Effect of Reduced Height Genes (Rht) on Components of the Root System in Bread Wheat

It is accepted that there is an interaction between dwarfing genes and the overall size of a plant because of the gene’s interaction with gibberellic acid pathways. The main objective of this project was to determine the effect of four stem dwarfing genes (Rht1, Rht2, Rht3 and rht) on components of the root system in near-isogenic lines of Maringa and Nainari 60. The working hypothesis was that since the Rht genes affect the plant hormone gibberellic acid (GA3) metabolic pathway (Spielmeyer et al. 2004), these genes will also dwarf or affect root characters, as well as shoot characters, and this may in turn affect water and nutrient uptake and grain yield in these wheat lines. Introduction: Agricultural geneticists and crop physiologists rarely study plant roots because of the difficulty of extracting them from field or pot soil containing organic matter. This project aims to study the effect of the reduced height genes (Rht) that were used to dwarf wheat stems in the green revolution, on root size and other root characters in bread wheat. There are few reports of such studies in the literature, and few using isogenic lines of wheat (Triticum aestivum L.) relevant to California growing conditions. 'Nainari 60' bread wheat is a tall (rht), spring cultivated variety (cultivar) released by the International Maize and Wheat Improvement Center, (CIMMYT) at Ciudad Obregon, Sonora, Mexico, to be grown in northwest Mexico and southwest USA. ‘Maringa’ bread wheat is an old, tall (rht) spring cultivar from southern Brazil which has similar growing conditions to Southern California. ‘Nainari 60’ was one of the earliest wheats released in 1960 by Dr. Norman Borlaug during the green revolution to be grown in irrigated conditions (Borlaug 1968). Both cultivars lodged (fell down) when too much irrigation water and fertilizer was applied to the fields, and this reduced grain yield. To overcome lodging, Borlaug introduced the dominant reduced height genes Rht1, Rht2 and Rht3 that dwarf the stems, into tall, recessive Nainari rht/rht, which allowed the plants to be grown with high water and fertilizer input. This prevented stems from lodging and increased grain yield considerably. Later, the same was done to dwarf the stems of Maringa wheat. Thus there are two near-isogenic series in Maringa and Nainari 60 where lines differ only by the dominant genes Rht1, Rht2, Rht3 or the recessive allele rht. Near-isogenic lines are lines of wheat that are nearly genetically identical except for the gene which carries the trait being studied. The effect of these genes on shoot characters and root dry matter has been studied (Ehdaie and Waines, 1994, 1996). But, no one has studied the effect of these dwarfing genes on components of the root system, which include: length of longest root, number of roots greater than 30 cm, total root length greater than 30 cm, shallow root biomass (less than 30 cm), deep root biomass (greater than 30 cm), and total root biomass. Expected Results: We expected the tall rht/rht lines to have a greater and longer root system than the dwarf Rht3/Rht3 lines, and the semi-dwarf genotypes to be intermediate. Previous studies (Ehdaie and Waines, 1996) demonstrated that mature Maringa rht/rht had taller stems than mature Nainari 60 rht/rht by 29cm. We expected the 45 day old Maringa isogenic lines to have a larger root system than the Nainari 60 isogenic series, if root characters reflect shoot characters at this early stage of plant growth. The length of the longest root may be similar in each line, but the total length of all seminal roots longer than 30 cm may be significantly different among lines. Genotypes with a larger root system may take up more water and fertilizer than those with small root systems. Results: The effect of the high temperatures in spring and summer on the plants may have had a role in root production because it was offseason for normal planting of these wheat cultivars. However, since the experiment was repeated in both spring and summer producing consistent data, the offseason heat had little or no effect on the expression of reduced height genes. A summary of the results is reported in Table 1. Table 1. Maringa and Nainari 60 near-isogenic lines for plant height evaluated for root traits in sand-tube experiments. Root Trait Spring 2006 Planting Summer 2006 Planting Maringa rht, Rht1, Rht2, Rht3 Nainari 60 Rht Rht2, Rht3 Maringa rht, Rht1 Rht2, Rht3 Nainari 60 rht Rht2,Rht3 Number of roots > 30 cm Similar (range: 12-15) Similar (range: 8-9) Similar (range: 12-14) Similar (range: 8-11) Longest root (cm) Similar (range: 87-96) Similar (range: 78-85) Similar (range: 83-92) Similar (range: 79-82) Total root length > 30 cm Similar (range: 665-880) Similar (range: 410-445) Similar (range: 595-723) Similar (range: 450-518) Shallow root biomass (g/plant) Mar 1 = Mar 2 > mar > Mar 3 (1.01, 1.03, 0.73, 0.48 respectively) Nai 2 > Nai 3 > nai (0.64, 0.37, 0.28 respectively) Mar 1 = mar = Mar 2 > Mar 3 (0.57, 0.48, 0.47, 0.40 respectively) Similar (range: 0.23-0.30)