Mapping of quantitative trait loci for seminal root morphology and gravitropic response in rice

In order to gain a better understanding of the complex root traits observed in previous studies using a mapping population derived from a Bala × Azucena cross, an experiment was conducted growing plants in agar-filled Perspex chambers with the aim of identifying quantitative trait loci (QTLs) for both seminal root morphology (SRM) and gravitropic response. A total of four main effect QTLs were detected for SRM (a measurement of the degree of a wavy/curly seminal root phenotype); two were located on chromosome 2, one at the top of chromosome 3 and one on chromosome 11. Two main effect QTLs were detected for the gravitropic response (the degree of bending of the growing seminal root when subjected to a 90° rotation); one on chromosome 6 and 1 on chromosome 11. As well as main effect QTLs, an epistatic interaction was observed for each of the traits. For SRM an interaction was detected between the top and the bottom of chromosome 4. For the gravitropic response an interaction was observed between a location on chromosome 6 and 11. Both these interactions were confirmed by analysis of variance using marker classes and the epistatic gravitropic response was also confirmed using a pair of near isogenic lines. All the SRM QTLs detected in this experiment co-localise with root growth QTLs (root penetration or morphology) detected previously in the mapping population. This information could prove valuable in attempts to identify candidate genes for these potentially valuable QTLs because we could postulate that the underlying genes should be involved in the pathway of gravity detection, signal transduction or the growth response to gravity.     

油菜素内酯诱导小白菜初生根不对称生长

油菜素内酯（Brassinolide,BL）能诱导单子叶植物初生根发生不对称生长,而在双子叶植物中还未见报道。本试验通过外源施加不同浓度2,4-表油菜素内酯（2,4-Epibrassinolide,2,4-eBL）观察小白菜（Brassica chinensis L.）初生根生长状况。结果表明：2,4-eBL能诱导小白菜初生根发生不对称生长形成卷曲。随着2,4-eBL浓度的升高,根卷曲比例和曲率逐渐增大,同时向地性逐渐降低。试验发现外源施加2,4-eBL可诱导双子叶植物根不对称生长的现象,为研究油菜素甾醇（Brassinosteroids,BRs）在根生长发育中的作用提供新途径。     

Negative gravitropism and growth stress in GA3-treated branches ofPrunus spachiana Kitamura f.spachiana cv.Plenarosea

One of the roles of growth stress in branch shape formation was investigated using a weeping-type Japanese cherry,Prunus spachiana. Negative released strains, caused by longitudinal tensile growth stresses, were detected in the upper side of gibberellin A₃-treated (GA₃-treated) and control branches. The mean value of the released strain in the upper side of the treated branches was –0.104%, which was larger than the value (–0.067%) observed in the control branches. Both branches formed tension wood in the upper side of the xylem, and the treated branches formed tension wood near the pith as well. This suggested that the treated branches generated larger tensile growth stress from the early growth stages. The successive generation of growth stress from the early growth stages was considered to generate forces large enough to bend the branch upward.     

Arabidopsis Phytochrome D Is Involved in Red Light-Induced Negative Gravitropism of Hypocotyles

The phytochrome gene family, which is in Arabidopsis thaliana, consists of phytochromes A-E（phyA to phyE）, regulates plant responses to ambient light environments. PhyA and phyB have been characterized in detail, but studies on phyC to phyE have reported discrepant functions. In this study, we show that phyD regulates the Arabidopsis gravitropic response by inhibiting negative gravitropism of hypocotyls under red light condition. PhyD had only a limited effect on the gravitropic response of roots in red light condition. PhyD also enhanced phyB-regulated gravitropic responses in hypocotyls. Moreover, the regulation of hypocotyl gravitropic responses by phyD was dependent upon the red light fluence rate.     

Genetic improvement for root growth angle to enhance crop production

The root system is an essential organ for taking up water and nutrients and anchoring shoots to the ground. On the other hand, the root system has rarely been regarded as breeding target, possibly because it is more laborious and time-consuming to evaluate roots (which require excavation) in a large number of plants than aboveground tissues. The root growth angle (RGA), which determines the direction of root elongation in the soil, affects the area in which roots capture water and nutrients. In this review, we describe the significance of RGA as a potential trait to improve crop production, and the physiological and molecular mechanisms that regulate RGA. We discuss the prospects for breeding to improve RGA based on current knowledge of quantitative trait loci for RGA in rice.