还珠h版小说令妃:pin formed

What are the mechanisms underlying differential distribution of theplant hormone auxin within plant tissues, and how do these processesregulate plant growth and architecture?

Plant development ischaracterized by a pronounced adaptability to different environmentalconditions. Extensive post-embryonic development, involving the activityof permanent stem cell populations (meristems), de novo organ formationand changes in growth direction, provides plants with exceptionalflexibility in terms of growth and survival. Differential distribution(gradients) of the plant signaling molecule auxin underlies many ofthese developmental events. These gradients are established andmaintained by a directional, intercellular auxin transport called polarauxin transport. Polar auxin transport provides positional anddirectional information for many aspects of plant development.

Classical models postulate that polar auxin transport requires theactivity of auxin influx and efflux carriers. Molecular genetic studiesin the model plant Arabidopsis thaliana have identified AUX1/LAX and PINgene families coding for components of influx and efflux carriersrespectively. Studies in cultured plant, mammalian and yeast cells showthat PINs mediate auxin efflux from cells. The PIN gene family inArabidopsis consists of eight members, and orthologs have been found inmost other plant species. Genetic studies have revealed the roles of thedifferent PIN proteins in the establishment of auxin gradientsmediating multiple developmental processes, including apicalorganogenesis and phyllotaxis, gravitropic and phototropic growth, rootmeristem patterning, vascular tissue development and embryonic axisformation.

The key feature of polar auxin transport, namely its controlleddirectionality, was postulated to result from the asymmetric,subcellular localization of the efflux carriers. Remarkably, aspredicted, PIN proteins display an asymmetric localization within auxintransport competent cells and determine the direction of auxin flow. Thedecision on PIN polar targeting depends on their phosphorylationregulated by PINOID kinase and PP2AA phosphatase. As cellular levels(and thus the activity of PINOID) are dependent on auxin itself, thisprovides a possible feedback regulation between auxin and PIN polarity.
Polartargeting of PIN proteins is related to their continuous subcellularmovement between endosomes and the plasma membrane. PIN internalizationoccurs by the clathrin-dependent endocytosis mechanism; its recyclingback to the plasma membrane requires ARF GEF regulators of vesicletrafficking. The constitutive cycling of auxin transport componentsprovides an entry point for internal and external signals, which in thismanner can rapidly modulate PIN polarity. Dynamic changes of PINpolarity in response to environmental and developmental signals havebeen observed to divert auxin flow during gravitropic response,embryogenesis, post-embryonic organogenesis and tissue regeneration. Inaddition, auxin itself can influence the subcellular distribution ofplasma membrane proteins, including PINs, by inhibiting theirendocytosis, and thus regulate activity of its efflux.

These data show that PIN proteins are key components of an intricateauxin distribution network that mediates local auxin gradients inmultiple developmental processes.
It also represents a unique modelsystem to study the functional link between basic cellular processes,such as endocytosis or cell polarity establishment, and theirdevelopmental outcome at the level of the multi-cellular plant organism.