Maize Brittle stalk2 Encodes a COBRA-Like Protein Expressed in Early Organ Development But Required for Tissue Flexibility at Maturity

Abstract

The maize (Zea mays) brittle stalk2 (bk2) is a recessive mutant, the aerial parts of which are easily broken. The bk2 phenotype is developmentally regulated and appears 4 weeks after planting, at about the fifth-leaf stage. Before this time, mutants are indistinguishable from wild-type siblings. Afterward, all organs of the bk2 mutants turn brittle, even the preexisting ones, and they remain brittle throughout the life of the plant. Leaf tension assays and bend tests of the internodes show that the brittle phenotype does not result from loss of tensile strength but from loss in flexibility that causes the tissues to snap instead of bend. The Bk2 gene was cloned by a combination of transposon tagging and a candidate gene approach and found to encode a COBRA-like protein similar to rice (Oryza sativa) BC1 and Arabidopsis (Arabidopsis thaliana) COBRA-LIKE4. The outer periphery of the stalk has fewer vascular bundles, and the sclerids underlying the epidermis possess thinner secondary walls. Relative cellulose content is not strictly correlated with the brittle phenotype. Cellulose content in mature zones of bk2 mature stems is lowered by 40% but is about the same as wild type in developing stems. Although relative cellulose content is lowered in leaves after the onset of the brittle phenotype, total wall mass as a proportion of dry mass is either unchanged or slightly increased, indicating a compensatory increase in noncellulosic carbohydrate mass. Fourier transform infrared spectra indicated an increase in phenolic ester content in the walls of bk2 leaves and stems. Total content of lignin is unaffected in bk2 juvenile leaves before or after appearance of the brittle phenotype, but bk2 mature and developing stems are markedly enriched in lignin compared to wild-type stems. Despite increased lignin in bk2 stems, loss of staining with phloroglucinol and ultraviolet autofluorescence is observed in vascular bundles and sclerid layers. Consistent with the infrared analyses, levels of saponifiable hydroxycinnamates are elevated in bk2 leaves and stems. As Bk2 is highly expressed during early development, well before the onset of the brittle phenotype, we propose that Bk2 functions in a patterning of lignin-cellulosic interactions that maintain organ flexibility rather than having a direct role in cellulose biosynthesis.

Silencing CHALCONE SYNTHASE in Maize Impedes the Incorporation of Tricin into Lignin and Increases Lignin Content

“…The brittle culm (bc) mutants in rice show reduced tissue mechanical strength due to significant reductions in cellulose content, which is often accompanied by increased levels of lignin, as in the case of bc1, bc10, and bc12, each of which is affected in a different aspect of cellulose deposition (Li et al, 2003b;Zhou et al, 2009;Zhang et al, 2010). Similarly, the reduced cellulose content in mature stems of the spontaneous maize mutant brittle stalk2 (bk2), disrupted in a gene orthologous to the BC1 gene in rice, goes along with a marked enrichment in lignin deposition, resulting in brittleness of all aerial plant organs (Sindhu et al, 2007). In agreement with its corresponding mutant in rice, bk2 and wild-type plants were indistinguishable in all aspects of plant growth and development (Ching et al, 2006;Sindhu et al, 2007).…”

“…Similarly, the reduced cellulose content in mature stems of the spontaneous maize mutant brittle stalk2 (bk2), disrupted in a gene orthologous to the BC1 gene in rice, goes along with a marked enrichment in lignin deposition, resulting in brittleness of all aerial plant organs (Sindhu et al, 2007). In agreement with its corresponding mutant in rice, bk2 and wild-type plants were indistinguishable in all aspects of plant growth and development (Ching et al, 2006;Sindhu et al, 2007). In brachypodium, the dwarf spaghetti1 (spa1) mutant presents a unique phenotype combining brittleness with increased elasticity of the internodes (Timpano et al, 2015).…”

Molecular characterization, expression pattern, and function analysis of the OsBC1L family in rice

“…AtCOBL9 is required for tip-directed growth in root hair development in Arabidopsis (Parker et al 2000;Jones et al 2006). The rice brittle culm1 (bc1) mutant and the maize brittle stalk 2 (bk2) mutant were found to have mutations in the putative orthologs of AtCOBL4 that affected the mechanical strength of plant tissues and secondary cell wall biosynthesis Ching et al 2006;Sindhu et al 2007). Moreover, recent research has indicated that ZmBk2L1 is required for the root hair development in maize (Hochholdinger et al 2008).…”

Investigation of cell wall composition related to stem lodging resistance in wheat (Triticum aestivumL.) by FTIR spectroscopy

“…11,13 The localized accumulation of cell wall-bound phenolics was visualized in cross-sections from equivalent root regions using UV-fluorescence microscopy 14 and by in situ staining for lignins. 15,16 Here, we used a surgical approach to determine whether the localized accumulation of wall cellulose or lignin occurred in stem tissues that provide stem strength.…”