Role of silicon in Plasmalemma H+-ATPase hydrolytic and pumping activity in maize (Zea mays L.)

摘要

Silicon is the second most abundant element in the earth crust and all plants grown in soil contain some amount of Si in their body. Despite of the ubiquitous nature of Si in soil-plant systems the essentiality of Si is not yet proven for higher plants. The beneficial effects of Si are more pronounced in plants grown under various biotic and abiotic stresses. Unfortunately, there is little evidence that Si application takes part in some of the physiological and biochemical processes in plants. Some earlier studies suggested that Si can increase the expansion growth in various plant species by changing the cell-wall extensibility in young growing shoot and root tissues. Moreover, some studies also suggested that Si uptake and translocation are energy requiring processes and may require a proton gradient. Plasma membrane H+-ATPase is a master enzyme and it extrudes H+ out of the cytosol and creates an electrochemical H+ gradient. The plasma membrane H+-ATPase generated H+ gradient is responsible for cell wall extensibility and expansion growth. Moreover, the electrochemical H+ gradient energizes various proteins involved in nutrient and solute uptake and translocation. Although few attempts have been made to elucidate the role of Si in plant growth and cell-wall extensibility, mechanisms lying behind are not fully understood. Therefore, it was assumed that Si-enhanced plant growth and uptake and translocation of Si from nutrient solution may require increased plasma membrane H+-ATPase activity. To determine the effect of Si nutrition on maize growth and its relationship with the plasma membrane H+-ATPase regulation the following hypotheses were tested: (1) Exogenously supplied Si in nutrient solution improves maize (Zea mays L cv. Amadeo) growth under normal growth conditions (2) Changes in plasma membrane H+-ATPase are responsible for the Si-induced maize growth (3) Silicon-nutrition increases maize shoot growth by inducing the apoplast acidification.The results are summarized as: 1.Silicon application in nutrient solution increased maize growth at all concentrations. The maximum increase in plant growth attributes was observed at 0.8 - 1.2 mM Si in nutrient solution. Furthermore, the results showed that Si nutrition had a balancing effect on other mineral nutrients in maize plants. Silicon application in nutrient solution changed the concentrations of different cations in maize roots and shoots tissues especially Ca, Zn, Mn, and Fe.2.Silicon nutrition changed the plasma membrane H+-ATPase characteristics that were: (I) Plasma membrane vesicles isolated from Si-treated shoots had 77% more enzyme protein. (II) The plasma membrane H+-ATPase isolated from Si-treated maize shoots showed inhibited hydrolytic and pumping activities by Si addition in the assay medium in comparison to the vesicles isolated from plants grown without Si. (III) Silicon nutrition induced differential transcription of plasma membrane H+-ATPase isforms. The plants supplied with Si had reduced MHA3 and MHAfam transcription. (IV) Similar to the shoots, Si nutrition brought some changes in the characteristics of plasma membrane H+-ATPase in roots as well. The addition of 1 mM Si in the assay medium significantly reduced the hydrolytic activity of plasma membrane H+-ATPase isolated from plants supplied with Si in nutrient solution. 3. Silicon-induced expansion growth in maize shoot showed no correlation with apoplastic pH. The plants grown with and without 1 mM Si supply in nutrient solution had a comparable value for their in vivo-measured apoplastic pH. Therefore, it is assumed that the Si-mediated increased shoot growth was due to unknown factors other than apoplastic pH.