QTL mapping of seedling tolerance to exposure to low temperature in the maize IBM RIL population

Abstract

Maize is a cold sensitive crop that exhibits severe retardation of growth and development when exposed to cold spells during and right after germination, including the slowdown in development of new leaves and in formation of the photosynthetic apparatus. Improving cold tolerance in maize would allow early sowing to improve crop yield by prolonging a growing season and by decreasing the negative effects of summer drought, diseases, and pests. Two maize inbreds widely incorporated into American maize germplasm, B73 and Mo17, exhibit different levels of tolerance to low temperature exposure at seedling stage. In addition, thirty seven diverse inbred maize lines showed large variation for seedling response to low temperature exposure with lines with extremely low tolerance to seedling exposure to low temperatures falling into stiff stalk, non-stiff stalk, and tropical clades. We employed the maize intermated B73×Mo17 (IBM) recombinant inbred line population (IBM Syn4 RIL) to investigate the genetic architecture of cold stress tolerance at a young seedling stage and to identify quantitative trait loci (QTLs) controlling this variation. A panel of 97 recombinant inbred lines of IBM Syn4 were used to measure, and score based on several traits related to chlorophyll concentration, leaf color, and tissue damage. Our analysis resulted in detection of two QTLs with high additive impact, one on chromosome 1 (bin 1.02) and second on chromosome 5 (bin 5.05). Further investigation of the QTL regions using gene expression data provided a list of the candidate genes likely contributing to the variation in cold stress response. Among the genes located within QTL regions identified in this study and differentially expressed in response to low temperature exposure are the genes with putative functions related to auxin and gibberellin response, as well as general abiotic stress response, and genes coding for proteins with broad regulatory functions.

Fig 1. Effect of low temperature exposure of maize seedlings on leaf phenotype.

A. The scale for visual scoring of tolerance to low temperature exposure was as follows: 1 –severe leaf damage, large proportion of dry / dead tissue, severe wilting; 2 –intermediate level of leaf damage, some discolored / dry tissue, primarily on the edges and tips of the leaf; 3 –minimal / no leaf damage, no discoloration, minimal wilting. B. Diverse maize lines vary in their response to low temperature exposure.

Fig 2. Frequency distributions of leaf color—Related traits following low temperature exposure in the IBM Syn4 population of RIL lines.

A. Proportion of green leaf tissue following low temperature exposure; B. Visual cold tolerance score following low temperature exposure; C. Absorbance of methanol leaf extracts measured at 663 nm; D. Absorbance of methanol leaf extracts measured at 645 nm. Arrows show the relative position of B73 and Mo17 parents.

Fig 3. Profiles for QTLs for response to low temperature in maize seedlings in IBM Syn 4 population.

LOD score of 3.0 (green line) was used as a cutoff to determine the statistical significance of discovered QTLs. The location of QTLs across the genome (A) or on chromosomes 5 (B) and 1 (C) are shown for various approaches used to measure plant response to cold temperature. The QTL profile determined using visual tolerance score is shown in black, using the proportion of green leaf tissue is shown in blue, while the QTL profile determined by absorbance at 645 nm is shown in red. The QTL profile determined using absorbance at 663 nm closely resembles the profile for visual tolerance score and is not shown.