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Wrinkles on Sepals: Cuticular Ridges Form when Cuticle Production Outpaces Epidermal Cell Expansion

2017; Elsevier BV; Volume: 10; Issue: 4 Linguagem: Inglês

10.1016/j.molp.2017.02.008

1674-2052

David Smyth,

Tree Root and Stability Studies

The cuticle is a waterproof coat that covers the outer surface of plant organs exposed above the ground, such as leaves, stems, floral organs, and fruits (but not bark) (Yeats and Rose, 2013Yeats T.H. Rose J.K.C. The formation and function of plant cuticles.Plant Physiol. 2013; 163: 5-20Crossref PubMed Scopus (780) Google Scholar). It prevents the escape of water vapor and can protect the plant from pathogen attack and UV-B exposure. The cuticle is made from cutin, a complex polyester embedded with hydrophobic waxes, and covers the outer cell wall of the epidermis. Cutin is polymerized from ω-hydroxy fatty acids by a small subfamily of GDSL lipase/hydrolases called cutin synthases (CUS) (Yeats et al., 2014Yeats T.H. Huang W.L. Chatterjee S. Viart H.M.F. Clausen M.H. Stark R.E. Rose J.K.C. Tomato Cutin Deficient 1 (CD1) and putative orthologs comprise an ancient family of cutin synthase-like (CUS) proteins that are conserved among land plants.Plant J. 2014; 77: 667-675Crossref PubMed Scopus (80) Google Scholar). There are four members in Arabidopsis and five in tomato where they were discovered by the observation of the near absence of cutin on tomato fruits in Slcus1 mutants. CUS genes are found across all land plants, indicating the importance of the cuticle in the colonization of the land. On many cells the cuticle has a smooth surface, but in others it forms narrow parallel ridges from 100 nm up to several micrometers in width, often called nanoridges (Li-Beisson et al., 2009Li-Beisson Y. Pollard M. Sauveplane V. Pinot F. Ohlrogge J. Beisson F. Nanoridges that characterize the surface morphology of flowers require the synthesis of cutin polyester.Proc. Natl. Acad. Sci. USA. 2009; 106: 22008-22013Crossref PubMed Scopus (182) Google Scholar). These often occur in floral organs, especially petals and sepals. Here their function is not fully understood, although in petals their regular spacing may act like a diffraction grating and is associated with iridescence (as occurs on some butterfly wings), serving to attract pollinators (Whitney et al., 2009Whitney H.M. Kolle M. Andrew P. Chittka L. Steiner U. Glover B.J. Floral iridescence, produced by diffractive optics, acts as a cue for animal pollinators.Science. 2009; 323: 130-133Crossref PubMed Scopus (285) Google Scholar). Other possible roles include prevention of wetting by promoting water droplet formation. Martens, 1933Martens P. Recherches sur la cuticule III. Structure, origine et signification du relief cuticulaire.Protoplasma. 1933; 20: 83-515Crossref Scopus (11) Google Scholar proposed that cuticular ridges arise through excessive generation of the cuticle that piles up in rows on the surface of underlying cells with more limited expansion. Recently, mathematical modeling based on differential growth of adjacent flat layers has been able to replicate the generation of such parallel ridges or wrinkles (Antoniou Kourounioti et al., 2013Antoniou Kourounioti R.L. Band L.R. Fozard J.A. Hampstead A. Lovrics A. Moyroud E. Vignolini S. King J.R. Jensen O.E. Glover B.J. Buckling as an origin of ordered cuticular patterns in flower petals.J. R. Soc. Interf. 2013; 10: 20120847Crossref PubMed Scopus (39) Google Scholar). Consistent with this idea, mutants of Arabidopsis defective in cuticle synthesis usually lack such ridges, at least on petal epidermal cells (e.g., Li-Beisson et al., 2009Li-Beisson Y. Pollard M. Sauveplane V. Pinot F. Ohlrogge J. Beisson F. Nanoridges that characterize the surface morphology of flowers require the synthesis of cutin polyester.Proc. Natl. Acad. Sci. USA. 2009; 106: 22008-22013Crossref PubMed Scopus (182) Google Scholar, Shi et al., 2011Shi J.X. Malitsky S. De Oliveira S. Branigan C. Franke R.B. Schreiber L. Aharoni A. SHINE transcription factors act redundantly to pattern the archetypal surface of Arabidopsis flower organs.PLoS Genet. 2011; 7: e1001388Crossref PubMed Scopus (161) Google Scholar, Mazurek et al., 2016Mazurek S. Garroum I. Daraspe J. De Bellis D. Olsson V. Mucciolo A. Butenko M.A. Humbel B.M. Nawrath C. Connecting the molecular structure of cutin to ultrastructure and physical properties of the cuticle in petals of Arabidopsis.Plant Physiol. 2016; 173: 1146-1163Crossref PubMed Scopus (27) Google Scholar). Now Hong et al., 2017Hong L. Brown J. Sergeson N.A. Rose J.K.C. Roeder A.H.K. CUTIN SYNTHASE2 maintains progressively developing cuticular ridges in Arabidopsis sepals.Mol. Plant. 2017; https://doi.org/10.1016/j.molp.2017.01.002Abstract Full Text Full Text PDF PubMed Scopus (35) Google Scholar provide new evidence in support of wrinkly cuticles arising as the consequence of a mismatch in the expansion of the cuticle versus that of underlying epidermal cells. Focusing on sepals in Arabidopsis, they followed the appearance of cuticular ridges in real time during sepal development (Figure 1). They showed that ridges appeared progressively down the sepal from apex to base, delineated by a moving cuticle ridge boundary. By mapping cell division and cell expansion using sophisticated imaging and MorphoGraphX software, they revealed that cell division ceased and cell expansion slowed directly above the boundary where the ridges first appeared. The authors tied this to the generation of the cuticle by mapping the expression of the cutin biosynthesis gene CUS2. This also moved in a wave down the sepals as they developed, and significantly it started in cells just before the cuticular ridge boundary reached them on its progression down the sepal (Figure 1). The most surprising result came when the authors looked at what happened to cuticle ridge formation when CUS2 activity was lost. As predicted, there were very few ridges in fully mature sepals of cus2 mutants. Unexpectedly, however, the ridges still appeared earlier in sepal development, but they were progressively lost as the cells aged. This was consistent with cuticle formation still occurring in cus2 mutants at first, but slowing down significantly later, apparently allowing the amount of cuticle generated to be matched by underlying cell expansion. Thus, the nanoridges were smoothed out. It looks like the cuticle is not firmly attached to the underlying cell wall, but that it fits over it somewhat like a loose rug. These conclusions were further tested by Hong et al., 2017Hong L. Brown J. Sergeson N.A. Rose J.K.C. Roeder A.H.K. CUTIN SYNTHASE2 maintains progressively developing cuticular ridges in Arabidopsis sepals.Mol. Plant. 2017; https://doi.org/10.1016/j.molp.2017.01.002Abstract Full Text Full Text PDF PubMed Scopus (35) Google Scholar using two other cutin biosynthesis genes. Mutants of the cytochrome P450 CYP77a6 also had ridges at first in sepal cells but lost them on older cells, showing that modification of cuticle production was likely the primary cause of the phenomenon. In addition, expression of CUS1, the cutin synthase gene most closely related to CUS2 (Yeats et al., 2014Yeats T.H. Huang W.L. Chatterjee S. Viart H.M.F. Clausen M.H. Stark R.E. Rose J.K.C. Tomato Cutin Deficient 1 (CD1) and putative orthologs comprise an ancient family of cutin synthase-like (CUS) proteins that are conserved among land plants.Plant J. 2014; 77: 667-675Crossref PubMed Scopus (80) Google Scholar), also occurred in developing sepals but earlier than CUS2. This suggests that CUS1 and CUS2 have sub-functionalized following their origin by gene duplication and that redundant CUS activity may be generating the early ridges seen in cus2 mutants. On the other hand, this is different in petals where the nanoridges still arise and are maintained when either CUS1 or CUS2 function is lost, whereas a simultaneous knockout of both genes eliminates the ridges (Shi et al., 2011Shi J.X. Malitsky S. De Oliveira S. Branigan C. Franke R.B. Schreiber L. Aharoni A. SHINE transcription factors act redundantly to pattern the archetypal surface of Arabidopsis flower organs.PLoS Genet. 2011; 7: e1001388Crossref PubMed Scopus (161) Google Scholar). Thus, the proposed sub-functionalization of CUS1 and CUS2 in sepals has apparently not occurred in petals where their functions are still fully redundant. Consistent with this, loss of the biosynthesis of a cutin precursor in cyp77a6 single mutants revealed that it was sufficient to generate ridges alone, without any redundant partners (Li-Beisson et al., 2009Li-Beisson Y. Pollard M. Sauveplane V. Pinot F. Ohlrogge J. Beisson F. Nanoridges that characterize the surface morphology of flowers require the synthesis of cutin polyester.Proc. Natl. Acad. Sci. USA. 2009; 106: 22008-22013Crossref PubMed Scopus (182) Google Scholar). Interesting questions remain. What coordinates the synthesis of cutin precursors in epidermal cells, their polar transport to the extracellular matrix, their delivery to the cutin synthases already located there, and the assembly of cutin and waxes into the mature cuticle (Yeats and Rose, 2013Yeats T.H. Rose J.K.C. The formation and function of plant cuticles.Plant Physiol. 2013; 163: 5-20Crossref PubMed Scopus (780) Google Scholar)? What is the role of the two other cutin synthase genes, CUS3 and CUS4, in Arabidopsis? Is CUS gene expression generally regulated by AP2 transcription factors of the SHINE clade (Shi et al., 2011Shi J.X. Malitsky S. De Oliveira S. Branigan C. Franke R.B. Schreiber L. Aharoni A. SHINE transcription factors act redundantly to pattern the archetypal surface of Arabidopsis flower organs.PLoS Genet. 2011; 7: e1001388Crossref PubMed Scopus (161) Google Scholar)? How is cuticle ridge formation controlled in other tissues, especially petals (Mazurek et al., 2016Mazurek S. Garroum I. Daraspe J. De Bellis D. Olsson V. Mucciolo A. Butenko M.A. Humbel B.M. Nawrath C. Connecting the molecular structure of cutin to ultrastructure and physical properties of the cuticle in petals of Arabidopsis.Plant Physiol. 2016; 173: 1146-1163Crossref PubMed Scopus (27) Google Scholar)? And what about cutin formation in leaves, stems, and fruit? The pattern of maturation of Arabidopsis leaves is similar to sepals, consistent with the likely orthology of these organs. In Arabidopsis leaves, too, a differentiation boundary moves as a progressive front from the apex to the base (Hepworth and Lenhard, 2014Hepworth S. Lenhard M. Regulation of plant lateral-organ growth by modulating cell number and size.Curr. Opin. Plant Biol. 2014; 17: 36-42Crossref PubMed Scopus (96) Google Scholar), and its molecular and genetic regulation may parallel that in sepals. Further questions can also be addressed. Is post-genital fusion between organs prevented by the presence of the cuticle (when cuticle formation is disrupted, vegetative and floral organs are often “sticky”) (e.g. Shi et al., 2011Shi J.X. Malitsky S. De Oliveira S. Branigan C. Franke R.B. Schreiber L. Aharoni A. SHINE transcription factors act redundantly to pattern the archetypal surface of Arabidopsis flower organs.PLoS Genet. 2011; 7: e1001388Crossref PubMed Scopus (161) Google Scholar, Mazurek et al., 2016Mazurek S. Garroum I. Daraspe J. De Bellis D. Olsson V. Mucciolo A. Butenko M.A. Humbel B.M. Nawrath C. Connecting the molecular structure of cutin to ultrastructure and physical properties of the cuticle in petals of Arabidopsis.Plant Physiol. 2016; 173: 1146-1163Crossref PubMed Scopus (27) Google Scholar)? Finally, does biophysical or other feedback occur to coordinate the size of the cutin-synthesizing cells and the amount of cuticle generated so that they remain in balance, either to generate a smooth and regular cuticular surface, or the specialized nanoridges shown here to arise by excess cuticular buckling? No conflicts of interest are declared.