Response Of Border Cells To Toxicity Of Ferrous Iron In Rice

Iron toxicity,a physiological disorder of rice,is widely spread in tropical and subtropical and causes severe rice yield reduction.Two rice varieties Fe(2+) tolerant Xieyou 9308 and Fe²⁺ sensitive IR64 screened from 18 rice varieties were used to study the biological characteristics of rice root border cells,their toxicity responses to ferrous iron stress and to reveal the effects of root border cell on alleviation of Fe²⁺ toxicity to root tip.The main results as follows:1.Most of rice root border cells have an elliptic shape or a quadrate shape.The root border cell number of Xieyou9308 reached the maximum when the root length was 20 mm(about 1795).The root border cell number of IR64 reached the maximum when the root length was 35 mm(about 1772).The activity of root border cell was 66%-90%.The greatest activity of root border cells of two rice varieties was observed at 20 mm root of length.With the developing root,the activity of pectin methyl esterase(PME) decreased.The highest relative activity of PME was achieved when the root was 3 mm long(0.166 H⁺ / root cap·h for Xieyou9308 and 0.164 H⁺ / root cap·h for IR64).Difference was observed between two varieties of rice in the change of nember,survival rate of root border cells,PME activity and exudation of mucilage in different root length.2.The toxicity responses to ferrous iron stress of root border cells between Xieyou9308 and IR64 were much more difference.With the increasing concentration of Fe²⁺,the number of root border cells in Xieyou9308 increased and reached the maximum at 100 mg / L of Fe²⁺,then decreased.The number of root border cells in IR64 decreased when Fe²⁺ concentration in excess of 50 mg / L.The activity of two rice varieties root border cells was reduced when concentration of Fe²⁺ increased.The activity of Xieyou9308 root border cells was reduced grievously at 200 mg/ L of Fe²⁺. When Fe²⁺ concentration ascended from 50 mg / L to 100 mg / L,the activity of IR64 root border cells was reduced 33.90%.With the increasing treatment time,the activity of two rice varieties root border cells was reduced.There was more serious harm under leave-body treatment.PME activity in root caps of Xieyou9308 increased with the increasing concentration of Fe²⁺,and reached the highest at 200 mg / L,then declined with the continue increasing concentration.We also observed the analogous phenomena in IR64 PME viability,but it reached the highest at 100 mg / L.Ferrous iron stress can induce secretion of mucilage.Mucilage layer become much thicker with increasing of Fe²⁺ concentration.The ultrastructural alterations in the root tip cells and root border cells were destroyed seriously under ferrous iron stress.3.Programmed cell death(PCD) was observed through PI-Hoechst33342 fluorescent staining.The result of PI-Hoechst33342 fluorescent staining showed that the root border cell of IR64 was programmed cell death when Fe²⁺ concentration was 50 mg / L,and the root border cell of Xieyou9308 was programmed cell death when Fe²⁺ concentration was 100 mg / L.The progressive delineation of fragmented DNA was appearance of DNA "ladder" under 100 mg / L Fe²⁺ treatment of Xieyou9308. The root border cells POD activity of Xieyou9308 increased with the increasing Fe²⁺ concentration,and reached its maximum when Fe²⁺ concentration was 200 mg / L, then decreased with the Fe²⁺ concentration farther increased.IR64 root border cells POD activity reached its maximum at 50 mg / L.With the increasing Fe²⁺ concentration,the root border cells NADH activity of Xieyou9308 increased.The root border cells NADH activity of IR64 increased with the increasing Fe²⁺ concentration, and reached its maximum when Fe²⁺ concentration was 100 mg / L,then decreased with the Fe²⁺ concentration farther increased.4.To study the recovery function of rice border cells in ferrous iron,we used static culture(to maintain border cells adhered to the root tips) and shaking culture(to remove border cells from root tips completely) under 200 mg / L Fe²⁺ concentration treatment.The relative length rates and water content of rice root was restrained by ferrous iron,and shaking culture was more serious.At the same time,POD,SOD and CAT activities was decreased in 200 mg / L Fe²⁺ treatment except the POD activity of Xieyou9308 with static culture.The restraining rate of POD,SOD and CAT activities was higher when rice root treated with shaking culture.The content of Fe in rice root was increased heavily under Fe²⁺ concentration treatment,and had significant difference between CK and treatment group.The Fe content in five segments(0-2 mm, 2-4 mm,4-6 mm,6-8 mm,8-10 mm) of rice root was gradual increased under static culture.There was no significant difference between four segments of rice root(0-2 mm,2-4 mm,4-6 mm,6-8 mm) under shaking culture.The results indicate that root border cells play a significant role in ferrous iron of rice.