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Agronomy Day 2009

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Tour B

Do Small Genes Make Big Hybrids?

Steve Moose
Steve Moose
Associate Professor
Department of Crop Sciences
(217) 244-6308
smoose@illinois.edu
Wes Barber
Wes Barber
Graduate Research Fellow
Department of Crop Sciences
(217) 244-6146
barber4@illinois.edu
Matt Hudson
Matt Hudson
Assistant Professor
Department of Crop Sciences
(217) 244-8096

An annual activity for Illinois corn farmers is the purchase and planting of hybrid seed.  The commercial development of hybrid corn by 20th century plant breeders has sprouted into a multi-billion dollar industry.  The benefits of corn hybrids are obvious: larger plants, higher grain yields, and stronger stress tolerance (Fig. 1).  However, these advantages require the controlled crossing of defined parent inbred lines, meaning farmers must purchase expensive new hybrid seed each season.  Have you ever wondered why corn hybrids show superior performance to their parents?

Scientists would also like to know the answer to this question, not only to understand how hybrid vigor works, but also to better predict which parents will make the best hybrids and to further increase corn yields.  Although we still do not yet fully understand the genetic basis for hybrid vigor, or heterosis, research has established four general rules.  First, hybrids grow faster than their inbred parents at all developmental stages (see Figure 1).  Second, heterosis is greatest when the parents are genetically unrelated to each other.  Third, heterosis is due to the influence of many genes.  Finally, hybrid vigor depends upon combinations of genes contributed by both parents, leading to interactions where gene effects can be much greater or less than that observed in the parental inbred lines. 

One of the most exciting biological discoveries during the past decade was the importance of small RNAs, recognized by the awarding of the Nobel Prize for Medicine in 2006.  Small RNAs are produced in the cells of all higher organisms, including corn and people, to turn off other genes.  The small RNAs can be divided into two major functional groups.  The first group, termed microRNAs (“micro” means small), regulate the rate of development and response to environmental stress.  The second group of small RNAs is named “small-interfering RNAs”, because their main job is to interfere with the activity of viruses.  The small interfering RNAs provide immunity to new viral infections or shut down viruses that have previously invaded the DNA genome, which can cause mutations or disease if left unchecked.

This new class of small RNA genes may be key to unlocking the mystery of hybrid vigor.  Because they are small, the small RNA genes are likely to have been missed in previous studies that have looked for genes influencing heterosis.  With support from the University of Illinois Critical Research Initiative and the National Science Foundation, we are using new laboratory technologies to rapidly characterize the small RNAs produced from many of the “founding fathers” for today’s commercial corn hybrids.  Combining this information with the sequence of the corn genome, produced with the support of the National Corn Grower’s Association, is providing valuable insights into hybrid vigor.  We are learning that those microRNAs controlling the speed of development and flowering time in corn differ greatly in hybrids compared their inbred parents.  We also observe a greater amount of small interfering RNA production in hybrids, which could explain their greater stress tolerance.  In addition, many different types of small RNAs are expressed in hybrids that are not detected in either inbred parent, which fits very well with the explanations for hybrid vigor proposed from earlier experiments.

We hope that our small RNA analysis will enable predictions of which parents will produce the highest heterosis and grain yield.  If those small RNAs most important to hybrid vigor can be identified, it may be possible to amplify their effects for even greater yields.  In the future, our research could help provide the benefits of hybrid vigor at lower cost, by reducing the amount of effort spent on finding and crossing the best parental lines, which is the most expensive aspect of producing hybrid corn seed.

Disease organisms can be founf in different plant tissues
Corn hybrids grow faster and larger from the seedling stage (top),
throughout vegetative growth (middle),
and produce higher grain yields (bottom).
 

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