Genetic and mechanistic characterisation of rootstock traits conferring abiotic stress tolerance to grapevines

Abstract

Modelling of climate change predicts that the grape-growing areas of southern Australia are likely to experience higher temperatures, and reduced quality and quantity of irrigation water and rainfall resulting in increased salinity problems in the future. We utilised the Plant Accelerator as a tool for screening young rootstock rooted cuttings for their ability to withstand these stresses. As a result, we mapped and characterised genes involved in sodium exclusion. We compared the water use of rootstocks grown in the Plant Accelerator and the field. Other experiments identified candidate genes for chloride exclusion traits and more essential information about mechanisms of ion exclusion.

Summary

Predictions of the effects of climate change on southern Australia suggest that there is likely to be an increase in temperatures and extreme temperature events, a reduction in rainfall and increased evaporation leading to reduced water availability for irrigation. The quality of the available water is also likely to be reduced. As a result, salinity will be an increasing problem, impacting on many of the irrigated grape growing areas. Hence, vineyards will probably be faced with a combination of abiotic stress factors: higher temperatures, reduced water and increased salinity. To assist in the protection of the Australian winegrape industry, CSIRO, in partnership with Wine Australia, is developing new rootstocks that can tolerate these stresses to a higher degree than currently available commercial rootstocks.

This project is built on a solid foundation of salinity tolerance research in grapevines carried out at CSIRO over many years and took advantage of a unique new facility, the Australian Plant Phenomics Centre’s Plant Accelerator, to facilitate the screening of large numbers of young rooted cuttings for their ability to withstand imposed stresses of salt, high temperature and water deficit, alone or in combination.

Salinity tolerance has generally been considered the result of ion exclusion, a trait conferred by rootstocks, as previously identified in many studies. By utilising the progeny of a cross between K51-40 (that takes up chloride) and 140 Ruggeri (a chloride excluder) and amazing serendipity, we were able to map a gene responsible for sodium exclusion, which proved to be highly variable in this population. In collaboration with our colleagues at the University of Adelaide, Dr Sam Henderson and Prof. Matt Gilliham, functional characterisation of the major gene encoding a protein essential for sodium exclusion, HKT1;1 has led to a publication in New Phytologist. However, further studies in glasshouse-grown plants have also implicated other mechanisms of sodium exclusion between different tissues, so this trait does require elucidation. The identification of this gene is a major step forward in developing a suite of markers for abiotic stress tolerance to aid in the breeding of new resilient rootstocks.

The Plant Accelerator experiments also confirmed that chloride exclusion is a complex trait, a conclusion previously reached by other studies. A second approach delivered a breakthrough in identifying a genetic locus responsible for chloride exclusion. Description of this work has been redacted from this report, as it is commercial-in-confidence.

Tolerance of soil water deficit is another trait well known to be complex and multi-genic. The Plant Accelerator experiments that examined this found significant differences between genotypes in their water use and their water use efficiency (in terms of growth per unit water transpired), but they did not find any difference between genotypes in their response to water deficit. In other words, a high vigour genotype when well-watered was a high-vigour genotype when water stressed, even though growth was reduced by water deficit. This may have been a result of the cross used for this work (V. cinerea x V. vinifera) or may be a feature of Vitis spp more generally.

A unique feature of this project was our ability to examine multiple combinations of abiotic stresses that are likely to be more common in a future Australian climate. Results from a glasshouse experiment brought a greater understanding of the interactions between root temperature and ion exclusion traits; however, it raised questions about how to discover the genes that identify good exclusion at high root temperature for use in a marker assisted approach to rootstock breeding.

While we have made significant strides towards providing markers for salt tolerance to the rootstock breeding program, we have also examined the extent to which traits observed in the Plant Accelerator are conferred to grafted mature vines in the field. For example, the relative growth rate of salt treated rooted cuttings was significantly and strongly correlated with pruning weights of 10 year old grafted vines in a salt treated vineyard. Our ability to test tolerance of water deficit in the same way was hampered by difficulties in generating soil moisture differences in field experiments. These experiments did demonstrate that ungrafted rootstock vigour in the Plant Accelerator was a poor indicator of conferred vigour in grafted plants in the field, demonstrating that using the Plant Accelerator for assessment of rootstock genotypes is best employed for traits that are clearly based in the root system with a minimal impact of the shoot.

Despite this, it is clear that the unique imaging facilities and scale of the Plant Accelerator provide a valuable new tool for predicting abiotic stress tolerance of new material in the CSIRO rootstock breeding program.