Regulation Of NPA And H₂O₂ On Non-gravitropism Of Pea Primary Roots

With the deepening research of root to gravity, H2O2 attracted more and more attention as a kind of stress factor and signal molecule in the gravitropic growing process of root, which could not only induce the decline of plant as the stress factor but also have effect on signal transduction of cells to stress response and regulation of cell growth and development. It was found that H2O2 could regulate the non-gravitropism of the plant root by affecting the asymmetric distribution of auxin. However, the way how H2O2 affects the asymmetry distribution of the hormone is still unclear. In this study, pea seeds were treated with different concentrations of NPA(1-Naphthylphthalamic acid) and 40 mmol/L H2O2, the physiological and biochemical indexes and the IAA and GA3 contents in pea primary roots with non-gravitropism growth condition were investigated in order to explore the regulation of NPA on the non-gravitropic bending of pea primary roots induced by H2O2. Results were summarized as follows: 1. Validation of the non-gravitropic bending pea primary roots induced by H2O2The pea seeds were cultured in vertically placed Petri dish in order to exclude the blocking effect of Petri dish bottom on the non-gravitropic bending of pea primary roots. Results showed that pea primary roots were still non-gravitropic bending without the block effect of Petri dish, which led to a conclusion that H2O2 could induce the non-gravitropic bending of pea primary roots. 2. Observation of external coloration of the curved primary roots induced by H2O2The non-gravitropic bending pea primary roots were stained and observed, results showed that with the increase of H2O2 concentration, the bending degree of the pea primary roots increased gradually, the hydrolysis of starch was exacerbated. The coloring deepened in some inside part of curved root, but the coloring of whole root was gradually shallower. 3. Effect of H2O2 on the arrangement of starch granules in pea primary bending rootThe stained pea primary roots were cut and made into slice, then observed under microscope. Results found that the average value of starch grains was found to be 30.5 pieces in the inner and outer sides of pea root that treated with distilled water. The arrangement of starch granules changed obviously in pea primary root that treated with H2O2, starch grains distributed more densely in the inner side with greater bending degree. Furthermore, the average value of the starch granules in inner side was greater than that of outer side and the difference was gradually increasing with the increase of H2O2 concentration. Therefore, the bending of pea root was related to the uneven distribution of starch grains for that the pea root always bent towards the side with more starch grains. 4. The mitigative effect of NPA on H2O2-induced non-gravitropic bending pea rootsWith the increase of NPA concentration, the bending degree of pea root decreased gradually, but still not restored to the control group level. The ?-amylase activity decreased gradually, the hydrolysis of starch slowed down, starch content increased, soluble sugar content decreased and the IAA content was also decreased, GA3 content did not show a gradual decline with the increase of NPA concentration, but decreased too. The increase of starch content led to the recovery of root tip to the perception of gravity. Thus, NPA could alleviate the non-gravitropic bending of pea primary roots induced by H2O2. 5. The inhibition of NPA on IAA polar transport under H2O2 stressThe IAA content in different parts was analyzed, and the highest content of IAA was found in bud where the IAA was synthesized, then followed by root tip, the lowest for cotyledon and the elongation zone where the IAA content was similar. With the increase of NPA concentration, the content of IAA in elongation zone and root tip decreased gradually; but the IAA content in bud and cotyledon did not show a relevant change with the increase of NPA. Therefore, with the treatment H2O2, the inhibition effect of NPA on IAA polar transport occurred only in pea root during the early germination of pea, the transport of IAA from shoots to other parts did not show inhibitory effect after germination.