The root stem cell niche defines the area that specifies and maintains the stem cells and is essential for the maintenance of root growth. Here, we characterize and examine the functional role of a quiescent center (QC)-expressed RAC/ROP GTPase activator, RopGEF7, in Arabidopsis thaliana. We show that RopGEF7 interacts with At RAC1 and overexpression of a C-terminally truncated constitutively active RopGEF7 (RopGEF7DC) activates RAC/ROP GTPases. Knockdown of RopGEF7 by RNA interference causes defects in embryo patterning and maintenance of the QC and leads to postembryonic loss of root stem cell population. Gene expression studies indicate that RopGEF7 is required for root meristem maintenance as it regulates the expression of PLETHORA1 (PLT1) and PLT2, which are key transcription factors that mediate the patterning of the root stem cell niche. Genetic analyses show that RopGEF7 interacts with PLT genes to regulate QC maintenance. Moreover, RopGEF7 is induced transcriptionally by auxin while its function is required for the expression of the auxin efflux protein PIN1 and maintenance of normal auxin maxima in embryos and seedling roots. These results suggest that RopGEF7 may integrate auxin-derived positional information in a feed-forward mechanism, regulating PLT transcription factors and thereby controlling the maintenance of root stem cell niches.

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ROP GTPases are crucial for the establishment of cell polarity and for controlling responses to hormones and environmental signals in plants. In this work, we show that ROP3 plays important roles in embryo development and auxin-dependent plant growth. Lossof-function and dominant-negative (DN) mutations in ROP3 induced a spectrum of similar defects starting with altered cell division patterning during early embryogenesis to postembryonic auxin-regulated growth and developmental responses. These resulted in distorted embryo development, defective organ formation, retarded root gravitropism, and reduced auxin-dependent hypocotyl elongation. Our results showed that the expression of AUXIN RESPONSE FACTOR5/MONOPTEROS and root master regulators PLETHORA1 (PLT1) and PLT2 was reduced in DN-rop3 mutant embryos, accounting for some of the observed patterning defects. ROP3 mutations also altered polar localization of auxin efflux proteins (PINs) at the plasma membrane (PM), thus disrupting auxin maxima in the root. Notably, ROP3 is induced by auxin and prominently detected in root stele cells, an expression pattern similar to those of several stele-enriched PINs. Our results demonstrate that ROP3 is important for maintaining the polarity of PIN proteins at the PM, which in turn ensures polar auxin transport and distribution, thereby controlling plant patterning and auxin-regulated responses.

AISI 304 austenitic stainless steel was cast into a water cooled copper mould with wedge shaped slot to investigate the effect of cooling rates on its microstructural evolution and solidification sequences between δ ferrite and austenite phases. Dendritic δ ferrite forms at the base of the sample where the cooling rate is low. With increasing cooling rate, the solidification mode is transformed from primary ferrite mode to primary austenite mode resulted from the phase selection. The metastable austenite solidifies as the primary phase due to its kinetic advantage when the growth rate is sufficiently high. With the higher cooling rate at the tip of the sample, the morphology of the austenite phase transforms from dendritic to cellular.

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