A common adaptation in angiosperms is the deposition of hydrophilic mucilage into the apoplast of seed coat epidermal cells during the course of their differentiation. Upon imbibition, seed mucilage, composed mainly of carbohydrates (i.e. pectins, hemicelluloses and glycans) expands rapidly, encapsulating the seed and aiding in seed dispersal and germination. The FEI1/FEI2 receptorlike kinases and the SOS5 extracellular GPI-anchored protein were previously shown to act on a pathway regulating cellulose biosynthesis during Arabidopsis root elongation. In the highlighted study, we demonstrated that FEI2 and SOS5 regulate the production of the cellulosic rays deposited across the inner adherentlayer of seed mucilage. Mutations in either fei2 or sos5 disrupted the formation of rays, which was associated with an increase in the soluble, outer layer of pectin mucilage and accompanied by a reduction in the inner adherent-layer. Mutations in CELLULOSE SYNTHASE 5 also led to reduced rays and malpartitioning of the pectic component of seed mucilage, further establishing a structural role for cellulose in seed mucilage. Here, we show that FEI2 expressed from a CaMV 35S promoter complemented both root and seed mucilage defects of the fei1 fei2 double mutant. In contrast, expression of FEI1 from a 35S promoter complemented the root, but not the seed phenotype of the fei1 fei2 double mutant, suggesting that unlike in the root, FEI2 plays a unique and non-redundant role in the regulation of cellulose synthesis in seed mucilage. Altogether, these data suggest a novel role for cellulose in anchoring the pectic component of seed mucilage to the seed surface and indicate that the FEI2 protein has a function distinct from that of FEI1, despite the high sequence similarity of these RLKs.
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inner layer of seed mucilage, resulting in Do not35S promoter in fei1dsfei2tribidoubleumutantet .FEI2 and SOS5 as regulators of cellulosea phenotype similar to that observed in background and assessed the ability of deposition into the rays radiating the seed fei1 fei2 and sos5. Consistent with this, each to complement the root and seed across the inner layer of seed mucilage. Sullivan et al., 2011 and Mendu et al., phenotypes. As each transgene is expressed Other recent studies have defined addi-2011 recently observed similar phenotypes from the same constitutive promoter, any tional, partially redundant functions for for other mutant alleles of cesA5, as well as difference in the ability to complement CESA2, CESA5 and CESA9 in cellulose additional, partially redundant roles for these phenotypes must be due to differ-deposition into the radial cell walls of CESA2, CESA5 and CESA9 in cellulose ences in the function of the encoded mucilage secretory cells.23-25These studies deposition during seed development.23,24 proteins. Consistent with the results set the stage for the use of mucilage Together, these studies demonstrate that obtained by the expression from its native secretory cells as a novel tool for further cellulose is an essential component of seed promoter, expression of FEI2 from the research aimed to uncover new players mucilage and that it acts to anchor the CaMV 35S promoter complemented regulating cellulose biosynthesis. pectic component of seed mucilage to the both the root and seed phenotypes in theseed surface, resulting in an inner layer of double fei1 fei2 mutant backgroundseed mucilage that remains attached to the (Fig. 1). Likewise, expression of FEI1 We thank Gyeong Mee Yoon for fruitful seed most likely in order to maintain a from the CaMV 35S promoter comple-discussions. This project was supported hydrated environment during the first mented the root phenotype of the double by National Science Foundation grant phase of seed germination. fei1 fei2 mutant (in all ten different lines IOS 0624377 (to J.J.K.) and by a Natural examined). However, the CaMV 35S:: Sciences and Engineering Research FEI2, But Not FEI1, Plays a FEI1 transgene failed to complement the Council Discovery Grant (to T.L.W.). Unique and Non-Redundant Role seed mucilage phenotype of the fei1 fei2 S.H-S. is a recipient of a Vaadia-BARD in Seed Mucilage double mutant (Fig. 1). These data postdoctoral fellowship award No. FI-indicate that despite the high sequence 411-2008 from BARD, The United The fei1 fei2 double mutant displays a similarity shared by these LRR-RLKs, the States—Israel Binational Agricultural swollen root phenotype as a result of FEI2 protein has a distinct function in the Research and Development Fund.
- Seed coat