Cactus seed lignin. Different polymers in similar coats

The seed as a distinct chemical system

Lignin is a phenolic polymer deposited in cell walls. Its structure is determined by both the type of aromatic units and how they are linked. Guaiacyl and syringyl units, designated G and S, predominate in typical angiosperm lignin. They arise from coniferyl and sinapyl alcohol. Chen and co-authors compared seed walls of 130 cactus species representing 47 genera in the classification used at the time. The article appeared in volume 73 of The Plant Journal in 2013, following online publication in November 2012. [Chen et al. 2013, pp. 201–204, Table 1]

The seeds came from collections and commercial batches with identified names. The study compared the chemical composition of this material. Typical G/S lignin was found in vegetative cactus tissues, including stems, roots and spines. Seed composition could be entirely different. Comparing organs within one plant group showed that the pathway forming seed walls can produce a different product from the pathway operating in vegetative parts. [Chen et al. 2013, pp. 202–204, 208–209]

What chemical degradation shows and what NMR shows

The main test was thioacidolysis of small samples, with released products analysed by gas chromatography coupled with mass spectrometry, GC-MS. The treatment cleaves specific polymer bonds and allows the resulting monomers to be identified. Two-dimensional NMR spectra of whole seed walls were also obtained for selected species. Carbon and hydrogen atom correlations identified aromatic units and portions of linkages retained in the polymer. [Chen et al. 2013, pp. 203–207, 210–211]

Thioacidolysis yield depends on lignin structure. A typical G/S polymer releases some of its units relatively readily in this test, while structures with numerous benzodioxane linkages give a low monomer-release yield. Few degradation products can therefore coexist with an extensive polymer. Combining GC-MS and NMR allowed this case to be distinguished from an actual absence of detectable lignin in the walls studied. [Chen et al. 2013, pp. 205–208]

Catechyl lignin

Catechyl lignin, C-lignin, arises from caffeyl alcohol. Its aromatic ring has adjacent hydroxyl groups, so coupling reactions produce characteristic benzodioxane units. The analysis indicated chain growth through the addition of successive monomers in a β–O–4 arrangement followed by ring closure. The Melocactus salvadorensis seeds studied yielded almost homogeneous C-lignin, with approximately 99% of analysed linkages of the benzodioxane type. [Chen et al. 2013, pp. 204–207, Fig. 4A]

Among the sixteen Mammillaria species studied, M. lasiacantha was distinguished by a high content of exclusively C-lignin. Four others had high contents of typical G/S lignin, and eleven had low G/S contents. The selected spectrum of M. densispina showed G units predominating over S. Such differences occurred within the genus, so genus identification alone does not predict the polymer in the seed of a particular plant. [Chen et al. 2013, pp. 204–206, Table 1, Fig. 4B]

Unusual polymers in former Escobaria

Three species described as Escobaria dasyacantha, E. lloydii and E. zilziana had 5-hydroxyguaiacyl units, 5H. Their current POWO names are Pelecyphora dasyacantha, P. lloydii and P. zilziana (formerly in Escobaria). The precursor of 5H is 5-hydroxyconiferyl alcohol. The paper documented natural accumulation of this polymer type, previously known mainly from studies of plants with modified lignin biosynthesis. [Chen et al. 2013, pp. 205–209][2-WWW]

In the analysed P. dasyacantha sample, almost all units in the lignin fraction corresponded to 5H, and approximately 98% of analysed linkages were benzodioxane linkages. In P. zilziana, the spectrum gave approximately 63% 5H and 37% S, with traces of G. Benzodioxane linkages accounted for approximately 84%. These are estimates of proportions within the polymer from NMR spectra. They do not express the percentage mass of lignin in the whole seed. The presence of 5H and S in P. zilziana left open the question of whether both unit types form a common polymer or different polymers in the same coat. [Chen et al. 2013, pp. 206–209, Fig. 4D–E]

The historical name Escobaria albicolumnaria is a synonym of Pelecyphora sneedii subsp. sneedii in POWO. The G/S result therefore needs to be attributed to material of this subspecies. The names Escobaria vivipara var. arizonica and var. neomexicana are now synonyms of Pelecyphora vivipara. In the publication, they retain a more detailed description of the batches studied. Updating the species name can link historical identifications while preserving their scope in the sample description. [2-WWW][2-WWW][Chen et al. 2013, pp. 206–208]

Seeds without detectable lignin

In seeds of Epithelantha micromeris and E. greggii, lignin was detected by neither chemical degradation nor NMR. The detailed E. micromeris spectrum contained polysaccharide signals, including glucose and xylose wall components. Unidentified aromatic signals also appeared. The result describes an absence of a typical lignin signature with the methods used, while seed cell walls remain complex in structure. [Chen et al. 2013, pp. 203–206, Fig. 4C]

The authors observed the surface and stained seed sections of four selected cacti representing C-lignin, 5H and S, and typical G/S lignin. Seed-coat ornamentation and cell shape did not form a simple pattern corresponding to polymer type. Autofluorescence was also not a simple measure of its amount. Image brightness and surface sculpture may therefore provide anatomical information, while chemical identification rests on analysis of specific wall components. [Chen et al. 2013, pp. 207–209, Fig. 5]

What may explain seed specificity

C, G, 5H and S units differ, among other features, in the number and position of hydroxyl and methoxyl groups on the aromatic ring. During biosynthesis, methoxyl groups arise through O-methylation reactions. The authors considered regulation of the methyltransferases COMT and CCoAOMT in seeds as a possible explanation for the pathway stopping at a C or 5H precursor. This paper identified end products, while specific expression or inactivation of these genes in the cacti studied was not determined. [Chen et al. 2013, pp. 208–209]

Such broad diversity within one family reveals the distinctness of seed-wall biosynthesis from that of vegetative organs and provides material for studying lignin evolution. The paper establishes unit and linkage types. The significance of these differences for germination, coat permeability, resistance or material applications requires separate measurements. The chemical structure of the polymer is a starting point for such experiments, and seed data remain assigned to the species and material batches studied. [Chen et al. 2013, pp. 208–209]