Investigating solar protection during plant growth

Project Highlights:
- Acquire diverse skills set centred on understanding of the 'Biophysics of Life' by working with Life Scientists and Chemists at Warwick University
- Unravel the energy transfer mechanisms occurring in Garden Cress Sprout (a member of the Brassicaceae family), specifically within light absorbing molecules (eg sinapoyl malate) used as antioxidants and solar protection in the cosmeceutical industry
- Develop a better appreciation of when to harvest Garden Cress Sprout for its light absorbers, ultimately benefitting the environment in terms of using less natural resources (eg water) and growth additives

Overview
Ultraviolet radiation that reaches the Earth's surface has extensive impact on the biosphere [1,2]. Of particular interest is the high energy component of solar UV radiation spectrum, the UV-B (280-315 nm) and shorter wavelengths of the UV-A spectrum (<340 nm) termed, henceforth, UV-radiation (UVR). In plants, UVR acts as a signal transducer for numerous processes including immune response, plant morphology and the phenylpropanoid pathway [2,3]. Deleterious effects of UVR exposure to organisms are widely known, for example, reduction of photosynthesis, growth inhibition and susceptibility to pathogens [4,5]. As such, plants synthesize and deposit UVR-absorbing compounds in epidermal tissue via the phenylpropanoid pathway to protect against too much UVR [2,3]

Specifically, studies of gene mutations in the plant Arabidopsis thaliana, a member of the Brassicaceae family, have found that the phenylpropanoid pathway could be disrupted such that the concentration of sinapate esters present in the epidermal layers of the plant are reduced, rendering the plant hypersensitive to UVR exposure [6-11]. Such experiments point towards sinapate esters as being the likely class of UVR screening molecules used by Brassicaceae plants. Sinapate esters are a derivative of sinapic acid, and closely related to sinapoyl malate (Figure 1) which has been the dominant ester- constituent deposited in the upper epidermis of Arabidopsis plant leaves [7,8,11].

We have recently proposed that solar protection of sinapate esters is driven by a molecular isomerisation [12-14]; the esters absorb UVR and, through a molecular twist, they convert potentially damaging absorbed UVR into vibrational motion (or heat). Having transferred this heat energy to the surrounding environment, the esters are then ready to reabsorb further UVR and hence provide the plant with the necessary solar protection. However, intriguing questions arise including: (1) what is the composition of these ester-species during the growth cycle of the plant; and (2) how do their composition vary with geographic location of the plant?

In this PhD proposal, our aim is to address these questions. We will specifically focus on another member of the Brassicaceae family, that of the Garden Cress Sprout. We will determine the composition of these esters (in the form of 'extracts') during the Garden Cress Sprout growth cycle and establish how geographic location (eg northern vs southern parts of UK) impacts this composition. Importantly, we will use state of the art analytical techniques based on laser spectroscopy to determine their solar protection mechanisms in as close to their natural environment as possible. Our insight could impact which stage of the growth cycle one harvests the plant for these extracts, which are finding widespread use, for example, in the cosmeceutical industries as antioxidants and SPF boosters.