These new discoveries illustrate the complexity of a highly redundant gravity-signalling process in roots, and help to elucidate the global mechanisms that govern auxin transport and morphogenetic regulation in roots.

Adenosine kinase (ADK) is a key enzyme that regulates intra-and extracellular levels of adenosine, thereby modulating methyltransferase reactions, production of polyamines and secondary compounds, and cell signaling in animals. Unfortunately, little is known about ADK's contribution to the regulation of plant growth and development. Here, we show that ADK is a modulator of root cap morphogenesis and gravitropism. Upon gravistimulation, soluble ADK levels and activity increase in the root tip. Mutation in one of two Arabidopsis (Arabidopsis thaliana) ADK genes, ADK1, results in cap morphogenesis defects, along with alterations in root sensitivity to gravistimulation and slower kinetics of root gravitropic curvature. The kinetics defect can be partially rescued by adding spermine to the growth medium, whereas the defects in cap morphogenesis and gravitropic sensitivity cannot. The root morphogenesis and gravitropism defects of adk1-1 are accompanied by altered expression of the PIN3 auxin efflux facilitator in the cap and decreased expression of the auxin-responsive DR5-GUS reporter. Furthermore, PIN3 fails to relocalize to the bottom membrane of statocytes upon gravistimulation. Consequently, adk1-1 roots cannot develop a lateral auxin gradient across the cap, necessary for the curvature response. Interestingly, adk1-1 does not affect gravity-induced cytoplasmic alkalinization of the root statocytes, suggesting either that ADK1 functions between cytoplasmic alkalinization and PIN3 relocalization in a linear pathway or that the pH and PIN3-relocalization responses to gravistimulation belong to distinct branches of the pathway. Our data are consistent with a role for ADK and the S-adenosyl-Lmethionine pathway in the control of root gravitropism and cap morphogenesis.

Aim Endophytic bacterial diversity in four rice cultivars grown in two soil-types, their plant-probiotic features and rhizospheric deployment under P-stress were investigated. Methods Oryza sativa cvs. TCN1, TCS10, TK8, and TN71 were cultivated in greenhouse using non-sterile acidic and near-neutral paddy soils for 60 days. Root, stem and leaf tissues were screened for culturable bacterial endophytes using nutrient agar. Endophytes were identified and profiled for plant-probiotic features. The richness, Shannon-Weiner diversity, evenness and Venn's distribution in terms of endophytic strains were evaluated. Seed-borne endophytes were characterized through DGGE. The deployment of endophytes into the rhizosphere in TCN1 and TK8 under gnotobiotic P-stress was assessed.Results A total of 52 distinct endophytic bacterial strains affiliated to 5 classes and 20 discrete genera exhibiting differential plant-probiotic features were isolated from various tissues of four different rice cultivars. The diversity and distribution of endophytes fluctuated with soil-type, tissue-type and rice genotype. Gnotobiotic insoluble P treatment revealed significantly enhanced deployment of P-solubilizing rhizobacteria in TCN1 as compared to soluble P and P-lacking control. Conclusions Rice endophytic bacteria are diverse, and their distribution within the plant and deployment as rhizobacteria were found to be influenced by host genotype, edaphic factors and nutrient stress.