Situación de las cadenas de suministro en Cuba

Polygenes that control many agronomic traits of important such as yield, quality and other forms of disease resistance have been reported (Asins, 2002). The principal method of identification of genomic region that contributes to a phenotypic variation is referred to as quantitative trait loci (QTLs) mapping. QTLs have been typically linked to or shown to contain genes that have been invariably involved in controlling observed phenotypic variation (Mikiko et al., 2001). Fragments of DNAs identified as genetic markers can be used to identify particular points within the genome that represents genetic differences (Asins, 2002).

Genetic markers have been categorised based on their technical requirements; those genetic markers that have shown to detect the whole chromosomes, and those that have a specific (fragments) of genetic variation have been reported (Mikiko et al., 2001). Among the earliest type of DNA markers discovered were Restriction Fragment Length Polymorphisms (RFLPs), which were shown to detect variation in restriction fragment size using southern hybridization. This technique, enables the detection and or removal of a restriction endonuclease recognition site, which resulted from a single nucleotide base alteration which caused a shift in fragment length (Horst and Wenzel, 2007). RFLP markers remain valuable tools that are used in breeding programs, however, advances and

development of simple single repeat (SSR) markers superseded the use of RFLP markers. SSR markers allow detection of variation in the number of short repeat sequences, they could be of two or three base pairs that are repeated in multiple occurring sequences in the genome of a species (Horst and Wenzel, 2007).

Other reported DNA markers that have been developed are the expressed sequence tag (EST) databases and single nucleotide polymorphisms (SNPs) that occur at varying frequencies in the different regions of the genome of any given species (Horst and Wenzel, 2007).

1.3.9.1 Mapped Genes Related to vacuolar (NHX1) and plasma

membrane (SOS1) Antiporter Genes Associated to Plant Salt

Stress Tolerance

Ion exclusion is one of the essential mechanisms involved in salt stress tolerance. Factors that have been reported to enhance low cytoplasmic Na+ _{concentrations in plant cells have}

been shown to be associated with the tonoplast Na+_/H+ _{exchanger 1 (NHX1) (Yamaguchiet}

al., 2013) (Figure 1.5). The activity of the plasma membrane-bound protein SALT OVERLY SENSITIVE (Figure 1.5) (NHX7 in Arabidopsis) Na+_/H+ _{antiporters have also}

been implicated in Na+ _{ion exclusion (Blumwald and Poole, 1987; Qui et al., 2002;}

Yamaguchiet al.,2013). The role critically played by NHXs gene has been primarily shown to do with Na+ _{exclusion through sequestration of Na}+ _{within the vacuole (detoxification}

process), while the SOS pathway has been shown to be involved in the exporting Na+ _ions

out of the cell. Additionally, overexpression of AtNHX1 and its analogues in Arabidopsis and other plant species, such as tomato (Solanum lycopersicum), and rice have been reported where they have been shown to improved salinity tolerance (Apse et al., 1999; Zhang and Blumwald, 2001). Recent studies have demonstrated other roles played by NHX-type

proteins in K+ _{compartmentalisation into the vacuole for cellular pH homeostasis and}

maintenance (Barragan et al., 2012). Furthermore, overexpression of AtNHX1 in tomato has been shown to improved vacuolar K+ _{and its transport from the root into the shoot (Leidi et}

al., 2010; Bassil et al., 2011; Barragan 2012).

A successful QTL mapping of increased leaf Na+ _{accumulation in tomato has identified}

LeNHX3 (Villalta et al., 2007). This demonstrates that NHX antiporters have more than one role in osmoregulation, cell growth, and plant development (Bassil et al., 2011; Barragan et al., 2012). Other studies have suggested an involvement of endosomal transport proteins, including NHXs genes in plant salt tolerance that show its involvement in pH regulation, especially in the subcellular organelles and ion homeostasis (Krebs et al., 2010; Bassil et al., 2011; Yamaguchi et al.,2013). In addition, two isoforms of NHXs, NHX5, and NHX6, have been reported that are localised in Golgi and trans-Golgi networks. nhx5 nhx6 double knockout plants have been shown to demonstrate hypersensitivity to salt stress (Leidi et al., 2010). Likewise, studies involving loss of function have shown that vacuolar transporter H+

-ATPase in a mutated Arabidopsis have been attributed to unimpaired salt tolerance, i.e., not significantly shown to alter the salinity stress in Arabidopsis. Other studies, however, have reported the reduction of V-ATPase activity in the trans-Golgi network/early endosome (TGN/EE), which resulted in an increase salt sensitivity (Villalta et al., 2007). Overexpression studies involving vacuolar-type I H+ _{-PPase AVP1 have also been shown}

to improve plant salt tolerance via vacuolar Na+ _{sequestration (Undurraga et al., 2012).}

Furthermore, overexpression of AtAVP1 in crop barley (Hordeum vulgare) have implicated H+ _{-PPase in the improvement of salinity tolerance under greenhouse controlled}

environment by substantially improving shoot biomass and grain yield (Schilling et al., 2013).

1.3.9.2 Overexpression of High Potassium Membrane Transporter Genes

(HKT) and Plant Salt Stress Tolerance

In order to find the role of HKT in salt tolerance, several studies have been carried out. These have involved identification of the role of HKT1 in wheat (Triticum aestivum) using gene (TaHKT2;1), which was shown to mediate in Na+_/K+ _{cation transport (Schachtman}

and Schroeder, 1994; Rubio et al., 1995). Further studies in the characterization of different isoforms of HKT genes that have been found present in different plants species have been reported (Horie et al., 2007). Different methods used in the analysis and characterisation of HKT have shown the presence of other copies of HKT transporters isoforms (Uozumi et al., 2000; Mäser et al.,2002). Subsequently, two distinct subgroups have been identified; class I and II, which are believed to mediate Na+ _{-selective transportation (Uozumi et al.,2000;}

Mäser et al., 2002), and are involved in co-transportation of Na+_-K+ _{(Rubio et al., 1995).}

Studies have reported that mutated HKT gene in Arabidopsis, AtHKT1;1, encodes for a class I transporter, which has shown to causes high Na+ _{sensitivity in the mutants (Mäser et al.,}

2002; Horie et al.,2007). Some detail analyses using mutant have indicated that AtHKT1;1, and its orthologues in rice, OsHKT1;5, play a critical role in the efflux of Na+ _{from the}

xylem sap into the surrounding xylem parenchyma cells. Thereby, enhancing the protection of leaf from Na+ _{toxicity (Ren et al., 2005; Sunarpi et al., 2005; Horie et al., 2006; Davenport}

et al., 2007). An overexpression of AtHKT1;1 in a target stele has shown to improves salt tolerance (Moller et al., 2009). Studies involving in vivo electrophysiological analyses using stellar cells from wild-type and Athkt1;1mutant plants have shown to demonstrate the role played by AtHKT1;1 mediating Na+ _{transport through channel-like transporters (Xue et al.,}

2013; Xue et al., 2016). It has been suggested that AtHKT1;1 gene play a role in facilitating Na+ _{removal from the xylem and indirectly stimulating K}+ _{loading into xylem vessels. The}

K+ _{filling was supposed to be carried out through outward rectifying K}+ _{channels, leading}

to the improvement of K+_/Na+ _{ratio in the leaves (Ren et al., 2005; Sunarpi et al., 2005).}

QTL analyses on Na+ _{resistance have indicated that xylem Na}+ _{unloading mechanisms have}

been essential for salt tolerance on both rice and wheat (Triticum turgidum) (James et al., 2006). Salt tolerance QTLs have been linked to the regions containing to HKT1;5 orthologues, which encodes for increased Na+ _{- exclusion and in particular for a class I HKT}

transporter (Ren et al., 2005; James et al., 2006; Byrt et al., 2017). Another QTL linked to Na+ _{tolerance has been correlated with a critical role played by the Nax1 gene for salt}

tolerance (Munns et al.,2012). Further work on Nax1 locus, have been linked to the region of the TaHKT1;4, gene that encodes for a class I HKT transporter (Huang et al., 2006).

HKT marker-assisted studies have shown to record some successes through the introduction of a wheat HKT1;5 from an old wheat relative Triticum monococcum into commercial durum wheat (T. turgidum ssp. durum var. Tamaroi). Studies on transgenic varieties of this gene have shown to improve tolerance to salt stress (Munns et al., 2012). Finally, it has been suggested that the maintenance of K+ _{acquisition with the exclusion of Na}+ _{from leaves have}