Mapping genes controlling root morphology and root distribution in a doubled-haploid population of rice

 A deep thick root system has been demonstrated to have a positive effect on yield of upland rice under water stress conditions. Molecular-marker-aided selection could be helpful for the improvement of root morphological traits, which are otherwise difficult to score. We studied a doubled-haploid population of 105 lines derived from an indica×japonica cross and mapped the genes controlling root morphology and distribution (root thickness, maximum root length, total root weight, deep root weight, deep root weight per tiller, and deep root to shoot ratio). Most putative QTL activity was concentrated in fairly compact regions on chromosomes 1, 2, 3, 6, 7, 8 and 9, but was widely spread on chromosome 5 and largely absent on chromosomes 4, 10, 11 and 12. Between three and six QTLs were identified on different chromosomes for each trait. Individual QTLs accounted for between 4 and 22% of the variation in the traits. Multiple QTL models accounted for between 14 and 49%. The main QTLs were common between traits, showing that it should be possible to modify several aspects of root morphology simultaneously. There was evidence of interaction between marker locations in determining QTL expression. Interacting locations were mostly on different chromosomes and showed antagonistic effects with magnitudes large enough to mask QTL detection. The comparison of QTL locations with another population showed that one to three common QTLs per trait were recovered, among which the most significant was in one or other population. These results will allow the derivation of isogenic lines introgressed with these common segments, separately in the indica and japonica backgrounds. Received: 12 August 1996 / Accepted: 15 November 1996  

Plant growth promoting rhizobacteria as biofertilizers

Numerous species of soil bacteria which flourish in the rhizosphere of plants, but which may grow in, on, or around plant tissues, stimulate plant growth by a plethora of mechanisms. These bacteria are collectively known as PGPR (plant growth promoting rhizobacteria). The search for PGPR and investigation of their modes of action are increasing at a rapid pace as efforts are made to exploit them commercially as biofertilizers. After an initial clarification of the term biofertilizers and the nature of associations between PGPR and plants (i.e., endophytic versus rhizospheric), this review focuses on the known, the putative, and the speculative modes-of-action of PGPR. These modes of action include fixing N₂, increasing the availability of nutrients in the rhizosphere, positively influencing root growth and morphology, and promoting other beneficial plant–microbe symbioses. The combination of these modes of actions in PGPR is also addressed, as well as the challenges facing the more widespread utilization of PGPR as biofertilizers.  

作物根系形态与非生物胁迫耐性关系的研究进展

从水分、铝、磷等非生物胁迫方面综述了作物根系形态与非生物胁迫耐性之间的关系及主要研究进展，阐明了根系形态在作物逆境胁迫中的重要作用，对根系形态性状进行改良将是进一步提高作物产量潜力的重要方面之一。  

Mapping QTLs associated with drought resistance in rice: Progress, problems and prospects

The use of molecular markers in the mapping of traits of agronomic importance holds great promise for speeding the development of improved plant varieties and increasing our understanding of the physiological or molecular mechanisms behind biological phenomena. The technique is now being applied to drought resistance in rice (Oryza sativa L.). Drought is important because a considerable proportion of the world rice area is not irrigated and can be prone to water deficit. A large number of people, particular some of the poorest rice farmers, stand to gain if new varieties which combine high yield and drought resistance can be developed. Rice should be particularly useful for the molecular genetic analysis of drought resistance because of its growing role as a model monocot species and the diversity of drought resistance mechanisms which are found in the germplasm. We briefly review the traits which might be considered important in improving drought resistance in rice, before explaining the molecular mapping approach. We review progress at locating quantitative trait loci (QTLs) for individual mechanisms of drought resistance in controlled environment conditions. This includes a detailed comparison of reported QTLs related to root morphological characters. The search for QTLs associated with field performance under drought stress is analysed and the problems associated with understanding the genetic control of a complex physical and physiological phenomenon under conditions of substantial environmental variation are highlighted. We emphasise that the use of near isogenic lines in overcoming some of the problems offers considerable promise for the future.(seconded from CIRAD-CA, France)  

玉米根系形态性状和空间分布对水分利用效率的调控

玉米根系形态性状(总根长、根系表面积和根系干物质重)与植物整体水分利用效率间具有显著或极显著的相关性,回归曲线趋势基本相同,均呈二次曲线关系,只是相关系数不同。说明从提高水分利用效率来说,根系需要维持适宜的大小。其中根长对水分利用效率的贡献是第一位的,而根系干物质重的贡献最小,根系表面积介于二者之间。从空间分布来说,玉米每层节根数、节根长度和直径在父母本和杂交种间也具有显著或极显著的差异。与中下层根量相比,母本与不抗旱的父本处于上层干土中的根系数量明显较多,且根系直径大,吸水困难。而杂交种在干旱条件下上层根重和数量维持不变,或略高于不抗旱品种,但中层和下层根系数量和长度明显高于不抗旱品种,且根系直径小于不抗旱品种,这样从多的有效根系数量和低的吸水阻力两方面保证了水分的吸收,从而使其产量和水分利用效率均最高,说明通过根系形态特性和空间分布的优化能够调节作物整体的水分利用效率。  

小麦幼苗根系形态与反复干旱存活率的关系

以35个不同栽培类型的小麦品种（系）作为试验材料，根据其6叶幼苗的根系形态性状进行聚类分析，供试材料的根系类型分为3种：大根系、小根系和中间型根系。具有中间型根系的材料反复干旱存活率最高，这些材料的根系特点是单株根数7－8．5条，最大根长20-22cm，根总干重44－48mg,其中10cm以下根干重占36％－45％，根冠比范围在0.22-0.24。一些水地栽培的育成品种苗期抗旱性较强，旱地栽培的育成品种苗期抗旱性差异较大，个别旱地栽培的地方品种在土壤水分胁迫条件下反而比在正常水分条件下的根系发育更好，可能是长期适用干旱条件的结果。  

The Roles of Auxins and Cytokinins in Mycorrhizal Symbioses

Abstract Most land plant species that have been examined exist naturally with a higher fungus living in and around their roots in a symbiotic partnership called a mycorrhiza. Several types of mycorrhizal symbiosis exist, defined by the host/partner combination and the morphology of the symbiotic structures. The arbuscular mycorrhiza (AM) is ancient and may have co-evolved with land plants. Emerging results from gene expression studies have suggested that subsets of AM genes were co-opted during the evolution of other biotrophic symbioses. Here we compare the roles of phytohormones in AM symbiosis and ectomycorrhizas (EC), a more recent symbiosis. To date, there is little evidence of physiologic overlap between the two symbioses with respect to phytohormone involvement. Research on AM has shown that cytokinin (CK) accumulation is specifically enhanced by symbiosis throughout the plant. We propose a pathway of events linking enhanced CK to development of the AM. Additional and proposed involvement of other phytohormones are also described. The role of auxin in EC symbiosis and recent research advances on the topic are reviewed. We have reflected the literature bias in reporting individual growth regulator effects. However, we consider that gradients and ratios of these molecules are more likely to be the causal agents of morphologic changes resulting from fungal associations. We expect that once the individual roles of these compounds are explained, the subtleties of their function will be more clearly addressed.  

Growth,photosynthesis and storage of carbohydrates and nitrogen in Phaseolus lunatus in relation to resource availability

External hyphae of vesicular-arbuscular mycorrhizal (VAM) fungi were quantified over a growing season in a reconstructed tallgrass prairie and an ungrazed cool-season pasture. In both sites, hyphal lengths increased throughout the growing season. Peak external hyphal lengths were 111 m cm^–3 of soil in the prairie and 81 m cm^–3 of soil in the pasture. These hyphal lengths calculate to external hyphal dry weights of 457 g cm^–3 and 339 g cm^–3 of soil for prairie and pasture communities, respectively. The relationships among external hyphal length, root characteristics, soil P and soil moisture were also determined. Measures of gross root morphology [e.g., specific root length (SRL) and root mass] have a strong association with external hyphal length. Over the course of the study, both grassland communities experienced a major drought event in late spring. During this period a reduction in SRL occurred in both the pasture and prairie without a measured reduction in external hyphal length. Recovery for both the pasture and prairie occurred not by increasing SRL, but rather by increasing external hyphal length. This study suggests that growth is coordinated between VAM hyphae and root morphology, which in turn, are constrained by plant community composition and soil nutrient and moisture conditions.