Grapevine Potassium Nutrition and Fruit Quality in the Context of Climate Change

Abstract

Potassium (K⁺) nutrition is of relevant interest for winegrowers because it influences grapevine growth, berry composition, as well as must and wine quality. Indeed, wine quality strongly depends on berry composition at harvest. However, K⁺ content of grape berries increased steadily over the last decades, in part due to climate change. Currently, the properties and qualities of many fruits are also impacted by environment. In grapevine, this disturbs berry properties resulting in unbalanced wines with poor organoleptic quality and low acidity. This requires a better understanding of the molecular basis of K⁺ accumulation and its control along grape berry development. This mini-review summarizes our current knowledge on K⁺ nutrition in relation with fruit quality in the context of a changing environment.

Keywords: climate change; fruit quality; grape berries; potassium nutrition; potassium transport.

Figure 1

K⁺ transporter and channel families. (A) Structure and function of different transporter families. The family HAK/KUP/KT (left panel) is present in all plant genomes and contains 13 members in A. thaliana and 18 in vine distributed into five clades. Arabidopsis has just one member belonging to cluster I, AtHAK5, which has been extensively studied and its structure is shown in the top. The general structure of the HAK/KUP/KT transporters is conserved. The number of transmembrane segments (TMS) is ranged from 10 to14, with the most common being 11–12 TMS. The HKT (High-affinity K⁺ Transporter) transporters (middle panel) are distributed into two families depending of the presence of a glycine into the first pore loop proposed as a K⁺ selectivity determinant. All members of HKT family are composed of eight transmembrane domains. The last family of K⁺ transporters is the CPA family (right panel) for Cation Proton Antiporters. CPAs are composed of 10 to 12 transmembrane domains. This family is divided in two sub-families (CPA1 and CPA2). CPA1 mainly consists in NHX antiporters (Na⁺(K⁺)/H⁺ exchangers). CPA2 is mainly constituted by CHXs (cation/H⁺ exchangers). Information on the CPA family is still fragmentary. (B) Description of K⁺ Shaker channels. The structure of different members is strictly conserved with six transmembrane domains. The fourth transmembrane segment (S4) harbors positively charged amino acids and acts together with S1, S2, and S3 as voltage sensor. The pore domain is located between the TMS5 and TMS6 domains. The C-terminal extremity of Shaker sub-unit is composed of three distinct domains: a cyclic nucleotide-binding domain (CNBD), an ankyrin domain allowing protein-protein interactions and a KHA domain rich in hydrophobic and acidic amino acids. Functional Shaker channels are multimeric proteins formed by the assembly of four Shaker gene products (Dreyer et al., 2019). In plants, three types of K⁺ Shaker channel localized at the plasma membrane have been exhaustively characterized: the inwardly, weak inwardly or outwardly rectifying K⁺ channels which drive inward or outward K⁺ fluxes across the plasma membrane according to the membrane potential.

Figure 2

Map of K⁺ transport in grape berry. Seven K⁺ transport systems have been identified and characterized to be involved in K⁺ fluxes into or out of the berry cells. The first identified K⁺ transport systems belonging to the HAK/KUP-KT family are VvKUP1 and VvKUP2. These transporters are involved in K⁺ transport into the berry skin only during the first phase of berry development. In contrast, three K⁺ Shaker channels and one antiporter belonging to the CPA family have been characterized to be involved in K⁺ berry loading during its ripening. Recently, two other Shaker channels have been characterized in phloem cells. VvK5.1, which is a typical outwardly rectifying K⁺ channel, is involved in the repolarization of the plasma membrane of phloem cells (Villette et al., 2019). The second one, VvK3.1, is a weakly rectifying K⁺ Shaker channel that can switch between two gating modes driving either inwardly rectifying or instantaneous currents. The latter mode allows to drive K⁺ influx or efflux through phloem cell membranes according to membrane potentials and K⁺ gradients. At the unloading site, the K⁺ gradient is in favor of K⁺ efflux (100 mM in the cytosol of phloem cells and 1 mM in the apoplast). The VvK3.1 channel is the main molecular actor involved in K⁺ unloading into berries, thanks to massive K⁺ efflux into the apoplast (Nieves-Cordones et al., 2019). Then, apoplastic K⁺ is directly recovered by the inwardly rectifying K⁺ channel VvK1.2 expressed in pulp cell plasma membranes. The CPA transporter, VvNHX1, expressed in the tonoplast of pulp cells, is involved in K⁺ storage in the vacuole during berry ripening (Hanana et al., 2007). It is important to note that some members of the CIPK/CBL families increase the functional activity of VvK3.1 and VvK1.2. Finally, another inwardly rectifying K⁺ channel, named VvK1.1, involved in K⁺ uptake from the soil in roots, is up-regulated in berries in drought stress conditions (Cuéllar et al, 2010).