Job-sharing by the barley cuticle

Plant overlay their aerial tissues, and also some of their ground tissues, with a cuticle, a structure with a complex molecular makeup. Various functions for this structure have been studied in plants and include protection from herbivorous insects, protection from plant pathogens and protection from electromagnetic radiation. An additional well known function is prevention of water loss from the plant in hot, dry conditions. Plants may also lose nutrients from their body surface. In contrast to water loss however, this may occur in damp and humid conditions where water accumulation on the plant surface promotes leaching of nutrients out of the plant. The way in which the plant cuticle may protect against nutrient leaching is less understood compared to its role in preventing water loss. This is particularly important when thinking about how we might maximise how commercially important plants perform in the field. In a recent Annals of Botany paper, Penny von Wettstein-Knowles of the University of Copenhagen examines the effects of a range of known mutants affecting barley cuticle structure to different degrees of severity on nutrient leaching. The findings in the paper provide a starting point for understanding how we may get the best out of the barley cuticle in the future.

The author exposed normal and mutant cuticle barley plants to high amounts of moisture through spraying them with differing amounts of rain per day, raising the humidity. When asked why she chose the range of cuticle mutants she did, Penny von Wettstein-Knowles told Botany One: ‘If one wants to determine if wax crystal structure has an effect on a phenomenon then one should start with mutants that exhibit the maximum difference, and about which as much other is known as possible, e.g. chemistry, gene being cloned etc.  This is true for the cer.c and .j mutants, two of the mutants I used in the paper. The cer.u, .e, and .i mutants that I also investigate, on the other hand, are intermediates having modified tube crystals representing phenotypes between their total absence on cer.c mutantsand a dense covering on their wild type’. The author uses the differing properties of these mutants to investigate how different aspects of the barley cuticle may contribute to its functions in varying conditions.

Under the highest rain conditions, kernel (seed) weight of the cuticle mutant plants decreasing more in the all of the cuticle mutant variants compared to normal barley, albeit to differing degrees between different the mutants. The number of kernels produced also decreased for all but one of the mutants. In order to link this to nutrient leaching rather than other possible off-effects of the humidity increase, Wettstein-Knowles increased the nutrient availability to both normal and cuticle mutant plants. This increase in nutrient availability could partially offset the decrease in kernel yield caused by the high humidity in some cases, but could not entirely compensate. The author also measures non-stomatal water loss in both normal and cuticle mutant barley plants but find that only two of the barley cuticle mutants lose water at a substantially greater rate. These two barley mutants lose water 18.3 times faster than some of the other barley lines studied here, highlighting that different cuticle mutants with varying effects on cuticle properties have differing effects on the water retention properties of the barley cuticle.

The data presented in this study therefore indicates that the barley cuticle, and quite likely the cuticle of other plant species, function partially to both prevent water loss during dry periods and to also prevent nutrient loss by leaching during humid periods. Very few of the barley mutants used in this study have been mapped to specific genes. The results in this study, particularly the variable  responses to the different environment conditions between mutant lines, highlight the need for greater understanding of the genetic control of cuticle formation in commercially-important plants such a barley. The plant cuticle is therefore capable of protecting plants from a variety of problems in different conditions, and different features of its structure appear to contribute to different tasks. Not only a complex structure then, but also a useful one.