Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - 3F-FutureFreshFruit (Elucidating the molecular basis of fruit resistance to desiccation: The cuticle as a key factor)

Teaser

Water stress is a key environmental factor that, in a horticultural context, causes important quality and economical losses before and after fruit harvest. As water depletion is becoming increasingly problematic, improving water efficiency and tolerance of plants to...

Summary

Water stress is a key environmental factor that, in a horticultural context, causes important quality and economical losses before and after fruit harvest. As water depletion is becoming increasingly problematic, improving water efficiency and tolerance of plants to dehydration is a priority of the European Union, as reflected in the societal challenge of Climate action, environment and resource efficiency. Likewise, food waste reduction is another key issue in the framework of Horizon 2020, as described in Food security and sustainable agriculture and forestry challenge. Indeed, a large proportion of fresh produce is wasted during postharvest (25-50%) of horticultural crops, which is largely caused by dehydration. Therefore, this project deals with the urgent need of understanding factors influencing water loss to develop innovative solutions geared to saving water and reducing food waste.
Cuticular waxes are critical for limiting water loss from fruit, and thus for resisting desiccation and spoilage. We propose to uncover the structural and regulatory pathways that mediate its biosynthesis, transport and assembly in response to drought stress both in climacteric and non-climacteric fruit. Tomato and citrus fruit have been selected as model systems. The availability of mutants of both fruits, as well as the application of stress-related hormones during postharvest, allowed comparative surveys to better understand the regulation of these processes. The outcomes of this proposal will represent a major advance in the understanding of the associations between water stress, wax metabolism and fruit biology, and will suggest strategies for enhancing shelf life, drought hardiness, water use efficiency and fruit quality in both climacteric and non-climacteric fruits.

Work performed

The outgoing phase of this project has been divided in two main work packages:

First, we addressed the hypothesis that abscisic acid (ABA), in addition to play a pivotal role as drought-responsive hormone in plants, mediates cuticle formation during organ development. We assessed the effect of ABA deficiency on cuticle formation in three ABA biosynthesis impaired tomato mutants (flacca, notabilis and sitiens) and in two different tissues, leaves and fruit. The mutant leaf cuticles were thinner and a substantial reduction in cutin levels. ABA deficiency also resulted in differences in the composition of leaf cutin and cuticular waxes. Exogenous application of ABA partially rescued these phenotypes, confirming that they were a consequence of reduced ABA levels. The ABA mutants also showed reduced expression of genes involved in cutin or wax formation. This difference was again countered by exogenous ABA, further indicating regulation of cuticle biosynthesis by ABA. The fruit cuticles were affected differently by the ABA associated mutations, but in general were thicker. However, no structural abnormalities were observed and the cutin and wax compositions were less affected than in leaf cuticles, suggesting that ABA action influences cuticle formation in an organ dependent manner.
The ABA signaling and the water stress response in plants are tightly related, although ABA-independent molecular mechanisms have been also described in the response of plants and fruits to dehydration.We wondered whether dehydrating conditions could affect fruit cuticle metabolism and modify fruit quality. We studied the effect of water limiting growing conditions on the cuticle formation and properties of tomato fruit and its relationship with the shelf life and fruit quality during postharvest storage.
Softening of fleshy fruits is a complex process involving cell wall metabolism and cellular water adjustments. In this regard, a role for fruit cuticle was also suggested by studying the delayed fruit deterioration (DFD) tomato fruit, which exhibit extended shelf life and are palatable for many months after ripening. Long shelf life trait of DFD depends on water stress growing conditions (WSGC), although the relationship between watering regime, cuticle properties, and effects on fruit quality remained unknown. We have observed that WSGC increased fruit firmness, cuticle thickness, cutin and wax load, and the expression of cuticle biosynthetic genes in the normally softening tomato fruit, Ailsa Craig and M82. In addition, cuticle permeability, fruit transpiration rate and the frequency of microbial infection were reduced in response to the deficient water availability. DFD fruit did not show such changes in cuticle properties, what we interpret as ‘pre-adapted’ cuticles to minimize fruit deterioration under WSGC.
To better understand the influence of water stress on cuticle biology and its relationship with fruit quality, we also performed a postharvest assay that allowed us to discriminate between the effect on fruit softening and transpirational weight loss caused by the watering regime or the dehydrating conditions during storage. Postharvest dehydrating conditions did not affect fruit cuticle properties or composition in any cultivar, suggesting that preharvest conditions play a far more important role in cuticle metabolism than the postharvest water stress applied. We conclude that limiting water availability during tomato cultivation affects fruit cuticle biosynthesis and metabolism, thereby reducing cuticle permeability, diminishing transpirational water loss and extending shelf life.

Final results

This proposal deals with subjects of importance for worldwide Agriculture and Food Science using cutting-edge technologies developed during this research. On one hand, this multidisciplinary research project provides results that will contribute to avoiding loss of fruit quality and, consequently, loss of economic value of fresh fruit. On the other hand, results obtained so far establish basic knowledge in tomato fruit for future development of biotechnological strategies aimed to enhance drought hardiness and water efficiency in horticultural crops and open new research lines focusing on disease resistance by improving epicuticular waxes, which will lead to reduction of pesticides. Both applied and basic research components are integral to those strategies that the EU are currently promoting, as reflected in Horizon 2020 designating the food security, sustainable agriculture and resources efficiency as key societal challenges.
Furthermore, translation of basic research into the breeding of drought-tolerant crops is a very important challenge for food security under global warming conditions. Because the global population continues to increase, and longevity has also been rapidly increasing, it is not only important for human health to increase food production but also to improve food quality. The successful application of basic knowledge from plant science and genomics will be crucial for future food security and agroindustry in the world. Such translational research for the production of more healthy and better foods is a visible contribution of plant science not only to European but also to worldwide society.