Lignin and/or suberin? Lignin!

For as long as the Casparian strip (CS) – an impervious component of the endodermal cell wall that blocks the passive flow of water and solutes into the vascular tissues of a plant – has been known (discovery credited to Robert Caspary in the mid 1860s), it has been considered to contain lignin and/or suberin. And, since both of those materials have waterproofing properties, does it really matter whether one or other or both were found there? Well, it certainly mattered to Sadaf Naseer et al., whose investigations in arabidopsis allow the unequivocal conclusion that the CS is made of lignin (…or a ‘closely related lignin-like polymer’). Furthermore, Naseer et al. unambiguously demonstrated that ‘completely abrogating suberin accumulation’ (i.e. using suberin-deficient plants) still allows the establishment of an efficient barrier to penetration of PI (propidium iodide, used as an apoplast tracer in the study). This elegant structure–function investigation presents precise developmental staging of the appearance of various histochemical stains for suberin and lignin using whole-mount staining procedures, combined with functional assays, reporter gene expression analysis and various pharmacological and novel genetic manipulations of lignin and suberin production. But for those who like a news items with no loose ends, this study is not quite as definitive as one might like, because the CS does contain suberin… And with almost precision timing this internal water-proofing revelation follows the announcement that Trevor Yeats et al. have determined the enzymatic basis of the polymerisation of cutin, the polyester polymer of plant cuticles (which famously waterproof the exterior of aerial plant parts). Experimenting with cutin deficient 1 (cd1), a cutin-deficient (!) mutant of tomato, the team propose that not only is an acyltransferase – CD1 – the principal catalyst of cutin polymerisation, but also that the polymerisation process is extracellular and occurs at the site of cuticle deposition.