Proceedings of the XLVI Italian Society of Agricultural Genetics - SIGA Annual Congress

Giardini Naxos, Italy - 18/21 September, 2002

ISBN 88-900622-3-1

Poster Abstract - 4.17

COMPARATIVE MAPPING OF QTLs FOR ROOT TRAITS IN MAIZE AND RICE

SALVI S., GIULIANI S., MACCAFERRI M.

Department of Agroenviromental Science and Technology, University of Bologna, Via Filippo Re 6-8, 40126 Bologna, Italy

maize, rice, root traits, QTLs, synteny

In cereals, little progress has been achieved using root traits as a selection criteria to improve yield; this is due to the difficulty in investigating roots and the limited information available on their genetic control. A better knowledge of the genetic determinants of root traits and how they influence yield would allow for more targeted approaches (e.g. marker-assisted selection) within breeding programs and, eventually, for the cloning of the gene/s underlying such QTLs. Because of the large investment of resources necessary for cloning a QTL, it is advisable to focus on major QTLs affecting the trait of interest in a number of genetic backgrounds within the same species and, when comparative mapping is possible, across species. Among cereals, rice is the model species, due to the relatively small genome size and the availability of its entire sequence. The extensive synteny between rice and the other cereals provides a powerful means to exploit the genetic information available in rice. Relying on these syntenic relationships, our long-term goal is to identify major QTLs controlling root traits in maize and rice and clone the genes underlying such QTLs using rice as a model. As a preliminary step, we have analysed the QTL data available in the literature for root traits in maize and rice to identify a number of syntenic regions which could be targeted with a cloning approach. Herein, we present a summary of this comparative analysis and summarize the possible approaches for the cloning of such QTLs in rice. In maize, four mapping populations have so far been investigated for QTLs for root traits under controlled conditions and/or in the field (Tuberosa et al. 2002, Annals of Botany 89: 941-963). Several chromosome regions affected root traits in two or even three populations. A number of these regions also affected grain yield under well-watered and/or drought-stressed conditions. The most important QTL effects were detected on chromosome bins 1.03, 1.06, 1.08, 2.03, 2.04, 7.02, 8.06 and 10.04. Exploiting the syntenic information available for maize and rice, we compared the maize QTL results with those available from five studies describing QTLs in rice. Synteny information was based on data retrieved from http://ars-genome.cornell.edu/rice. Twenty morphological root traits (e.g. total root weight, maximum root length, root/shoot ratio, etc.) were considered among the five rice mapping populations, for a total of 166 QTLs. Sixty-three (out of 100) maize bins were identified as syntenic to one or more regions with QTLs for root traits in rice. The five maize bins most frequently identified were: bin 2.04 (syntenic to regions carrying QTLs for 16 out of 20 traits, in all five populations), bin 5.03 (14 out of 20 traits; four out of five populations), bin 8.05 (12 out of 20 traits; four out of five rice populations), bin 5.04 (11 out of 20 traits; four out of five populations) and bin 4.05 (10 out of 20 traits; three out of five populations). It should be noted that bin 2.04 is important for controlling variation in root traits in maize. Six rice QTLs for root traits map in regions syntenic to maize bin 1.06, another region influencing variation in root traits in maize. Based on these findings, it could be envisaged that a set of orthologous genes may also control, at least in some cases, quantitative traits (and therefore root traits) in rice and maize. For two of these regions in maize, we have started to develop NILs (see Salvi et al.). Presently, positional cloning appears to be the main strategy toward QTL cloning in cereal species. After the QTL is confined to a region <1-2 cM in the source cereal species, the availability of the rice genome sequence and established information on synteny relationships among cereals allow us to use the molecular markers closest to the QTL to cross-reference the genetic map to the rice genome sequence. The rice genome will then serve as source of new markers to increase the mapping resolution and will provide candidate genes at the target QTL, eliminating or strongly reducing the need of establishing contigued BAC/YAC libraries. Along with positional cloning, insertional mutant collections and TILLING mutants provide further opportunities to identify allelic series of known genes.