Dissecting Seed Mucilage Adherence Mediated by FEI2 and SOS5

Abstract

The plant cell wall is held together by the interactions between four major components: cellulose, pectin, hemicellulose, and proteins. Mucilage is a powerful model system to study the interactions between these components as it is formed of polysaccharides that are deposited in the apoplast of seed coat epidermal cells during seed development. When seeds are hydrated, these polysaccharides expand rapidly out of the apoplastic pocket, and form an adherent halo of mucilage around the seed. In Arabidopsis, mutations in multiple genes have similar loss of mucilage adherence phenotypes including CELLULOSE SYNTHASE 5 (CESA5)/MUCILAGE-MODIFIED 3 (MUM3), MUM5/MUCI21, SALT-OVERLY SENSITIVE 5 (SOS5), and FEI2. Here, we examine the interactions between these factors to better understand how they participate to control mucilage adherence. Double mutant phenotypes indicated that MUM5 and CESA5 function in a common mechanism that adheres pectin to the seed through the biosynthesis of cellulose and xylan, whereas SOS5 and FEI2, encoding a fasciclin-like arabinogalactan protein or a receptor-like kinase, respectively, function through an independent pathway. Cytological analyses of mucilage indicates that heteromannans are associated with cellulose, and not in the pathway involving SOS5 or FEI2. A SOS5 fluorescent protein fusion (SOS5-mCITRINE) was localized throughout the mucilage pocket, consistent with a structural role in pectin adhesion. The relationship between SOS5 and FEI2 mediated mucilage adherence was examined in more detail and while sos5 and fei2 mutants show similar phenotypes, key differences in the macromolecular characteristics and amounts of mucilage polymers were observed. FEI2 thus appears to have additional, as well as overlapping functions, with SOS5. Given that FEI2 is required for SOS5 function, we propose that FEI2 serves to localize SOS5 at the plasma membrane where it establishes interactions with mucilage polysaccharides, notably pectins, required for mucilage adherence prior to SOS5 being released into the apoplast.

Sticking to cellulose: exploiting Arabidopsis seed coat mucilage to understand cellulose biosynthesis and cell wall polysaccharide interactions

“…Reduced cellulose crystallinity could increase interactions with xylan and thereby the amount of RG‐I in the adherent layer, which would also be coherent with cesa5 having a reduced adherent layer, as cellulose production is reduced. GGM labelling with antibodies has a considerably increased signal in cesa5 mucilage (Griffiths et al ., ). Furthermore, the compact mucilage phenotypes of clsa2 and muci10 closely resemble that of cesa3 ixr1‐2 , which exhibits similar alterations to cellulose crystallinity and increased pectin adherence (Griffiths et al ., ).…”

“…e), and is distinguished from the first class based on the different distribution of cellulose in the adherent layer, with a marked absence of cellulosic rays, while diffuse cellulose staining remains largely intact (Fig. o; Griffiths et al ., , ). When both cesa5 and sos5 mutants are combined together, an enhanced phenotype is observed with no cellulose evident in the adherent mucilage layer, demonstrating that these mutants are affected in two different pathways that mediate mucilage adherence independently (Figs f,k,p, 2; Griffiths et al ., , ).…”

“…o; Griffiths et al ., , ). When both cesa5 and sos5 mutants are combined together, an enhanced phenotype is observed with no cellulose evident in the adherent mucilage layer, demonstrating that these mutants are affected in two different pathways that mediate mucilage adherence independently (Figs f,k,p, 2; Griffiths et al ., , ). The size of mucilage polymer aggregates in nonadherent mucilage also differs between the cesa5 and sos5 mutant classes, with cesa5 ‐like mutants lacking a population of large polymer aggregates while this fraction is retained in the sos5 class (Macquet et al ., ; Sullivan et al ., ; Griffiths et al ., ; Hu et al ., ,b).…”

“…Both sos5 and fei2 enhance the phenotypes of cesa6 and cob1 without salt treatment (Xu et al ., ), and reactive oxygen species are elevated in sos5 roots before salt treatment (Xue & Seifert, ). Finally, the mucilage phenotype of sos5 and fei2 is independent of salt, further demonstrating the value of the SCE model in revealing novel phenotypes and clues to gene function (Griffiths et al ., , ).…”

“…Both layers are composed primarily of unbranched RG-I, while the adherent layer also contains small amounts of other types of pectin, hemicellulose and cellulose (Macquet et al, 2007;North et al, 2014;Yu et al, 2014). The adherence of the pectin component of the adherent layer depends on cellulose, hemicellulose and other proteins (Harpaz-Saad et al, 2011;Mendu et al, 2011;Sullivan et al, 2011;Griffiths et al, 2014Griffiths et al, , 2016Yu et al, 2014;Voiniciuc et al, 2015a,b;Ralet et al, 2016).…”