Deciphering Resistance to Root-Knot Nematodes in Prunus for Rootstock Breeding: Sources, Genetics and Characterization of the Ma Locus

Root-knot nematode (RKN) species are predominant pests of crops, attacking stone fruit crops Prunus spp. under Mediterranean climate conditions worldwide. Natural resistance for rootstock breeding is a control method that is gaining interest as an alternative to the highly toxic nematicides. This review first reports an outline of the root-knot nematodes parasitizing stone fruit crops and the Prunus species and rootstocks. It then describes the main sources of resistance detected among the Prunus germplasm and focuses on the major resistance genes identified and their characteristics (spectrum, durability, histological mechanism, effect of temperature, interaction with other pests and diseases, etc.). In peach, besides the RMia reference gene, the new genes PkMi and Mf, also located on chromosome 2, need to be characterized regarding their spectrum and relationship. The two other Prunus reference genes, Ma from plum (complete spectrum) and RMja from almond (more restricted spectrum), are orthologs that belong to a TIR-NB-LRR (TNL) cluster on chromosome 7. The review finally summarizes the positional cloning of the Ma gene and the characterization of its unique TNL structure, encompassing a five-times repeated post-LRR domain. Deciphering how this structure is functionally involved in Ma’s remarkable biological properties is a real challenge for the future.

Advances in Rootstock Breeding of Nut Trees: Objectives and Strategies

The production and consumption of nuts are increasing in the world due to strong economic returns and the nutritional value of their products. With the increasing role and importance given to nuts (i.e., walnuts, hazelnut, pistachio, pecan, almond) in a balanced and healthy diet and their benefits to human health, breeding of the nuts species has also been stepped up. Most recent fruit breeding programs have focused on scion genetic improvement. However, the use of locally adapted grafted rootstocks also enhanced the productivity and quality of tree fruit crops. Grafting is an ancient horticultural practice used in nut crops to manipulate scion phenotype and productivity and overcome biotic and abiotic stresses. There are complex rootstock breeding objectives and physiological and molecular aspects of rootstock–scion interactions in nut crops. In this review, we provide an overview of these, considering the mechanisms involved in nutrient and water uptake, regulation of phytohormones, and rootstock influences on the scion molecular processes, including long-distance gene silencing and trans-grafting. Understanding the mechanisms resulting from rootstock × scion × environmental interactions will contribute to developing new rootstocks with resilience in the face of climate change, but also of the multitude of diseases and pests.

Advanced in Rootstock Breeding of Nut Trees: Objectives and Strategies

The production and consumption of nuts are increasing in the world due to strong economic returns and the nutritional value of their products. With the increasing role and importance given to nuts (i.e., walnuts, hazelnut, pistachio, pecan, almond) in a balanced and healthy diet and in the prevention of various diseases, breeding of the nuts species has also been stepped up. Most recent fruit breeding programs have focused on scion genetic improvement. However, the use of locally adapted grafted rootstocks also enhanced the productivity and quality of tree fruit crops. Grafting is an ancient horticultural practice use in nut crops to manipulate scion phenotype and productivity and overcome biotic and abiotic stresses. There are complex rootstock breeding objectives and physiological and molecular aspects of rootstock–scion interactions in nut crops. In this review, we provide an overview of these, considering the mechanisms involved in nutrient and water uptake, regulation of phytohormones, and rootstock influences on the scion molecular processes, including long-distance gene silencing and trans-grafting. Understanding the mechanisms resulting from rootstock × scion × environmental interactions will contribute to developing new rootstocks with resilience in the face of climate change, but also of the multitude of diseases and pests and of the possible increase of their aggressiveness. They will also have to offer the premises of economic production, respectively yield and the quality, according to multiple destinations of nuts in the current consumption and food industry, but also the increasing exigencies of the consumer market and the profile industry.

‘Garnem’ and Myrobalan ‘P.2175’: Two Different Drought Responses and Their Implications in Drought Tolerance

One of the challenges in rootstock breeding programs is the combination of tolerances to different abiotic stresses in new interspecific hybrids adapted to a wide range of environmental conditions. In this work, two Prunus L. rootstocks: Myrobalan ‘P.2175’ (P. cerasifera Ehrh.) and the almond × peach hybrid ‘Garnem’ (P. amygdalus Batsch × P. persica (L.) Batsch) were subjected to drought during 24 h to understand their drought response mechanisms. The study was conducted monitoring leaf water potential (LWP), stomatal conductance (gs), relative water content (RWC), and electrolyte leakage (EL); as well as the abscisic acid (ABA) content in roots. The relative expression of five drought-relative genes was also studied. The obtained results allowed examining the drought tolerance potential of ‘Garnem’ and Myrobalan ‘P.2175’, demonstrating the great potential of ‘Garnem’ as drought tolerance source in future selections in breeding. Furthermore, based on the obtained data, the transcription factor Myb25-like could be a good biomarker of drought sensitivity for use in Prunus rootstock breeding programs.

Intermediate Inheritance with Disomic Tendency in Tetraploid Intergeneric Citrus × Poncirus Hybrids Enhances the Efficiency of Citrus Rootstock Breeding

Rootstocks are crucial for the sustainability of the citrus industry worldwide. Diploid intergeneric Citrus × Poncirus hybrids have contributed considerably to citrus rootstock improvement and their tetraploid (doubled-diploid) forms are important resources for the creation of a new generation of tetraploid rootstocks. To optimize the efficiency of tetraploid rootstock breeding strategies, more knowledge is required on inheritance in the allotetraploid genitors. A set of 159 new SNP markers that fully distinguish Poncirus trifoliata (L.) Raf. from Citrus species was developed from polymorphisms mined in GBS data and used to establish a genetic map of tetraploid citrumelo (C. × paradisi Macfad. × P. trifoliata) and to analyze the meiotic behavior of tetraploid citrumelo and citrandarin (C. reticulata Blanco × P. trifoliata). The tetraploid citrumelo genetic map was highly syntenic and collinear with the clementine reference genome. The apparent intergeneric recombination rate was strongly limited by high preferential chromosome pairing, resulting in intermediate inheritance with disomic tendency. Such inheritance, also observed in tetraploid citrandarin, results in the transmission by the diploid gametes of a high rate of intergeneric heterozygosity. It is therefore expected that a large part of the genetic value selected in the original diploid intergeneric rootstock is transmitted to the tetraploid sexual progenies.