperm is regularly present without parthenogenesis, suggesting that the parthenogenesis locus does not also control endosperm formation. However, the high recovery of autonomous endosperm is inconsistent with this phenotype segregating as the third dominant locus. These results highlight the genetic complexity underlying apomixis in the dandelion and underline the challenge of introducing autonomous apomixis into sexual crops.

Bacterial-induced calcium oscillations are common to nitrogen-fixing associations of nodulating legumes and nonlegumes

Plants that form root-nodule symbioses are within a monophyletic 'nitrogen-fixing' clade and associated signalling processes are shared with the arbuscular mycorrhizal symbiosis. Central to symbiotic signalling are nuclear-associated oscillations in calcium ions (Ca(2+) ), occurring in the root hairs of several legume species in response to the rhizobial Nod factor signal. In this study we expanded the species analysed for activation of Ca(2+) oscillations, including nonleguminous species within the nitrogen-fixing clade. We showed that Ca(2+) oscillations are a common feature of legumes in their association with rhizobia, while Cercis, a non-nodulating legume, does not show Ca(2+) oscillations in response to Nod factors from Sinorhizobium fredii NGR234. Parasponia andersonii, a nonlegume that can associate with rhizobia, showed Nod factor-induced calcium oscillations to S. fredii NGR234 Nod factors, but its non-nodulating sister species, Trema tomentosa, did not. Also within the nitrogen-fixing clade are actinorhizal species that associate with Frankia bacteria and we showed that Alnus glutinosa induces Ca(2+) oscillations in root hairs in response to exudates from Frankia alni, but not to S. fredii NGR234 Nod factors. We conclude that the ability to mount Ca(2+) oscillations in response to symbiotic bacteria is a common feature of nodulating species within the nitrogen-fixing clade.

IPD3 controls the formation of nitrogen-fixing symbiosomes in pea and Medicago Spp

A successful nitrogen-fixing symbiosis requires the accommodation of rhizobial bacteria as new organelle-like structures, called symbiosomes, inside the cells of their legume hosts. Two legume mutants that are most strongly impaired in their ability to form symbiosomes are sym1/TE7 in Medicago truncatula and sym33 in Pisum sativum. We have cloned both MtSYM1 and PsSYM33 and show that both encode the recently identified interacting protein of DMI3 (IPD3), an ortholog of Lotus japonicus (Lotus) CYCLOPS. IPD3 and CYCLOPS were shown to interact with DMI3/CCaMK, which encodes a calcium- and calmodulin-dependent kinase that is an essential component of the common symbiotic signaling pathway for both rhizobial and mycorrhizal symbioses. Our data reveal a novel, key role for IPD3 in symbiosome formation and development. We show that MtIPD3 participates in but is not essential for infection thread formation and that MtIPD3 also affects DMI3-induced spontaneous nodule formation upstream of cytokinin signaling. Further, MtIPD3 appears to be required for the expression of a nodule-specific remorin, which controls proper infection thread growth and is essential for symbiosome formation.

Strigolactone biosynthesis in Medicago truncatula and rice requires the symbiotic GRAS-type transcription factors NSP1 and NSP2

Legume GRAS (GAI, RGA, SCR)-type transcription factors NODULATION SIGNALING PATHWAY1 (NSP1) and NSP2 are essential for rhizobium Nod factor-induced nodulation. Both proteins are considered to be Nod factor response factors regulating gene expression after symbiotic signaling. However, legume NSP1 and NSP2 can be functionally replaced by nonlegume orthologs, including rice (Oryza sativa) NSP1 and NSP2, indicating that both proteins are functionally conserved in higher plants. Here, we show that NSP1 and NSP2 are indispensable for strigolactone (SL) biosynthesis in the legume Medicago truncatula and in rice. Mutant nsp1 plants do not produce SLs, whereas in M. truncatula, NSP2 is essential for conversion of orobanchol into didehydro-orobanchol, which is the main SL produced by this species. The disturbed SL biosynthesis in nsp1 nsp2 mutant backgrounds correlates with reduced expression of DWARF27, a gene essential for SL biosynthesis. Rice and M. truncatula represent distinct phylogenetic lineages that split approximately 150 million years ago. Therefore, we conclude that regulation of SL biosynthesis by NSP1 and NSP2 is an ancestral function conserved in higher plants. NSP1 and NSP2 are single-copy genes in legumes, which implies that both proteins fulfill dual regulatory functions to control downstream targets after rhizobium-induced signaling as well as SL biosynthesis in nonsymbiotic conditions.