Synthetic Polyploidy in Grafted Crops

Abstract

Synthetic polyploids have been extensively studied for breeding in the last decade. However, the use of such genotypes at the agronomical level is still limited. Polyploidization is known to modify certain plant phenotypes, while leaving most of the fundamental characteristics apparently untouched. For this reason, polyploid breeding can be very useful for improving specific traits of crop varieties, such as quality, yield, or environmental adaptation. Nevertheless, the mechanisms that underlie polyploidy-induced novelty remain poorly understood. Ploidy-induced phenotypes might also include some undesired effects that need to be considered. In the case of grafted or composite crops, benefits can be provided both by the rootstock's adaptation to the soil conditions and by the scion's excellent yield and quality. Thus, grafted crops provide an extraordinary opportunity to exploit artificial polyploidy, as the effects can be independently applied and explored at the root and/or scion level, increasing the chances of finding successful combinations. The use of synthetic tetraploid (4x) rootstocks may enhance adaptation to biotic and abiotic stresses in perennial crops such as apple or citrus. However, their use in commercial production is still very limited. Here, we will review the current and prospective use of artificial polyploidy for rootstock and scion improvement and the implications of their combination. The aim is to provide insight into the methods used to generate and select artificial polyploids and their limitations, the effects of polyploidy on crop phenotype (anatomy, function, quality, yield, and adaptation to stresses) and their potential agronomic relevance as scions or rootstocks in the context of climate change.

Keywords: grafting; polyploid; rootstock; scion; stress tolerance.

Figure 1

Illustration of the phenotypic differentiation between diploid (2x) and polyploid (triploid, 3x and tetraploid, 4x) citrus at the plant, organ and cellular level. (A) Diploid and 4x seeds of Citrumelo (bar = 0.5 cm). (B) Diploid and 4x Citrumelo trees planted at the same age at the INRAe – Cirad germplasm of San Giuliano, France (bar = 0.5 m). (C) Diploid and 4x Carrizo citrange fibrous roots (bar = 1 cm). (D) Scanning electron microscopy pictures of the leaf palisade parenchyma of 2x and 4x Volkamer lemon (bar = 10 μm). (E) Light micrographs of cross-sections of internodes and roots of Rangpur lime plants (bar = 25 μm) from Allario et al. (2011). (F) Light micrographs of leaf epidermis showing stomata from 2x and 4x Citrumelo (bar = 10 μm). (G) Leaf and fruit of Mexican lime (2x), Tahiti lime (3x), and Giant Mexican lime (Autotetraploid; bar = 0.5 cm).

Figure 2

Physiological implication of the polyploidization when associating 2x and 3x scion with 2x or 4x rootstock. On one hand, 4x rootstock confers a more limited vigor than its respective 2x. On the other hand, a 3x scion will induce a greater vigor and larger leaves than in the 2x scion. At root level, tetraploidy will limit the tree vigor but will favor an increase of the size of the secondary roots. At the root and scion level, polyploidy may induce large phenotypical changes.

Figure 3

Physiological and molecular implications for stress tolerance when associating a 2x or 3x scion with a 2x or 4x rootstock. Arrows indicate the influence of the different factors at root and scion level depending on the ploidy that may change the physiology or the adaptation of the scion/rootstock association.