Cytological Mechanisms Of Boron In Alleviating Aluminum Toxicity To Root Apex And Root Border Cells Of Pea(Pisum Sativum)

The soil in southern china is acidic soil, Aluminum(Al) toxicity and boron(B) deficiency are main limiting factors for plant growth in this soil. B and Al both have influence on cell wall and auxin transport, therefore, B can alleviate Al toxicity in plant. Our previous studies showed B could reduce the accumulation of Al in root and cell wall, and block Al transferred from root to shoot. However, the cytological mechanisms of B in alleviating Al toxicity is little known. Especially, the relationship between auxin transport and cell wall expansion as well as the relationship between the adsorption of Al in cell wall and Al cytotoxicity are worth further studied. Consequently, we proposed that B could alleviate Al cytotoxicity through participating in the regulation of auxin polar transport, the variation of apoplast p H and perception of multi-signal, eg: H+, ROS and Ca2+, to promote cell wall modifications and growth. In this study the root apices and root border cells(RBCs) of pea(pisum sativum) were employed, Auxin polar transport, cell wall ultrastructure and mitochondrial metabolic were used to explain cytological mechanisms of B in alleviating Al toxicity. The main results of this study are as follows:1. The effects of B on alleviating Al toxicity: B could reduce the accumulation of Al in the root apex, especially in the apical transition zone, simultaneously mitigate the inhibition of Al on the root apex and stimulate the root growth.2. Auxin participated in the regulation of Al toxicity: Exogenous auxin could effectively reduce the accumulation of Al in the apical transition zone, implying B reduced the accumulation of Al in transition zone may be regulated by auxin. IAA could inhibit the production of ROS. Under the Al stress, ROS would be more evenly distributed and could act as signaling molecule stimulating the root apex to resist from Al toxicity.3. B could alleviate Al induced inhibition of auxin polar transport: Under normal conditions, IAA flux along different zone of root apex showed typical changing wave, the wave peak appeared in the transition zone, which was notably higher than the meristem and elongation zone. B deficiency significantly reduced IAA peak in transition zone, while the waveform remained constant. Al blocked the polar transport of auxin, and alter the distribution pattern of auxin in the root apex which resulted in the auxin accumulation in meristem zone and disappeared wave peak in transition zone, finally restrained the growth of root apex. Under Al stress, B could promote the polar transport of auxin, increase the content of auxin in transition zone, and thereby maintain root growth and Al tolerance, ameliorate of Al toxicity4. B involved in the regulation of root surface p H and PM H+-ATPase by auxin: IAA induced decrease of root surface p H without Al, implying auxin could accelerate the efflux of H+. Furthermore, the result of auxin polar transport inhibitors indicated that auxin regulated p H mainly depend on efflux of IAA. Meanwhile, Different root zones presented different responses to exogenous auxin, which demonstrated the specific auxin distribution in root apex plays an important regulatory role of root surface p H and H+ flux variation. B supply stimulated the perceptions of cells to auxin, promoted the PM H+- ATPase activity and decrease of p H. Al induced rise of root surface p H, showing Al inhibited PM H+- ATPase activity and efflux of H+. Under Al stress, B supply promoted root surface p H alkalization, cells’ absorption and perceptions of auxin, therefore keep the normal growth of root apex.5. Al induced the burst of ROS and reduce of mitochondrial activity and membrane potential of RBCs, the continuous membrane potential depolarization led to the change of mitochondrial membrane permeability, triggered the increase of cytoplasmic Ca2+. Results of Transmission Electron Microscope(TEM) showed that the nuclear nucleolus disintegrated, chromatin condensed, intracellular contents disappeared, and appeared vacuolated under Al stress, all these evidence showed that Al induced ROS production through intervention in the mitochondria metabolism RBCs to arouse the programmed cell death. B supply, especially 25 μmol B could effectively maintain the activity of mitochondria, inhibit the mitochondrial membrane potential depolarization and ROS-induced programmed cell death of RBCs under Al stress.6. Under Al stress, cell wall components changed, causing cell wall thickening, cell membrane damage and accumulation of dense electronic content in cell wall. Meanwhile, the physical properties and ductility also varied, which brought about a large number of filamentous protrusions in RBCs surface and a rough surface, B supply kept the integrity and stability of the cell wall structure, and retained the cell wall actin smooth and toughness. B increases the fixing of Al in cell wall, and prevents Al entering into the cell from a direct damage.In root apex, B supply alleviate Al toxicity by regulating the inhibition of auxin efflux and cells’ perceptions to auxin and the variation of root suface p H, promoting polar transport of auxin, the modification and growth of cell wall, reducing accumulation of reactive Al in root apex, increasing more Al adsorbed and fixed in cell wall. In RBCs, B supply inhibited ROS explosion and depolarization of mitochondrial inner membrane potential(MMP), maintained Ca2+ homeostasis, and delayed Al-induced programmed cell death(PCD), therefor alleviated Al toxicity.