SUPRA-PHYSIOLOGICAL LEVELS OF GIBBERELLINS / DELLAS ALTER THE PATTERNING, MORPHOLOGY AND ABUNDANCE OF ROOT HAIRS IN ROOT TIPS OF ARABIDOPSIS THALIANA SEEDLINGS

: In spite of the role of gibberellins/DELLAs in leaf hair production, no investigations have assessed their function in the production of root hairs. To this aim, the effects of supra-physiological levels of GAs/DELLAs on the gene expression patterning of the root hair (CPC) and non-hair (GL2, EGL3 and WER) epidermal cell fate markers, and on the distribution, morphology and abundance of root hairs, were studied in root tips of 5-day-old A. thaliana seedlings. Results showed that excessive GAs/DELLAs misarranged the CPC , GL2 , EGL3 and WER gene expression patterning and the location, shape and frequency of root hairs. However, when the gai-1 (GA-insensitive-1) DELLA mutant protein was specifically over-expressed at the root epidermis, no changes in the patterning or abundance of root hairs occurred. Thus, results suggest that, in A. thaliana seedlings , the GAs/DELLAs might regulate the patterning, morphology and abundance of root hairs from the sub-epidermal tissues of the root. the effects of high levels of GAs/DELLAs on the spatial gene expression of the hair (CPC) and non-hair (GL2, WER and EGL3) markers of root epidermal cell fate, as well as on the distribution of root hairs, were analysed in seedlings of A. thaliana . Results showed that excessive levels of GAs/DELLAs impaired the spatial gene expression of the root hair/non-hair epidermal cell fate markers and disarranged the normal distribution of root hairs, what suggested that the GAs/DELLAs might be involved in regulating the root hair patterning in seedlings of A. thaliana. In fact, stable or inducible mutants with low ( gai-1, HSp::gai-1 , pGAI::gai-1:GR , SCR::gai-1:GR ) or high ( QD , 5X , GID1b-ox ) levels of GAs showed not only a random expression of GL2 at the MZ and EZ of the root, known as the cell fate-decision zones [PERNAS & al. 2010], but also a disarrangement of the root hairs. Because neither the spatial expression of GL2 nor the distribution of root hairs suffered changes when the gai-1 DELLA was over-expressed at the root epidermis ( ML1::gai-1 x GL2pro::GUS , ML1::gai-1 and UAS::gai-1 x J0951 transgenic lines), it was concluded that the GAs/DELLAs do not seem to affect the root hair patterning in A. thaliana seedlings by acting on this root cell layer, but on tissues placed underneath. In fact, over-expression of gai-1 at the cortex, endodermis or pericycle of the root MZ altered the root hair patterning.