Development of lateral roots (LRs) is promoted when parental root elongation is suppressed. Promotion of LR growth compensates for parental root growth, which contributes to the maintenance of total root length. However, the developmental processes underlying the compensatory growth of LRs are still unclear. In this study, we induced LR compensatory growth in rice by surgical excision of a parent (seminal) root tip, and analyzed the morphological and anatomical changes in LRs. Our analysis revealed that seminal root-tip excision increased the diameter of first-order LRs by increasing in the number of ground-tissue layers and the stele diameter and also promoted elongation, with higher order LRs emerging in the remaining proximal portions. We also found that LR primordia in early developmental stages increased in diameter markedly and then produced higher order LRs pronouncedly in response to seminal-root cutting. Furthermore, the degree of change in LR morphology differed depending on the distance from the cut site, with changes in morphologies weakening further from the cut site. Taken together, the morphological and anatomical changes of LRs that are induced by the root-tip excision of the parent vary continuously, and are dependent on both the developmental stage of LR primordia at the site of root-cutting and the distance from the cut site.
Pyridinedicarboxylic acid (PDCA) analogs, including 2,3-, 2,4-, 2,5-, 2,6-, 3,4- and 3,5-PDCA, accelerate flower opening and retard senescence of spray-type carnation flowers. In addition the present study revealed that 2,3-PDCA promoted root elongation in lettuce, carrot and rice seedlings, whereas 2,4-PDCA inhibited it. Then, the action of PDCA and pyridinecarboxylic acid (PCA) analogs on root elongation was further examined using rice seedlings. 2,3-, 3,4- and 3,5-PDCA promoted rice root elongation, whereas 2,4- and 2,6-PDCA inhibited, and 2,5-PDCA had little effect. 3-PCA (nicotinic acid) promoted rice root elongation, but 2- and 4-PCA did not. Moreover, 3-PCA amide (nicotinamide) did not promote root elongation. These findings indicated that a carboxyl group substituted on position 3 of the pyridine ring is necessary to promote root elongation, and that the promoting effect of 3-PCA was not from its action as vitamin B3, but from its intrinsic activity as a 3-COOH substituted pyridine. On the other hand, all the PCA and PDCA analogs tested in this study, except 2,6-PDCA and 4-PCA, promoted shoot elongation of rice seedlings.
T-DNA of Ri-plasmid from Agrobacterium rhizogenes is able to trigger the hairy root syndrome in infected plants. This natural phenomenon is used to generate hairy root cultures predominantly only in dicotyledonous plants. We propose a new method of hairy roots induction without Agrobacterium-mediated transformation. The 5461 bp T-DNA region from A. rhizogenes A4 strain with all four rol genes was amplified using primers containing sequences for left and right T-DNA borders on their 3’-ends. This amplicon was used for direct transformation of tobacco leaf discs without A. rhizogenes and binary vectors. We showed the possibility of generation of hairy roots on tobacco leaf discs by biolistic transformation utilizing only rol genes amplicon.
Rhizoboxes are soil-root compartments that may well provide the closest naturalistic conditions for studying root systems architectures (RSAs) in controlled environments. Rhizobox-based studies can however lead to mis-estimation of root traits due to poor recovery of roots and loss of fine root features during washing of roots. We used a novel scanner-based rhizobox system to evaluate: (i) RSA traits of Brassica rapa genotypes; (ii) the relationship between root traits recorded from rhizoboxes and those of harvested roots and (iii) genotypic variation of seedlings in response to external P ([P]ext) availability. Brassica rapa genotypes were grown in soil-filled rhizoboxes abutting flatbed scanners and were watered once with either deionised water or a solution of 600 ΜM KH2PO4 to approximately 80% field capacity on a weight basis. Shoot and root P concentrations ([P]shoot and [P]root) of the B. rapa lines grown on different [P]ext were quantified. Visible root length at the surface of rhizoboxes constituted 85% of the total root length recovered from harvested root samples. High P supply induced a strong increase in [P]shoot in all genotypes (P < 0.001) whereas low P supply generally led to greater partitioning to roots. Seed P concentration and tissue P concentration were correlated only at low [P]ext. Total root length was strongly correlated with tissue P content under both low [P]ext (r = 0.81, P < 0.05) and high [P]ext (r = 0.82, P < 0.05) conditions. The novel scanner-based rhizobox system used addresses the substantial limitations associated with current use of rhizoboxes to study root growth dynamics.
Polar auxin transport was inhibited in rice seedlings when they were treated with N-1-naphthylphthalamic acid (NPA). The treatment reduced total root length and the number of lateral roots, and negatively affected gravitropism. The auxin content at the base of the seminal root significantly increased in the NPA treatment seedlings compared to the control. Lateral roots elongated along the seminal root axis after NPA treatment, but growth remained within the cortex. Lignin content in the basal region also increased at the same time and accumulated in the epidermis. These results suggest that NPA treatment prevents lateral roots from penetrating the hypodermis due to the hardening of hypodermis cell walls through the enhanced lignification, and the disturbed gravitropism caused by NPA treatment affected auxin flow.
Graminaceous plants biosynthesize siderophores and secrete it into the rhizosphere, solubilize insoluble Fe(III) by chelation, and take up the Fe(III)-siderophore complex. Active uptake of Fe(III) in microorganisms such as Pseudomonas fluorescens is also based on siderophores. However, vegetative plants including the tomato, cannot synthesize siderophores and take up Fe(III) directly. The growth of tomatoes under Fe(II)-deficient conditions supplemented with the Fe(III)-siderophore complex is enhanced in comparison with the growth of the control. However, the mechanism of iron absorption has not been clarified yet.Two-week-old seedlings were transferred to a liquid medium inoculated with P. fluorescens. After incubating for four days, the iron concentration of plants increased compared with that of non-symbiotic seedlings. Moreover, the expression of the Fe(II) transporter IRT1 and Nramp3 was analyzed by qRT-PCR. The expression of IRT1 was not induced until day 7 but that of Nramp3 was induced in four days. In addition, Fe(III) chelate reductase (FRO) activity was high in symbiotic seedlings after four days. These results suggest that the chelated Fe(III) was reduced to Fe(II) by FRO and taken up by Nramp3. Siderophore-mediated Fe(III) uptake by tomato is believed to be a useful strategy for increasing iron uptake from the environment.
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