Many self-pollinating crops, including bread wheat, are particularly sensitive to abiotic stresses at the reproductive stage. Research has established that impairment of pollen development is a key factor in stress-induced yield losses.
Heat is a key abiotic stress and its effect on grain yield can be as important as drought and frost. Field trials have shown that a single day of high temperature at flowering time can cause a 25 per cent yield reduction.
However, in screening Australian wheat cultivars, pollen development in some varieties was found to be more tolerant to heat than in others, implying the existence of genes that confer degrees of heat tolerance on pollen development.
The La Trobe University research used a variety of techniques to identify these genes, with the aim of providing breeders with molecular tools to select for heat-tolerant germplasm.
The search focused on a single layer of cells within the male anther called the tapetum since this tissue plays a critical role in pollen development.
In addition to providing the developing pollen grains with essential nutrients, the tapetum also provides components of the pollen outer wall (the exine) and enzymes that release microspores (the developing pollen grains) from their enveloping callose wall.
The tapetal cells die at a specific stage of pollen development and the correct timing of this programmed cell death (PCD) is crucial for the production of viable pollen.
That means tapetal PCD is tightly regulated and pollen development is affected if the PCD is premature or delayed.
The tapetum is particularly sensitive to increased temperature, as well as cold and drought. In wheat, high temperature results in tapetal breakdown and the microspores (the developing pollen grains) are unable to complete the first cell division in the reproduction cycle (meiosis).
To identify gene networks that control tapetal PCD, researchers initially worked with Arabidopsis thaliana (thale cress), which has long provided a convenient model system of choice for plant molecular research.
This is a relevant model plant given that anther structures and functions have been highly conserved between Arabidopsis and modern cereals, such as wheat, and consequently so are the many genes that regulate pollen development.
Lines were isolated in which pollen development was either heat sensitive or heat tolerant. The contrast in the pattern of gene expression in these lines led to the identification of several genes that are specifically expressed in the tapetum. One of them encodes a transcription factor (TF) protein and heat stress was found to reduce TF expression. This results in premature tapetal PCD in the anthers, reducing or preventing pollen development.
The TF protein has been found to control the expression of genes involved in pollen development, which, in turn, can influence tolerance to heat stress.
Heat tolerance in wheat
To study the effects of heat on anther and pollen development in wheat, researchers created a method to type the anther developmental stages. Fifteen distinct developmental stages were identified and described.
The timing of tapetal PCD was also determined and TF gene expression was found to peak at the onset of PCD.
Heat stress, however, disrupted several mechanisms associated with this in heat-sensitive wheat varieties, but not heat-tolerant germplasm.
These findings indicate that high levels of TF gene expression may provide a useful way to select heat-tolerant wheat lines for use in wheat-breeding programs.
Also, an analysis of all genes expressed in heat-stressed wheat anthers identified candidates that confer heat-stress tolerance during pollen development. Counterparts of these genes exist in Arabidopsis and rice where they have a role in regulating pollen development, programmed cell death, hormone signalling and stress responses.
Detailed analysis of these genes will now be undertaken in Arabidopsis. The genes that pass the Arabidopsis test will then be tested in wheat. Those found to enhance heat tolerance can then be incorporated into breeding programs through the use of marker-assisted selection.
This could lead to a step change in wheat stress tolerance by providing genetic options and tools to strongly select for heat tolerance during flowering in elite breeding material.
In addition, a sensitive and a tolerant wheat cultivar have been crossed to map the genetic basis of this specific tolerance trait and could result in additional genetic findings.
- More information: Professor Roger Parish at firstname.lastname@example.org, Dr Song Li or email@example.com
GRDC Research Code: ULA00009