Differential response of cuticular wax and photosynthetic capacity by glaucous and non-glaucous wheat cultivars under mild and severe droughts

A GDSL-motif Esterase/Lipase Affects Wax and Cutin Deposition and Controls Hull-Caryopsis Attachment in Barley

The cuticle covering aerial organs of land plants is well known to protect against desiccation. Cuticles also play diverse and specialized functions, including organ separation, depending on plant and tissue. Barley shows a distinctive cuticular wax bloom enriched in β-diketones on leaf sheaths, stem nodes and internodes and inflorescences. Barley also develops a sticky surface on the outer pericarp layer of its grain fruit leading to strongly adhered hulls, 'covered grain', important for embryo protection and seed dispersal. While the transcription factor-encoding gene HvNUDUM (HvNUD) appears essential for adherent hulls, little is understood about how the pericarp cuticle changes during adhesion or whether changes in pericarp cuticles contribute to another phenotype where hulls partially shed, called 'skinning'. To that end, we screened barley lines for hull adhesion defects, focussing on the Eceriferum (= waxless, cer) mutants. Here, we show that the cer-xd allele causes defective wax blooms and compromised hull adhesion, and results from a mutation removing the last 10 amino acids of the GDS(L) [Gly, Asp, Ser, (Leu)]-motif esterase/lipase HvGDSL1. We used severe and moderate HvGDSL1 alleles to show that complete HvGDSL1 function is essential for leaf blade cuticular integrity, wax bloom deposition over inflorescences and leaf sheaths and pericarp cuticular ridge formation. Expression data suggest that HvGDSL1 may regulate hull adhesion independently of HvNUD. We found high conservation of HvGDSL1 among barley germplasm, so variation in HvGDSL1 unlikely leads to grain skinning in cultivated barley. Taken together, we reveal a single locus which controls adaptive cuticular properties across different organs in barley.

Dynamic changes to the plant cuticle include the production of volatile cuticular wax-derived compounds

The cuticle is a hydrophobic structure that seals plant aerial surfaces from the surrounding environment. To better understand how cuticular wax composition changes over development, we conducted an untargeted screen of leaf surface lipids from black cottonwood (Populus trichocarpa). We observed major shifts to the lipid profile across development, from a phenolic and terpene-dominated profile in young leaves to an aliphatic wax-dominated profile in mature leaves. Contrary to the general pattern, levels of aliphatic cis-9-alkenes decreased in older leaves following their accumulation. A thorough examination revealed that the decrease in cis-9-alkenes was accompanied by a concomitant increase in aldehydes, one of them being the volatile compound nonanal. By applying exogenous alkenes to P. trichocarpa leaves, we show that unsaturated waxes in the cuticle undergo spontaneous oxidative cleavage to generate aldehydes and that this process occurs similarly in other alkene-accumulating systems such as balsam poplar (Populus balsamifera) leaves and corn (Zea mays) silk. Moreover, we show that the production of cuticular wax-derived compounds can be extended to other wax components. In bread wheat (Triticum aestivum), 9-hydroxy-14,16-hentriacontanedione likely decomposes to generate 2-heptadecanone and 7-octyloxepan-2-one (a caprolactone). These findings highlight an unusual route to the production of plant volatiles that are structurally encoded within cuticular wax precursors. These processes could play a role in modulating ecological interactions and open the possibility for engineering bioactive volatile compounds into plant waxes.

Variation between glaucous and non-glaucous near-isogenic lines of rye (Secale cereale L.) under drought stress

Glaucous (811, L35, and RXL10) and non-glaucous (811bw, L35bw, and RXL10bw) near-isogenic lines (NILs) of rye (Secale cereale L.) forming three pairs of inbred lines were the subject of the research. The research aimed to study the relationship between wax cover attributes and the physio-biochemical drought reactions and yield of rye NILs and to uncover the differences in drought resistance levels of these lines. The greatest differences between glaucous and non-glaucous NILs were observed in the RXL10/RXL10bw pair. Of particular note were the stable grain number and the thousand grain weight of the non-glaucous line RXL10bw under drought and the accompanying reactions, such as an approximately 60% increase in MDA and a two-fold increase in wax amount, both of which were significantly higher than in the glaucous line RXL10 and in other NILs. The surprisingly high level of MDA in the RXL10bw line requires further analysis. Moreover, additional wax crystal aggregates were found under drought conditions on the abaxial leaf surface of the glaucous lines 811 and RXL10. The use of rye NILs indicated that line-specific drought resistance could be associated with wax biosynthetic pathways involved in physiological and biochemical responses important for increased drought resistance.

Conserved signalling components coordinate epidermal patterning and cuticle deposition in barley

Faced with terrestrial threats, land plants seal their aerial surfaces with a lipid-rich cuticle. To breathe, plants interrupt their cuticles with adjustable epidermal pores, called stomata, that regulate gas exchange, and develop other specialised epidermal cells such as defensive hairs. Mechanisms coordinating epidermal features remain poorly understood. Addressing this, we studied two loci whose allelic variation causes both cuticular wax-deficiency and misarranged stomata in barley, identifying the underlying genes, Cer-g/ HvYDA1, encoding a YODA-like (YDA) MAPKKK, and Cer-s/ HvBRX-Solo, encoding a single BREVIS-RADIX (BRX) domain protein. Both genes control cuticular integrity, the spacing and identity of epidermal cells, and barley's distinctive epicuticular wax blooms, as well as stomatal patterning in elevated CO₂ conditions. Genetic analyses revealed epistatic and modifying relationships between HvYDA1 and HvBRX-Solo, intimating that their products participate in interacting pathway(s) linking epidermal patterning with cuticular properties in barley. This may represent a mechanism for coordinating multiple adaptive features of the land plant epidermis in a cultivated cereal.

Correlation between leaf epicuticular wax composition and structure, physio-biochemical traits and drought resistance in glaucous and non-glaucous near-isogenic lines of rye

The objective of this research was to investigate the differences between glaucous and non-glaucous near-isogenic lines (NILs) of winter rye (Secale cereale L.) in terms of epicuticular wax layer properties (weight, composition, and crystal morphology), selected physiological and biochemical responses, yield components, above-ground biomass, and plant height under soil drought stress. An important aspect of this analysis was to examine the correlation between the above characteristics. Two different NIL pairs were tested, each consisting of a typical glaucous line and a non-glaucous line with a recessive mutation. The drought experiment was conducted twice (2015-2016). Our study showed that wax accumulation during drought was not correlated with higher leaf hydration and glaucousness. Environmental factors had a large impact on the response of the lines to drought in individual years, both in terms of physiological and biochemical reactions, and the composition of epicuticular leaf wax. The analysed pairs displayed significantly different responses to drought. Demonstration of the correlation between the components of rye leaf wax and the physiological and biochemical parameters of rye NILs is a significant achievement of this work. Interestingly, the study showed a correlation between the wax components and the content of photosynthetic pigments and tocopherols, whose biosynthesis, similarly to the biosynthesis of wax precursors, is mainly located in chloroplasts. This suggests a relationship between wax biosynthesis and plant response to various environmental conditions and drought stress.