Carotenoids are important micro-nutrients for humans present in fruits, vegetables and certain grains including bread wheat (Triticum aestivum). Several carotenoids including β-carotene and β-cryptoxanthin provide provitamin A activity making them key targets regarding biofortification of staple foods to address vitamin A deficiencies in developing countries. Due to their reactive nature, carotenoids are highly prone to oxidative degradation and have rapid turnover during the storage of crops, both in common varieties and biofortified lines. In commonly consumed foods carotenoids are often found esterified with fatty acids. Esterification positively influences stability, sequestration and accumulation of carotenoids. However, whilst the genes and regulation of carotenoid biosynthesis have been well studied, the molecular mechanisms underpinning esterification have remained unknown in all plant species to date.
Here I present data on the identification and genetic and biochemical characterisation of Xanthophyll Acyl-Transferase (XAT), a member of the GDSL esterase/lipase gene family, responsible for the production of carotenoid esters in mature grain of bread wheat. Intriguingly, despite carotenoids being synthesised in plastids, XAT was found to accumulate in the apoplast. I investigate the implications of this compartmentalisation and a potential alternative function for XAT in wheat by analysing substrate preferences and ester formation in vitro and in vivo using carotenoid-accumulating rice callus. I propose that disintegration of cellular structure during wheat grain desiccation facilitates access to carotenoids promoting their esterification. Given that the accumulation of carotenoids is governed by both the rates of synthesis and degradation, enhancing carotenoid stability could provide an opportunity to increase the nutritional status of crops.
Jacinta is a PhD candidate in Barry Posgon’s lab at the ANU. Prior to her PhD she was employed as a research assistant at the CSIRO Waite campus working in grapevine genetics. Jacinta studied for her undergraduate degree at Monash University followed by an honours year which was also conducted in Barry Pogson’s lab at the ANU.