A new source of root-knot nematode resistance from Arachis stenosperma incorporated into allotetraploid peanut (Arachis hypogaea)

Root-knot nematode is a very destructive pathogen, to which most peanut cultivars are highly susceptible. Strong resistance is present in the wild diploid peanut relatives. Previously, QTLs controlling nematode resistance were identified on chromosomes A02, A04 and A09 of Arachis stenosperma. Here, to study the inheritance of these resistance alleles within the genetic background of tetraploid peanut, an F2 population was developed from a cross between peanut and an induced allotetraploid that incorporated A. stenosperma, [Arachis batizocoi x A. stenosperma]4×. This population was genotyped using a SNP array and phenotyped for nematode resistance. QTL analysis allowed us to verify the major-effect QTL on chromosome A02 and a secondary QTL on A09, each contributing to a percentage reduction in nematode multiplication up to 98.2%. These were validated in selected F2:3 lines. The genome location of the large-effect QTL on A02 is rich in genes encoding TIR-NBS-LRR protein domains that are involved in plant defenses. We conclude that the strong resistance to RKN, derived from the diploid A. stenosperma, is transferrable and expressed in tetraploid peanut. Currently it is being used in breeding programs for introgressing a new source of nematode resistance and to widen the genetic basis of agronomically adapted peanut lines.

Molecular biogeographic study of recently described B- and A-genome Arachis species, also providing new insights into the origins of cultivated peanut

The cultivated peanut Arachis hypogaea is a tetraploid, likely derived from A- and B-genome species. Reproductive isolation of the cultigen has resulted in limited genetic variability for important traits. Artificial hybridizations using selected diploid parents have introduced alleles from wild species, but improved understanding of recently classified B-genome accessions would aid future introgression work. To this end, 154 cDNA probes were used to produce 1887 RFLP bands scored on 18 recently classified or potential B-genome accessions and 16 previously identified species. One group of B-genome species consisted of Arachis batizocoi , Arachis cruziana , Arachis krapovickasii , and one potential additional species; a second consisted of Arachis ipaënsis , Arachis magna , and Arachis gregoryi . Twelve uncharacterized accessions grouped with A-genome species. Many RFLP markers diagnostic of A. batizocoi group specificity mapped to linkage group pair 2/12, suggesting selection or genetic control of chromosome pairing. The combination of Arachis duranensis and A. ipaënsis most closely reconstituted the marker haplotype of A. hypogaea, but differences allow for other progenitors or genetic rearrangements after polyploidization. From 2 to 30 alleles per locus were present, demonstrating section Arachis wild species variation of potential use for expanding the cultigen’s genetic basis.

Resistance to Meloidogyne arenaria in Advanced Generation Breeding Lines of Peanut1

Levels of resistance to the root-knot nematode Meloidogyne arenaria in F2 individuals from the second, third, and fourth backcross (BC) generations were compared in seven separate tests to that of the root-knot nematode-resistant peanut germplasm line TxAG-7. Resistance of TxAG-7 was derived from the wild species Arachis batizocoi, A. cardenasii, and A. diogoi. Recurrent susceptible parents were Florunner and Tamnut 74 for the all backcrosses, Tamspan 90 for BC3 and BC4, and NC 7 and VC-1 for BC4. Resistance in these tests was defined as an inhibition of nematode reproduction relative to that of the susceptible recurrent parent. Numerous individuals with a level of resistance similar to that of TxAG-7 were identified from each backcross generation. In three field tests, the resistant BC2 genotype TP-223 supported a lower final nematode population density than did its susceptible recurrent parent Florunner. When rooted cuttings from selected BC4F2 individuals were retested to confirm the original resistance class, ratings were unchanged for those originally identified as resistant or susceptible. Of nine individuals originally identified as having moderate resistance (2.5 to 12.5% of the eggs/g roots as the susceptible recurrent parent), one was identified as susceptible, one as moderately resistant, and seven as resistant (<2.5% of the eggs/g roots) upon retest. These data are evidence that this source of resistance is readily recoverable from advanced back-cross generations.

Spore Production and Latent Period as Mechanisms of Resistance to Cercospora Arachidicola in Four Peanut Genotypes 1,3

Arachis batizocoi, A. monticola and two genotypes of A. hypogaea (‘Florigiant’ and PI 109839) chosen to represent differing levels of resistance to early leafspot (Cercospora arachidicola) were evaluated for their effects on production of conidia per lesion, conidia per unit area of lesion and latent period necessary for sporulation. The largest lesions and the most conidia per lesion and unit lesion area were produced on cultivar ‘Florigiant’. PI 109839 had smaller lesions than Florigiant. Fewer conidia per lesion and per unit lesion area were produced on PI 109839 than Florigiant C. arachidicola sporulated abundantly on lesions from both Florigiant and PI 109839 15 days after inoculation. Size of lesions and conidia per lesion did not differ between A. monticola and PI 109839 but conidia per unit lesion area were fewer on A. monticola. The smallest lesions and the fewest conidia per lesion and per unit lesion area were produced on A. batizocoi. C. arachidicola did not begin sporulating on A. monticola and A. batizocoi until 18 days after inoculation. Sporulation of C. arachidicola was observed on defoliated leaves of A. monticola and A. batazocoi 21 days after inoculation.

Cytology of Interspecific Hybrids in Section Arachis of Peanuts1

Interspecific hybrids between Avachis correntina (Burk.) Krap. et Greg. nom. nud. (coll. GKP 9530–31) and seven other diploid peanut species of section Arachis nom. nud. [syn. Axonomorphae (7)] were cytologically analyzed. Although hybrid plants were partially sterile, cytological barriers to gene exchange were nonexistent except for A. batizocoi Krap. et Greg, hybrids. Arachis batizocoi hybrids had between 0 and 4.67% pollen fertility, probably due to an average of 2.88 univalents per cell. Laggards and anaphase I bridges were observed in 85% of the hybrid cells. Because the cultigen, A. hypogaea L. (2n = 40), and the diploid wild species (2n = 20) are at different ploidy levels, hybridization results in sterile triploid plants. This is a major barrier to introgression from wild to cultivated varieties. In order to derive wild species of section Arachis at the same ploidy level as A. hypogaea, 572 three to six-day-old seedlings were colchicine-treated representing 34 interspecific Arachis section hybrid combinations. Eighty-one cytologically confirmed amphidiploids plus 49 probable ones based on plant morphology were isolated. After colchicine treatments, 26 autotetraploids were likewise produced from six species. The observations indicated that selection can occur at the diploid level in wild species for desired agronomic traits or for disease and insect resistances. Colchicine-treated selected hybrid seedlings would then serve as a pathway for overcoming the major sterility obstacle to introgressing germplasm into A. hypogaea.