Breeding for drought tolerance

What is a drought?

Seems like a pretty simple question doesn’t it? Especially after two pretty dry years. Clearly a drought is when it doesn’t rain enough – right? But I don’t think it is that clear cut – most farmers in Australia would be pretty happy if they could achieve a 5t wheat crop, but if you belonged to the 10t club in Europe, and you only got enough rain for a 5t crop, you would claim you had suffered a pretty bad drought.

So the definition of drought is going to change dependent on your specific environment. Not only that, but the reasons your crop suffers from drought symptoms may be due to a number of different factors.

What leads to a drought affected wheat crop?

Obviously if you don’t get enough rain, a crop will be drought affected. But there are other ways a crop may appear ‘drought’ affected. Firstly, the crop may not be able to access the water that is in the soil. This might be due to direct soil constraints such as: poor soil structure, low organic matter, or impenetrable soil layers (rock, clay, salt and boron etc). Alternatively, root diseases may have damaged the root mass, reducing its water harvesting capacity and efficiency. These may all lead to less water getting into the plant than should theoretically be possible. Unfortunately for farmers in southern Australia, these problems are all too often experienced by crops that are also facing general water shortage.

Even if the crop is able to extract all of the available water from the soil (before it is evaporated from the soil’s surface), some crops may be less efficient at converting that water into carbohydrates. Genetic improvement of water use efficiency has been a key target of the breeding strategies that produced the varieties Drysdale and Rees but these varieties have only shown benefits in some specific situations.

Other factors that stress the plant can lead to less water being processed into carbohydrates in the grain. Right when the crop is at its most vulnerable (around flowering), high spring temperatures start to kick in. Not only does this have the potential to damage the plant itself, it also leads to higher evaporation rates. If water is already a limiting factor, farmers will be well aware of the rapid wilting and desiccation that will take place in a crop that is then exposed to a 35oC+ day. An analysis of a set of AGT trials grown in southern Australia from 2002-2005 showed that the number of extreme heat events at a site had a large effect on grain yield (Table 1). This may have been a direct heat stress effect or perhaps the increased heat led to higher evaporation rates and therefore water stress. But it is interesting to note that contrary to what one might expect, the quantity of in-season rainfall was found to not be as important as the number of days exceeding 35oC .


Table 1. The effect of some environmental characters on grain yield in a set of southern Australian experimental trials (2002-2005).

So how do we breed something tolerant to drought?

As we can see, to breed a wheat variety that yields well in environments that suffer from drought, a lot of different factors need to be considered. A holistic approach is needed. Lots of traits need to be manipulated to cover the different types of droughts and related stresses. We are far from convinced that a one gene panacea will solve all the issues required for true drought tolerance in southern Australia.

By combining different parents of known good performance under drought, and then yield testing in the target environment we have been able to select varieties such as Gladius that perform exceptionally well under most drought conditions encountered by growers. The key to this approach has been a thorough understanding of the parents used for crossing and an effective selection system. But is there another way?

Can genomics help us?

Although a ‘plant it and see how it grows’ approach has worked well, is it possible that new advances in genomic analysis will help us to improve the adaptation of our varieties to the southern Australian drought? The challenge for us is to understand the various mechanisms that contribute to the robust nature of varieties such as Gladius. Once we have a better understanding of the range and value of each factor we can impose more direct selection pressure on these factors to enhance the rates of genetic gain for drought tolerance.

One way of examining the genes that may be involved in adaptation to drought tolerance is by genetic mapping. By crossing two parents that differ in their yielding ability under a southern Australian drought and then examining the performance of their progeny we can start to tease apart what specific genes should be targeted in a breeding strategy. This approach has been quite successful when trying to work out the genes for simple traits but we are still in the early days of applying this sort of analysis to highly complex traits like drought tolerance. A couple of early results are discussed below:

Does boron tolerance lead to higher yield?

It is well known that many varieties that have proven to be successful in southern Australia are tolerant to boron toxicity. The list of tolerant varieties includes varieties such as Halberd, Frame, Yitpi, Pugsley, Trident and Krichauff, and more recently; Correll, Derrimut, Peake and Gladius. However it is also interesting to look at the list of varieties that have performed well in southern Australia but are not boron tolerant. For example: Machete, Excalibur, Wyalkatchem, Scythe and Axe. So how important is boron tolerance? To answer this, we took a population of ~200 individuals from a cross between Trident and Molineux and rated each of them for their tolerance to boron using the standard method (by comparing the relative root length of seedlings grown in filter paper soaked in high and low boron solutions). These lines were then grown from 2002-2006 across a total of 22 environments and the grain yield recorded. What did we see? Those lines with boron tolerance only had higher grain yield at one of the 22 sites! At Pinnaroo in 2006, the lines with boron tolerance yielded 3% higher than those without tolerance. So what does this mean? Well it is far from conclusive, but it probably questions how important boron tolerance is as a means of improving the ability of wheat varieties to access more soil water. It certainly suggests that there must be a number of other factors that are more important to achieving higher WUE and therefore drought tolerance…..


It appears that not all genes controlling flowering time were created equal

We are all well aware of the advantage that early flowering varieties have when subjected to terminal drought stress. This was particularly evident in 2006 when one looked at the yields of early varieties such as Westonia, Young, H46 and Axe. Often however, there is a trade-off for growing early flowering varieties to achieve drought avoidance. There is the possibility of missing out on late spring rains in a ‘normal’ year and it may subject the crop to a greater likelihood of suffering frost damage . But perhaps there is a way we can get the best of both worlds?

In a breeding programme, varieties are selected that flower within an acceptable flowering window. But varieties get there by different routes. It may be through sensitivity to photoperiod, a vernalisation (cold) requirement or inherent earliness or a combination thereof. Recent analysis of progeny from two wheat populations (Kukri x Excalibur and Trident x Molineux) has shown that the grain yield effects of these genes are drastically different. We found the same set of three genes had the greatest effects on heading date in both populations (Edwards personal communication 2007; Kuchel et al., 2007). Two of these appear to respond to photoperiod, while one is sensitive to vernalisation. Although the magnitude of their flowering time effects is comparable, their effect on grain yield is not. One of the photoperiod sensitive genes (located on chromosome 7A), has a large and consistent effect on grain yield, whereas the grain yield effect of the other two genes is marginal. So what implications does this have when trying to breed a drought tolerant variety? Well firstly, simply selecting for a variety that flowers at the ‘right time’ may not be the answer. If one just assesses the plants on a phenotypic basis (what the plant looks like), wheat lines may be selected that flower at the ‘right time’, but do not improve grain yield. It may be better to use marker-assisted selection (DNA analysis) to select lines with the best genes. Secondly, it may be possible to pyramid various genes involved in the control of flowering time that result in a later maturing variety (that can therefore make the most of late spring rains), but that is also higher yielding when subject to terminal drought stress (capturing the benefits normally offered by earliness). Although still being completed, hopefully this sort of genomic analysis will help us to better design our breeding objectives and the strategies we use to get there.

A few questions we would like genomics to help answer
• How important is leaf waxiness to grain yield?
• What is the grain yield benefit of resistance to root diseases such as Pratylenchus neglectus?
• Do we have useful genes for improving rooting depth or architecture?
• What are the ‘real world’ effects of traits such as early vigour and leaf architecture in southern Australia?
• Are we selecting any genes that improve photosynthetic efficiency per se?

Conclusion

Given the myriad of factors that reduce the grain yield of wheat in southern Australia, particularly when water is limiting, it is unlikely that we will see large improvements in ‘drought tolerance’ simply by targeting a single gene (either by conventional breeding, marker-assisted selection or genetic modification). However, new technologies such as genomic analysis should help us to better understand the genetic basis to drought stress adaptation. When combined with an effective selection system this will hopefully lead to greater rates of genetic gain for drought tolerance.


Contact: Haydn Kuchel
Ph: 08 8303 7708
Email: haydn.kuchel@ausgraintech.com