Bioengineering inducible gene expression in leafy brassicas to address post-harvest-specific requirements

Abstract

Food loss and waste has a significant negative impact on the sustainability of the global food system. It is estimated that ~13% of harvested fresh horticultural crops are never consumed owing to deterioration of plant health and quality in the post-harvest period. Harvested leafy crops are living tissues which can remain edible for periods of one week to six months, depending on the variety; this is astonishing when we consider that they are unable to uptake nutrients or water, and are subject to the stress of the harvest and storage processes. Improvements to the health and quality of post-harvest leafy crops could increase their storage life, and consequently reduce food loss. The aim of this study is to identify changes in transcription and immunity caused by harvest, and bioengineer a harvest-inducible genetic system to bolster identified weaknesses in post-harvest health.

Post-harvest improvements to crop health have mostly focused on storage conditions and abiotic stress. However, disease can contribute significantly to food loss and waste, and the changes to immunity of leafy crops after harvest remain obscure. Accordingly, in Chapter 3 I examine post-harvest pattern-triggered immunity (PTI) and selected immune signalling pathways in an Arabidopsis model harvest system. Here I show that harvest suppresses both early PTI-responsive gene expression and the salicylic acid pathway, and harvested plants are more susceptible to a hemi-biotrophic pathogen. By contrast the jasmonic acid pathway is enhanced in harvested tissues, and growth of a necrotrophic fungus is suppressed. Therefore, this study identifies that harvest has a significant impact on the immune system of Arabidopsis rosettes, and presents attenuated immune pathways as potential targets for post-harvest enhancement.

The multiple stresses of harvest and storage can lead to physiological changes in harvested leafy crops, including accelerated tissue senescence. In Chapter 4 I explore the progression of post-harvest transcriptional changes in leafy brassicas, comparing the model brassica Arabidopsis with a long-storage leafy crop, pointed cabbage (Brassica oleracea var. capitata), and existing transcriptomic datasets in short-storage brassicas broccoli (Brassica oleracea var. italica) and salad rocket (Eruca sativa). I demonstrate that there is commonality of post-harvest abiotic responses in short-storage brassicas and the model system, which are not seen in the cabbage dataset. This study shows that the post-harvest needs of leafy crops are likely to be markedly different depending on the length of shelf-life.

The post-harvest shift in the transcriptome and immune system of harvested leafy brassicas underlines the requirement for post-harvest-specific interventions, which would avoid negative impacts on the health or yield of the plant while it is growing on soil. To this end, in Chapter 5 I identify likely regulatory control of harvest-inducible genes, which show low expression on soil, and high expression post-harvest. Promoter sequences from these harvest-inducible genes are used to drive reporter gene expression in a harvest-responsive manner.

Overall, our findings indicate that the changes in the immune system and transcriptome of harvested leafy brassicas necessitate different interventions from soil-growing plants. As such, I provide proof-of-concept of a harvest-inducible system that could form the future basis of bioengineering strategies to improve health and quality of harvested leafy brassicas, and thereby reduce food loss and waste.