Sun Nan,Huang Lin,Zhao Hongcheng,Zhang Nan,Lin Xianyong,Sun Chengliang
Although most cultivated soils have high levels of total phosphorus (P), the levels of bioavailable inorganic P (Pi) are insufficient. The application of plant-growth-promoting rhizobacteria (PGPR) is an eco-friendly strategy for P utilization; however, PGPR-mediated plant responses that enhance Pi acquisition remain unexplored. Here, we investigated the effect of Azospirillum brasilense on Arabidopsis adaptation to Pi deficiency. Results showed that A. brasilense inoculation alleviated Pi-deficiency-induced growth inhibition and anthocyanin accumulation and increased the total P content in Arabidopsis plants. A comprehensive analysis of root morphology revealed that A. brasilense increased root hair density and length under Pi-limited conditions. We further demonstrated that A. brasilense enhanced the acid phosphatase activity and upregulated the expression of several Pi transporter genes, such as PHOSPHATE1 (PHO1), PHOSPHATE TRANSPORTER 1:(PHT1:1) and PHT1;4. However, A. brasilense did not enhance the growth o total P content in pht1;1, pht1;4 and pht1;1pht1;4 mutants. Moreover, A. brasilense could not increase the P content and PHT1;1 expression in the root hairless mutant rsl4rsl2, because of the occurrence of low-Pi-induced PHT1;1 and PHT1;4 in root hairs. These results indicate that A. brasilense can promote root hair development and enhance acid phosphatase activity and Pi transporter expression levels, consequently improving the Pi absorption capacity and conferring plant tolerance to Pi deficiency.
Summary The molecular mechanism of beneficial bacterium Azospirillum brasilense -mediated root developmental remain elusive. A. brasilense elicited extensively transcriptional changes but inhibited primary root elongation in Arabidopsis . By analyzing root cell type-specific developmental markers, we demonstrated that A. brasilense affected neither overall organization nor cell division of primary root meristem. The cessation of primary root resulted from reduction of cell elongation, which is probably because of bacterially activated peroxidase that will lead to cell wall cross-linking at consuming of H 2 O 2 . The activated peroxidase combined with downregulated cell wall loosening enzymes consequently led to cell wall thickness, whereas inhibiting peroxidase restored root growth under A. brasilense inoculation. We further showed that peroxidase activity was probably promoted by cadaverine secreted by A. brasilense . These results suggest that A. brasilense inhibits root elongation by activating peroxidase and inducing cell wall modification in Arabidopsis , in which cadaverine released by A. brasilense is a potential signal compound.
Hongcheng Zhao,Jiarui Xu,Lin Huang,Yizhe Ding,Xianyong Lin,Chengliang Sun
Beneficial microbiomes significantly modulate root hair morphogenesis, yet the underlying signalling pathways remain unclear. Through comprehensive phenotypic analysis, we demonstrated that Azospirillum brasilense , a well-known plant-growth-promoting rhizobacteria (PGPR), promotes root hair development by prolonging the elongation period. The promotion of root hair growth by A. brasilense is attributed to increased auxin accumulation in root apices and enhanced shootward transport from the root tip, which is mediated by the accumulation of the auxin importer auxin transporter protein 1 (AUX1) in the lateral root cap and the overlying epidermis. Chemical or genetic disruption of auxin signalling abolished bacteria-induced root hair responses. Furthermore, A. brasilense upregulated several root hair-related basic helix-loop-helix family transcription factors, including root hair defective 6 (RHD6) and RHD6-LIKE 1 (RSL1), which control root hair development through RSL4. Morphological analysis revealed that while the rsl2rsl4 double mutant remained hairless after inoculation, A. brasilense successfully rescued root hair growth in rhd6rsl1 . These findings suggest that A. brasilense can bypass RHD6 to activate RSL4, which then drives root hair development dependent on ROS. Collectively, our study uncovers the core signalling components modulated by A. brasilense to fine-tune root hair responses.
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