Physiological characterization and genetic modifiers of aberrant root thigmomorphogenesis in mutants of Arabidopsis thaliana MILDEW LOCUS O genes

Abstract

Root architecture and growth patterns are plant features that are still poorly understood. When grown under in vitro conditions, seedlings with mutations in Arabidopsis thaliana genes MLO4 or MLO11 exhibit aberrant root growth patterns upon contact with hard surfaces, exemplified as tight root spirals. We used a set of physiological assays and genetic tools to characterize this thigmomorphogenic defect in detail. We observed that the mlo4/mlo11-associated root curling phenotype is not recapitulated in a set of mutants with altered root growth patterns or architecture. We further found that mlo4/mlo11-conditioned root curling is not dependent upon light and endogenous flavonoids, but is pH-sensitive and affected by exogenous calcium levels. Based upon the latter two characteristics, mlo4-associated root coiling appears to be mechanistically different from the natural strong root curvature of the Arabidopsis ecotype Landsberg erecta. Gravistimulation reversibly overrides the aberrant thigmomorphogenesis of mlo4 seedlings. Mutants with dominant negative defects in α-tubulin modulate the extent and directionality of mlo4/mlo11-conditioned root coils, whereas mutants defective in polar auxin transport (axr4, aux1) or gravitropism (pgm1) completely suppress the mlo4 root curling phenotype. Our data implicate a joint contribution of calcium signalling, pH regulation, microtubular function, polar auxin transport and gravitropism in root thigmomorphogenesis.

Comprehensive study of excess phosphate response reveals ethylene mediated signaling that negatively regulates plant growth and development

“…Several previously known genes for controlling the RSA were identified. We found an upregulation of MLO 14 like protein in root, a recent study of MLO mutant revealed its involvement in polar auxin transport which is one of the components jointly responsible for an aberrant root phenotype 65 . A channeling protein, Annexin (ANNAT7) is reported to be involved in lateral root development by modulating the Ca 2+ ion gradient 66, 67 .…”

Arabidopsis MLO2 is a negative regulator of sensitivity to extracellular reactive oxygen species

“…MLO4 and MLO11 are involved in modulating root thigmomorphogenesis. In mlo4 and mlo11 mutants, roots grow in a tightly curled pattern; this anisotropic chiral root expansion phenotype requires polar auxin transport and calcium signalling (Bidzinski et al, ; Chen et al, ). Mechanical stimulation of Arabidopsis roots results in a specific signature of calcium transients that further triggers apoplastic alkalinization and RBOHC ‐dependent ROS accumulation, transmitting touch stimuli into appropriate growth responses (Monshausen, Bibikova, Weisenseel, & Gilroy, ).…”

“…Beyond MLO2, MLO6, and MLO12, few functions of other Arabidopsis MLO genes are known. MLO4 and MLO11 are involved in modulating root thigmomorphogenesis (Bidzinski et al, 2014;Chen et al, 2009) and MLO7 in the regulation of pollen tube growth (Kessler et al, 2010).…”

Magical mystery tour: MLO proteins in plant immunity and beyond

“…1c). When grown in vitro, Atmlo4 and Atmlo11 null mutants display unusual root curvature, evident as severe root curling, upon a tactile stimulus (Chen et al, 2009;Bidzinski et al, 2014;Fig. 1b).…”