Branched Pectic Galactan in Phloem-Sieve-Element Cell Walls: Implications for Cell Mechanics.

A major question in plant biology concerns the specification and functional differentiation of cell types. This is in the context of constraints imposed by networks of cell walls that both adhere cells and contribute to the form and function of developing organs. Here, we report the identification of a glycan epitope that is specific to phloem sieve element cell walls in several systems. A monoclonal antibody, designated LM26, binds to the cell wall of phloem sieve elements in stems of Arabidopsis (Arabidopsis thaliana), Miscanthus x giganteus, and notably sugar beet (Beta vulgaris) roots where phloem identification is an important factor for the study of phloem unloading of Suc. Using microarrays of synthetic oligosaccharides, the LM26 epitope has been identified as a ß-1,6-galactosyl substitution of ß-1,4-galactan requiring more than three backbone residues for optimized recognition. This branched galactan structure has previously been identified in garlic (Allium sativum) bulbs in which the LM26 epitope is widespread throughout most cell walls including those of phloem cells. Garlic bulb cell wall material has been used to confirm the association of the LM26 epitope with cell wall pectic rhamnogalacturonan-I polysaccharides. In the phloem tissues of grass stems, the LM26 epitope has a complementary pattern to that of the LM5 linear ß-1,4-galactan epitope, which is detected only in companion cell walls. Mechanical probing of transverse sections of M x giganteus stems and leaves by atomic force microscopy indicates that phloem sieve element cell walls have a lower indentation modulus (indicative of higher elasticity) than companion cell walls.

Reduced terpene levels in cottonseed add food to fiber.

Using RNA interference (RNAi) technology, the levels of a toxic phytoprotectant have recently been reduced specifically in the seeds of cotton to generate a novel dual-purpose crop. By engineering an endogenous terpene pathway, there is now the exciting potential for an added-value, genetically modified crop with the cash value of the fiber supported by the improved nutritional value and expanded food and feed use for the cottonseed, which is normally a low-value by-product.

Antisense suppression of a (+)-delta-cadinene synthase gene in cotton prevents the induction of this defense response gene during bacterial blight infection but not its constitutive expression.

In cotton (Gossypium hirsutum) the enzyme (+)-delta-cadinene synthase (CDNS) catalyzes the first committed step in the biosynthesis of cadinane-type sesquiterpenes, such as gossypol, that provide constitutive and inducible protection against pests and diseases. A cotton cDNA clone encoding CDNS (cdn1-C4) was isolated from developing embryos and functionally characterized. Southern analysis showed that CDNS genes belong to a large multigene family, of which five genomic clones were studied, including three pseudogenes and one gene that may represent another subfamily of CDNS. CDNS expression was shown to be induced in cotton infected with either the bacterial blight or verticillium wilt pathogens. Constructs for the constitutive or seed-specific antisense suppression of cdn1-C4 were introduced into cotton by Agrobacterium-mediated transformation. Gossypol levels were not reduced in the seeds of transformants with either construct, nor was the induction of CDNS expression affected in stems of the constitutive antisense plants infected with Verticillium dahliae Kleb. However, the induction of CDNS mRNA and protein in response to bacterial blight infection of cotyledons was completely blocked in the constitutive antisense plants. These results suggest that cdn1-C4 may be involved specifically in the bacterial blight response and that the CDNS multigene family comprises a complex set of genes differing in their temporal and spatial regulation and responsible for different branches of the cotton sesquiterpene pathway.

Spatial and temporal regulation of a soybean (Glycine max) lectin promoter in transgenic cotton (Gossypium hirsutum).

The activity of a soybean (Glycine max L. Merrill) lectin gene promoter was investigated in transgenic cotton plants (Gossypium hirsutum L.) with the view to using this promoter for the seed-specific alteration of gossypol, a secondary metabolite in cotton that has adverse effects on the nutritional value of cottonseed products like oil and protein-rich meal. Agrobacterium-mediated transformation generated stable transformants containing a construct with the lectin promoter fused to the ß-glucuronidase reporter gene (pLeGUS). Fluorometric GUS assays and northern hybridization detected strong promoter activity during embryo development. GUS activity in developing embryos was detected as early as 10 d post-anthesis (dpa), peaking late in embryo maturation. Enzyme activity persisted in imbibed mature seed, and negligible activity remained detectable in the roots and cotyledons of 7-d-old seedlings. No GUS activity was detected in leaves and squares of mature plants. GUS transcripts increased during embryo development to peak about 35 dpa, declining to a low level in imbibed mature seed. No transcripts were detected in roots, cotyledons, leaves or squares. Histochemical GUS activity staining indicated promoter activity in all cells of the cotyledons, including the flattened cells of the gossypol glands, the presumed site of synthesis of gossypol. This study concluded that the soybean lectin gene promoter is a useful tool for the seed-specific expression of transgenes in cotton.