**1. Introduction**

Plant cell walls are dynamic compartments whose composition and chemical structure vary during plant development in response to environmental signals. Cell walls are composed of three layers: the middle lamella, and the primary and secondary cell walls. Primary cell wall deposition continues through cell growth and expansion. It is a very complex and dynamic structure composed mainly of three types of polysaccharides: cellulose, hemicelluloses, and pectins. Cellulose microfibrils and hemicelluloses constitute a network with structural proteins that are embedded in a gel-like matrix of pectins. Localization of different classes of polysaccharides within the cell wall appears to depend on species, organ, tissue, and cell type, and the pectin network is temporally and spatially regulated during plant development. Alterations of the cell wall structure and changes in properties during plant development are mainly the result of protein activities. Enzymatic activity and protein interactions respond to developmental and environmental signals and tightly regulate the complex and dynamic structure of plant cell walls.

In terrestrial plants, the chemical structure of homogalacturonans (HGs), a type of pectin that is comprised of chains of α-1,4-galacturonic acid, is modified by de-methyl-esterification processes during plant development by pectin methyl esterases, and pectin methyl esterase inhibitory proteins regulate the degree of HGs' esterification of the cell wall in different cell types. Consequently, cell or tissue-specific function could be influenced by the HGs' status [1]. An increase in complexity in the regulation of HGs' esterification status through plant evolution has been described including a diversification of the gene families involved either in pectin synthesis or in pectin modification [2,3]. The shift from an aquatic to a terrestrial environment constitutes a milestone in the complexity of pectin regulation. In the algae *Chara*, cell growth is a result of a calcium pectate cycle that does not involve enzymatic activities [4]. Charophytes are the closest related group to land plants, thus, this enzyme-less mechanism could be still present in terrestrial plants [5].

The DUF642 protein family is found exclusively in spermatophytes and is involved in regulating HGs esterification. The DUF642 family was first described in apoplast proteomes of rosette leaves in *Arabidopsis thaliana* (Arabidopsis) by Boudart et al. [6] and subsequently in cell wall proteomes from different tissues and plant species from both dicotyledons and monocotyledons [7,8]. This multigene family is represented by 10 genes in Arabidopsis as well as in other plant species [9]. DUF642 proteins contain two DUF642 domains (PF04642) that present two conserved motifs NGXFEXXP and CGPVXD, determined as the family signature. A phylogenetic analysis revealed four clades of orthologous genes for all spermatophytes (Clades A, B, C, and D). After the divergence of gymnosperms and angiosperms, a duplication of Clade A occurred, producing two subclades (A1 and A2), and a subsequent duplication occurred after the divergence of monocotyledons and eudicotyledons [9].

The DUF642 proteins are encoded by genes that contain three exons. The first exon encodes a signal peptide, and the other two exons encode the DUF642 domains. Proteomic and cellular localization studies of the Clade A proteins indicated they were located primarily in the cell wall [6,10]. However, the protein encoded by *At3g08030* was located in the nuclear pore, probably because this gene has an alternative first exon edition so one of the possible encoded protein lacks the signal peptide. The proteins in Clades B, C, and D have a GPI-anchor site at the carboxyl terminus and were detected in cell membrane proteomes [11].

The interactions of the DUF642 proteins with polysaccharides and cell wall enzymes have been demonstrated in vitro. The protein encoded by *At3g08030* (Clade A2) interacted specifically with cellulose [11]. The proteins encoded by *At4g32460*/*BDX* and *At5g11420* (Clade A1) interacted with the catalytic domain of a pectin methyl esterase (PME) [12]. In two transgenic Arabidopsis lines overexpressing *BDX* the PME activity increased in the seeds, seedlings, and the apical meristems, but no changes in PME activity were detected in the leaves [13]. PME activity was higher in the seeds of both overexpressing lines than it was in the control seeds during germination [13,14]. PME activity was also higher in seedlings, stems, leaves, and fruits in Arabidopsis plants overexpressing the *VqDUF642* gene (Clade A1) of *Vitis quinquangularis* [15]. The leaves of Arabidopsis plants overexpressing the *AhDGR2* gene (Clade A1) of *Amaranthus hypochondriacus* showed a decrease in PME activity, whereas an increase in PME activity was detected in the roots [16]. The phenotypes of all the Clade A1 protein mutants in Arabidopsis have been established. The *At5g25460*/*dgr2* mutant had a shorter root and smaller rosette leaves than the wild-type [17]. No alterations in the phenotypes of the *At5g11420* and *At1g80240*/*dgr1* mutants compared with the wild-type have been described. The *At4g32460*/*bdx* mutants had shorter siliques, seeds with altered development, and longer hypocotyl than the wild-type [18]. Among Arabidopsis Clade A2 mutants, the *At2g41800*/*teb* mutant is the only one for which a phenotype (longer hypocotyls) has been established [19]. No phenotypes have been established for the two Clade B genes *At1g29980* and *At2g34510*, or the Clade D gene *At5g14150*.
