Early modifications of Brassica napus root system architecture induced by a plant growth‐promoting Phyllobacterium strain

Abstract

Summary• Plant growth-promoting bacteria (PGPB) have been reported to stimulate root morphogenesis. To improve our knowledge of the PGPB effect, the early modifications of Brassica napus root system architecture induced by the PGPB Phyllobacterium sp. (29-15) were analysed.• Plants were grown in Petri dishes on a vertical medium supplemented with variable doses of Phyllobacterium sp. in gnotobiotic conditions. Root system elementary variables were measured in a nondestructive manner and the distribution of the bacteria throughout the primary root was quantified.• Phyllobacterium sp. in doses from 3 × 10 7 to 3 × 10 8 colony-forming units ml − 1 significantly promoted B. napus total root length up to 50% by increasing both lateral root density throughout the primary root and growth rate of mature lateral roots. The primary root was progressively colonized by the bacteria from the tip to the base and the number of colonizing cells was positively correlated with the inoculum density.• Relationships between inoculum density, root colonization and root system architecture emphasized the relevance of this approach to specify PGPB effects on plants.

Emended description of the genus Phyllobacterium and description of four novel species associated with plant roots: Phyllobacterium bourgognense sp. nov., Phyllobacterium ifriqiyense sp. nov., Phyllobacterium leguminum sp. nov. and Phyllobacterium brassicacearum sp. nov.

“…Strain W3 stimulates initial root growth and de novo root development in Picea spp. (Chanway et al, 1994), strain STM 196 (a synonym of isolate 29-15) was recognized as a plant-growth-promoting bacterium in plant culture of oilseed rape (Brassica napus) (Bertrand et al, 2001;Larcher et al, 2003) and Arabidopsis thaliana (Mantelin et al, 2006) and strain BOG-1-98 promotes growth of black mangrove seedlings in artificial sea water when co-inoculated with Bacillus licheniformis (Rojas et al, 2001). Interestingly, some strains have been isolated from root nodules (Sturz et al, 1997;Rasolomampianina et al, 2005), although the capacity of the isolates to induce nodulation was not clearly demonstrated.…”

PGPR-Arabidopsis interactions is a useful system to study signaling pathways involved in plant developmental control

“…Probably the most extensively described of these phenotypes consists of more numerous and/or longer lateral roots. [10][11][12] However, even more spectacular is the effect of rhizobacteria on root hair elongation: in our hands, depending on the rhizobacterial strain tested, the root hair length of Arabidopsis thaliana seedlings increased 2-to 3-fold upon inoculation. 13 Considering that ethylene is a key regulator of root hair elongation, 14 we used the rhizobacteria-triggered root air elongation phenotype in the model plant to examine the implication of ethylene in beneficial biotic interaction.…”

Bacteria from the rhizosphere and roots ofBrassica napusinfluence its root growth promotion byPhyllobacterium brassicacearum

“…navajo seeds were surface-sterilized in 3% calcium hypochlorite (w/v) for 5 min and washed 4 times with sterile water. Seeds were placed for germination in Petri dishes on e' (Larcher et al, 2003) semisolid medium diluted 10 times, for 48 h at 20°C in darkness.…”

“…the Petri dishes were stored vertically for eight days in a growth chamber set for a 22 °C/18 °C light/dark thermoperiod and 16 h/8 h light/dark photoperiod at a photon flux density of 130 µmol/m 2 /s. the following treatments were performed: P. brassicacearum alone [at 10 6 , 4x10 6 , 3x10 7 , 4x10 7 , 1.5x10 8 , 3x10 8 and 6x10 8 CFu/mL according to Larcher et al (2003)] or co-inoculated with either the 14 B. napus rhizosphere bacteria at ratios of 2:8, 7:3, 9:1 (P. brassicacearum: rhizosphere bacteria) or with the 9 B. napus root bacteria at ratios of 2:8, 6:4, 9:1 (P. brassicacearum: root bacteria). the whole bacterial densities ranged from 0.8 to 1.8x10 8 CFu/mL.…”

“…navajo (Bertrand et al, 2001;Mantelin et al, 2006). It was shown under gnotobiotic conditions to improve root growth by increasing both lateral root density and growth rate of mature lateral roots (Larcher et al, 2003). to define whether indigenous bacteria affected B. napus growth promotion mediated by P. brassicacearum, we first isolated and identified some bacteria associated with B. napus roots and rhizosphere.…”