M. Kleinhentz

The Ma gene for complete-spectrum resistance to Meloidogyne species in Prunus is a TNL with a huge repeated C-terminal post-LRR region.

Root-knot nematode (RKN) Meloidogyne species are major polyphagous pests of most crops worldwide, and cultivars with durable resistance are urgently needed because of nematicide bans. The Ma gene from the Myrobalan plum (Prunus cerasifera) confers complete-spectrum, heat-stable, and high-level resistance to RKN, which is remarkable in comparison with the Mi-1 gene from tomato (Solanum lycopersicum), the sole RKN resistance gene cloned. We report here the positional cloning and the functional validation of the Ma locus present at the heterozygous state in the P.2175 accession. High-resolution mapping totaling over 3,000 segregants reduced the Ma locus interval to a 32-kb cluster of three Toll/Interleukin1 Receptor-Nucleotide Binding Site-Leucine-Rich Repeat (LRR) genes (TNL1–TNL3), including a pseudogene (TNL2) and a truncated gene (TNL3). The sole complete gene in this interval (TNL1) was validated as Ma, as it conferred the same complete-spectrum and high-level resistance (as in P.2175) using its genomic sequence and native promoter region in Agrobacterium rhizogenes-transformed hairy roots and composite plants. The full-length cDNA (2,048 amino acids) of Ma is the longest of all Resistance genes cloned to date. Its TNL structure is completed by a huge post-LRR (PL) sequence (1,088 amino acids) comprising five repeated carboxyl-terminal PL exons with two conserved motifs. The amino-terminal region (213 amino acids) of the LRR exon is conserved between alleles and contrasts with the high interallelic polymorphisms of its distal region (111 amino acids) and of PL domains. The Ma gene highlights the importance of these uncharacterized PL domains, which may be involved in pathogen recognition through the decoy hypothesis or in nuclear signaling.

Terpene variations in maritime pine constitutive oleoresin related to host tree selection byDioryctria sylvestrella RATZ. (Lepidoptera: Pyralidae)

Gas chromatographic analysis of the volatile terpenes of the wood oleoresin of maritime pines showed that the relative proportions of terpenes within the same tree did not vary according to sampling height and date. Pruning and tree vigor did not affect terpene profiles, but oleoresin composition varied widely from tree to tree. The trees attacked byDioryctria sylvestrella contained a significantly higher percentage of limonene, longipinene, and copaene, and a discriminant analysis, using the relative percentage of six terpenes, significantly separated infested and uninfested trees. The olfactory sensitivity of adult moths to a series of 20 volatile chemicals, including maritime pine oleoresin components, was evaluated with the electroantennogram method. Both sexes were sensitive to monoterpenes, oxygenated terpenoid compounds, and oleoresin samples. The use of terpenes as host attractants and biochemical markers for resistance toD. sylvestrella is discussed.

Characterization of the RMja gene for resistance to root-knot nematodes in almond: spectrum, location, and interest for Prunus breeding

In Prunus spp., resistance genes to root-knot nematodes (RKN), Meloidogyne arenaria, Meloidogyne incognita, Meloidogyne javanica, and Meloidogyne floridensis, confer either a complete spectrum, e.g., the Ma and Rjap genes in Myrobalan and Japanese plums (subgenus Prunophora), respectively, or a more restricted spectrum, e.g., the RMia gene (M. arenaria + M. incognita) in peach (subgenus Amygdalus). We report here characterization data of the RMja gene from the almond Alnem1, another Amygdalus source. The study of its spectrum is hampered by the inability of almond to be propagated by cuttings; we overcame this problem by using F1 and BC1 crosses with previously genotyped Myrobalan plums that conferred their rooting ability to hybrids for simultaneous evaluation to different RKN. As expected from a homozygous dominant resistance, BC1 progenies of Alnem1 segregated for resistance to M. javanica but were uniformly susceptible to M. incognita and M. floridensis, demonstrating that RMja controlled M. javanica but not M. incognita nor M. floridensis. SSR markers covering the Prunus reference map placed RMja on LG7 in the same region as Ma and Rjap and thus showed its independence from the RMia gene (LG2) of the botanically closer peach. The spectrum of this gene allows the theoretical construction of interspecific rootstocks, Myrobalan plum × (almond × peach), which cumulate RMja with Ma and RMia and are protected from each of the predominant RKN affecting Prunus, i.e., M. arenaria, M. incognita, and M. javanica, by at least two genes. This pyramiding strategy should offer to rootstock material an unprecedented guarantee of durable RKN resistance.

Genetic dissection of resistance to root-knot nematodes Meloidogyne spp. in plum, peach, almond, and apricot from various segregating interspecific Prunus progenies

Sources of resistance in Prunus spp. exhibit different spectra to the root-knot nematodes (RKN) Meloidogyne incognita, Meloidogyne javanica and Meloidogyne floridensis. In this Prunus genus, two dominant genes, Ma with a complete spectrum from the heterozygous Myrobalan plums P.2175 and P.2980 (section Euprunus; subgenus Prunophora) and RMia with a more restricted spectrum from the peaches Nemared and Shalil (subgenus Amygdalus), have been identified. This study characterizes the resistance spectra of interspecific crosses involving (1) previous Myrobalan and peach sources, (2) two Alnem almonds (subgenus Amygdalus) resistant to M. javanica, and (3) the apricot A.3923, representing a species considered RKN-resistant (section Armeniaca; Prunophora). For both latter species, genetic data could be obtained through F1 crosses with genetically characterized Myrobalans that conferred their rooting ability for clonal multiplication of the hybrids and permitted their simultaneous evaluation to the three RKN. Crosses involving either Ma or RMia or both generated the expected resistance spectra. Nemared confirmed the species-specific resistance to M. incognita conferred by RMia. This rootstock, also previously considered resistant to M. javanica, was susceptible to the M. javanica isolate used, what illustrates an isolate-specific resistance to this species. Alnem accessions were shown homozygous resistant to M. javanica. In the progeny P.2980 × A.3923, Ma markers allowed to distinguish resistant individuals carrying that gene from resistant individuals lacking it. Distribution of non-Ma individuals in this cross suggested, in the apricot parent, (1) the absence of a major gene allelic to Ma and (2) the presence of a non RKN specific polygenic resistance.

Terpene Attractant Candidates of Dioryctria sylvestrella in Maritime Pine (Pinus pinaster) Oleoresin, Needles, Liber, and Headspace Samples

Capillary GC analysis was used to determine the proportion and quantity of terpenes in wood resin, pentane extracts from needles and liber, and headspace samples of needles and pruning wounds in 24 thirteen-year-old maritime pine. Fifteen different terpenes were identified in the samples. Germacrene D and β-pinene were the dominant terpenic compounds in the needles, while α-pinene and β-pinene were dominant in the liber and wood resin. Headspace samples of both needles and pruning wounds contained essentially monoterpenes. Only trace amounts of sesquiterpenes were found in pruning wound emissions. The presence of an oxygenated compound, linalool, in the pruning wound emissions is discussed, although this compound is not found in Maritime pine essential oil. Twelve of the 24 trees studied were infested by Dioryctria sylvestrella. Maritime pine susceptibility to this insect was related to the terpene composition of the different samples.