La rumba criolla

Initially named SUT1/SUC2 clade (Lalonde et al. 2004) according to A. thaliana AtSUC2

gene synonymy, which is also called AtSUT1 in some publications (Feuerstein et al. 2010),

the SUT1 clade is the largest and most studied clade and is dicotyledonous specific (Fig 8). It seems that this clade is composed of paralogs that have evolved through gene duplication and that each gene variant acquired different functions (Lynch and Force 2000; Ibraheem et al.

2010). H+/sucrose importers (Fig 12) from the SUT1 clade show high affinity for sucrose with

Fig 9. A single SUT protein is necessary for phloem loading in apoplasmic loader plants

a) Phenotype of wild type and NtSUT1 antisense tobacco plants (Burkle et al. 1998). b) Starch

accumulation in leaves of wild type and NtSUT1 antisense tobacco plants shown by Lugol's iodine

staining. c) Reduction of tuber yield in potato tubers in StSUT1 antisense line αSUT43 and αSUT13

(Riesmeier et al. 1994). d) Phenotype of wild type and ZmSUT1 insertional mutant maize plants

(Slewinski et al. 2009). e) Starch accumulation in leaves of wild type and ZmSUT1 mutant plants

shown by Lugol's iodine staining. f) Autoradiographs showing the blockage of [14C]sucrose export

from ZmSUT1 mutant leaves. g) No phenotype differences were observed in the symplasmic loader

3.2.2.1.1 A single SUT1 member necessary for phloem loading

A single member of the SUT1 clade is responsible for sucrose phloem loading for apoplasmic

loaders (Fig 4.3, see 2.2.1, Zhang and Turgeon 2009); for instance AtSUC2 in A. thaliana,

StSUT1 in potato, SlSUT1 in tomato or NtSUT1 in tobacco. The downregulation (Riesmeier et al. 1994; Kühn et al. 1996; Bürkle et al. 1998; Schulz et al. 1998; Hackel et al. 2006b) or total gene disruption (Gottwald et al. 2000) of the phloem loading protein caused sugar accumulation, reduced photosynthesis and caused chlorotic lesions in source leaves as well as reduced growth of sink organs and thereby an overall stunted plant growth (Fig 9a-c).

However, contradictive results were published about the fertility of AtSUC2 KO plants upon

their ability to complete a life cycle and produce viable seeds (Gottwald et al. 2000;

Srivastava et al. 2009b). In symplasmic loader species (Fig 4.2) such as Verbascum

phoeniceum, the disruption of VpSUT1 the phloem loader orthologs only had minor effect on sugar partitioning and plant morphology was not affected (Fig 9g).

Consistent with their loading functions, all loading proteins are high affinity sucrose importers and were tagged at the plasma membrane of SE (Kühn et al. 1997; Reinders et al. 2002a; Kühn et al. 2003; Hackel et al. 2006a; Weise et al. 2008) or of both CC and SE (Knop et al.

2004; Scofield et al. 2007b). As the expression of antisense StSUT1 under the control of a

CC-specific promoter prevented phloem loading (Lemoine et al. 1996) and as enucleate SE mostly depend on CC’s molecular machinery, transcription and translation of the SUT1 loading member are likely to occur in CC. Moreover, various GFP fusion proteins were shown to traffic towards SE with a large exclusion limit (>67kDa) between plasmodesmata connecting SEs and CCs (Stadler et al. 2005; Thompson and Wolniak 2008). However, the

presence of StSUT1 mRNAs in SE (Kühn et al. 1997) together with the discovery of a

translational machinery in angiosperm SE (Lin et al. 2009) led to an alternative model for protein synthesis in phloem (Kühn and Grof 2010).

In addition to this source localization, several phloem loader transporter orthologs were also

shown to be expressed in sink tissues where their roles remain obscure. Indeed, AtSUC2

expression was detected in roots and pods (Truernit and Sauer 1995), StSUT1 in potato tubers

(Kühn et al. 2003), NtSUT1 in roots, buds, sink leaves and flowers (Bürkle et al. 1998) and

SlSUT1 in roots, fruits and flowers (Hackel et al. 2006a; Boldt et al. 2011). Although Srivastava et al. 2008 pointed a major role for AtSUC2 in sugar retrieval but not in sucrose unloading other authors suggest a dual role of such members in both loading and unloading mechanisms (Kühn 2003; Carpaneto et al. 2005; Doidy et al. 2012a).

3.2.2.1.2 The other SUT1 clade members

In addition to the single phloem loading orthologs, the SUT1 clade is also represented by numerous multicopy SUT1 paralogs in all dicotyledonous species. For instance, in addition to

AtSUC2, the genome of A. thaliana possesses 6 other SUT1 members originating from gene

duplication including 2 pseudogenes (AtSUC6 and AtSUC7, Sauer et al. 2004).

These other SUT1 clade members are not as well characterized as the single loading member but are thought to have a role in carbon partitioning of sink organs, namely in flowers and

seeds where they are mostly expressed. Indeed, AtSUC1 expression was shown in anther

connective tissue, in funiculi, in fully developed pollen grains and in female gametophyte

(Stadler et al. 1999; Feuerstein et al. 2010), AtSUC8 along the transmitting tissue and at the

funiculi (Sauer et al. 2004), AtSUC9 in sepals, in style and in the filament vasculature (Sivitz

et al. 2007), PmSUC1 in pollen grains, inside the anthers and in pollen tubes (Lauterbach et

al. 2007) and NtSUT3 is pollen specific (Lemoine et al. 1999). Consequently, plant containing

a mutation of AtSUC1 showed impaired pollen germination (Sivitz et al. 2008) and AtSUC9

mutant showed an early flowering phenotype (Sivitz et al. 2007). In addition to its floral

localization, PmSUC1 was shown to be expressed in the innermost layer of the inner

integument (Lauterbach et al. 2007) and AtSUC1 in vascular tissues of siliques (Sivitz et al.

2007). AtSUC5 expression is endosperm specific and antisense lines showed reduction in fatty

acid composition of their seeds (Baud et al. 2005).

Moreover, members of the SUT1 clade were also present in other sink organs. RhSUC2 is

expressed in young bursting rose buds (Henry et al. 2011). AtSUC1 is expressed in trichomes

(Sivitz et al. 2007), in roots and in hypocotyls of young seedling (Sivitz et al. 2008; Hoth et

al. 2010) and mutation of an AtSUC1 key promoter motif revealed an important role for

seedling development (Hoth et al. 2010).

In conclusion, SUT1 Dicot clade is large and well studied, with a well characterized SUT1 protein that function as the main phloem loader in apoplasmic loading species. Numerous other SUT1 members are mostly present in sink organs but their exact physiological roles are still not deciphered.