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Glossary of Scientific Terms - What Is Life?

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Lecture Notes in Immunology: Function of the Human …

Based on the above observations, we hypothesized that AtMYB41 functions as a regulator of suberin biosynthesis. Suberin is not normally produced in leaves. Instead, a biosynthetically related but distinct cuticle, comprising cutin that is impregnated with waxes, covers the epidermal surfaces of leaves and other aerial plant organs. We postulated that overexpression of AtMYB41 might lead to the ectopic accumulation of suberin in aerial organs such as leaves where a cuticle is normally produced. Similar to a previous report, we found that plants overexpressing AtMYB41 driven by the 35S promoter (AtMYB41 OE-9) (Cominelli et al., ) had phenotypes associated with surface defects including stunted growth, glossy leaf surfaces, elevated permeability to toluidine blue stain, and altered pavement cell shape (Figures S1–S3). Analysis of leaf cross sections of stably transformed AtMYB41 OE-9 Arabidopsis plants by transmission electron microscopy (TEM) revealed the presence of lamellar structures, alternating light and dark bands deposited on the internal surfaces of the primary cell walls of epidermal cells (Figures and S4). These lamellar structures strongly resemble the lamellae typical of suberized endodermal and peridermal root cells (Figure S5) (Enstone et al., ; Ma and Peterson, ; Franke and Schreiber, ; Molina et al., ).

Bacteria Single cell organisms and most prevalent form of life on Earth

To date, only one report has described the transgenic production of suberin-type monomers in tissues normally covered with a cuticle (Li et al., ). Co-overexpression of the suberin biosynthetic genes GPAT5 and CYP86A1 resulted in the production of about half as many suberin-like monomers as cutin without apparent modification of cutin content. In contrast, overexpression of AtMYB41 resulted in the production of 4.5 times more suberin-type than cutin-type aliphatic monomers in Arabidopsis (Figure) and 22 times more suberin-type than cutin-type aliphatic monomers in N. benthamiana (Figure). TEM analysis showed distinct lamellar structures abutting the inner surfaces of cell walls in the leaves of both Arabidopsis and N. benthamiana overexpressing AtMYB41 (Figures and ), whereas ectopic co-overexpression of GPAT5 and CYP86A1 did not result in the production of lamellar structures abutting the primary cell wall nor a lamellate cuticle. This indicates that overproduction of suberin monomers via co-overexpression of multiple genes encoding activities for monomer synthesis alone is not sufficient to induce the ectopic assembly of suberin lamellae anywhere in or adjacent to the cell wall. It is striking that the overexpression of a single gene, AtMYB41, can activate and coordinate the large number of activities required to synthesize and assemble a complex macromolecular structure. Likewise, it is noteworthy that overexpression of AtMYB41 results in the assembly of lamellar structures at the subcellular site where suberin is normally deposited, adjoining the inner face of the primary cell wall.

DNA is a long polymer made from repeating units called nucleotides

Based on our results, several possibilities exist for the biological function of AtMYB41. First, production of suberin-type monomers and lamellar structures by AtMYB41 overexpression could be a pleiotropic effect of strong overexpression. However, the high degree of coordinated gene expression encoding enzymatic, transporter, and polymerizing activities required to assemble a complex lamellar structure like suberin strongly argues against this. Further, the lack of induced suberin-type monomer production by transient expression of other candidate MYBs (Figure S9) corroborates a specific role for AtMYB41 in the synthesis of suberin aliphatics. Overall, our results indicate that AtMYB41 acts as a component of the regulatory network underlying stress-induced aliphatic suberin biosynthesis. Several lines of evidence support this hypothesis: (i) AtMYB41 induces the deposition of suberin-like materials when overexpressed; (ii) the AtMYB41 promoter is active in suberizing tissues under abiotic stress but not unstressed conditions; (iii) augmented root suberization is a recognized response to NaCl and ABA treatment (Reinhardt and Rost, ; Karahara et al., ; Schreiber et al., ; Efetova et al., ; Franke et al., ); (iv) it is well-documented that AtMYB41 transcripts accumulate in roots and seedlings in response to various types of abiotic stress (Cominelli et al., ; Kosma et al., ; Lippold et al., ), and (v) stress-induced phosphorylation of AtMYB41 by a mitogen-activated protein kinase (MPK6) is required for the salt-tolerant phenotypes imparted by AtMYB41 overexpression (Hoang et al., ). Whether or not AtMYB41 acts via direct or indirect activation of suberin genes is not yet known. However, the coordination of multiple activities required for the deposition of such a lamellar structure (i.e. lipid and phenylpropanoid metabolism, transport to the cell wall, assembly, etc.) is reminiscent of the level of control exhibited by the transcriptional networks defined for secondary cell wall synthesis and seed maturation (Santos-Mendoza et al., ; Zhong et al., ). Specific MYB and NAC transcription factors act as master switches that regulate a downstream cascade of transcription factors more directly involved in activating the synthesis of lignin, cellulose, and hemicellulose components of secondary cell walls. Similarly, LEC2 (more globally involved in seed maturation processes) regulates seed oil biosynthesis by targeting WRI1, which is a direct regulator of fatty acid synthesis genes.

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