Aluminum Tolerance In Cultivars Of Pea(Pisum Sativum)and Its Relationship With Cell Wall Properties

Aluminum(Al)toxicity is the primary constraint of agriculture production on acid soils.Therefore,it is of great significance to carry out research on Al tolerance in plants to improve the yield and quality of crops,which is the major problem for food production in acidic soils.Previous studies have demonstrated that the transition zone is most sensitive,which plays an important role in plant Al tolerance.In this study,Al-tolerant and Alsensitive cultivars of pea(Pisum sativum)are employed to investigate the mechanisms of Al sensitive in transition zone and Al tolerance in cultivars in the distribution of Al,cell wall pectin,apoplast p H,and root border cells.The main results are below:1.Verification of Al tolerance in different pea cultivarsRoot relative elongation,Al content and callose content were used to identify Al tolerance in cultivars.And results show that Onward and Alaska are Al-tolerant cultivars,Hyogo and Sima are Al-sensitive cultivars.2)Distribution of Al in root zones of different pea cultivarsThe accumulation of Al in root zone decreases from root tip to root base,and it is lower in the transition zone than in the meristem zone.However,Morin stain shows that the active Al content in transition zone is higher than the other zones,and the active Al is distributed in the epidermis and cortex in the transition zone,rather than only in the epidermis in the meristem zone.Our results suggest that higher content and wider distribution of active Al in transition zone is the main reason for Al sensitivity in the transition zone.The root Al content and cell wall Al content in the transition zone of Al-sensitive cultivars is higher than that of Al-tolerant cultivars.The green fluorescence indicated by Morin are stronger in Al-sensitive cultivars than that in Al-tolerant cultivars.It indicates the active Al in the transition zone of the Al-sensitive cultivars is more and distributes wider than that of the Al-tolerant cultivars.The results of XPS are also show that more Al is present in ionic form in Al-sensitive cultivar than in Al-tolerant cultivar.The accumulation of Al on chelator soluble pectin and alkaline soluble pectin is the reason for the differences of Al accumulation on cell wall among cultivars,which is higher in Al-sensitive cultivar than that of Al-tolerant cultivar.Therefore,our results suggest that the Al content in transition zone of Al-sensitive cultivars is higher than that of Al-tolerant cultivars,and the distribution of Al in Al-sensitive cultivars is wider.3.Pectin properties in different zones of different pea cultivarsAl toxicity induces the increase of pectin content,pectin methylesterase(PME)activity and degree of methylesterification(DM).The pectin content of transition zone is higher than that of meristem zone.The PME activity and DM of transition zone is lower than that of meristem zone.The higher pectin content and lower DM resulted in the highest unmethylesterified pectin in the transition zone,which is the mechanism for Al sensitivity in the transition zone.The differences of pectin properties among cultivars are mainly in the transition zone.The pectin content and PME activity of transition zone in Al-sensitive cultivars is higher than that of Al-tolerant cultivars.The higher PME activity of the Al-sensitive cultivars results in lower DM.The higher pectin content and lower DM results in higher unmethylated pectin content in the transition zone of Al-sensitive cultivar than that in Altolerant cultivars.Our results provide evidence that the lower unmethylesterified pectin content in root transition zone of Al-resistant cultivars confers Al tolerance in cultivars of pea.4.Mechanism of apoplast p H difference in transition zone of different pea cultivarsThe result of Oregon stain showed that the apoplast p H of transition zone is higher than other root zones.This is the reason why Al accumulation in the transition zone is lower than that in meristem zone.Higher apoplast p H and PME activity results in the Al sensitivity of root transition zone.The rhizosphere p H of Al-tolerant cultivar is higher than that of Al-sensitive cultivar,which is consistent with the results of apoplast p H in transition zone.The p H difference was related to H~+ influx,which is higher in Al-tolerant cultivar.After adding vanadate(VA)to inhibit plasma membrane H~+-ATPase(PM-H~+-ATPase)activity,the difference of H~+ influx in transition zone between cultivars disappear,which indicates that PM-H~+-ATPase is involved in the regulation of H~+ influx in root transition zone in different cultivars.After addition of NPA,an auxin efflux inhibitor,there is little difference in H~+ influx in the cultivars,while the difference in H~+ influx in the cultivars increases after the addition of CHPAA,an auxin influx inhibitor.These results show that the apoplast p H in the transition zone of Al-resistant cultivars is higher than that of Al-tolerant cultivars.And it indicates that the difference in apoplast p H of cultivars is related to H~+ influx,which is regulated by auxin efflux and PM-H~+-ATPase.5.Root border cells of different pea cultivarsAfter removal of root border cells,the root relative elongation rate of Al-sensitive cultivar is significantly decreased under Al toxicity,but there is little change in the Altolerant cultivar.The Al content of root border cells of the Al-sensitive cultivar is higher than that of the Al-resistant cultivar,and the Al tolerance in the root border cells follows that in the cultivars.It indicate that the root border cells of Al-sensitive cultivar alleviate the Al toxicity by accumulating more Al.In summary,higher pectin content,higher Al accumulation and wider distribution of active Al in transition zone is the main mechasnim of Al sensitivity in root transition zone.And the Al sensitivity of the transition zone determines the Al tolerance of the cultivars.There are two reasons:(1)Under Al stress,lower pectin content and higher DM lead to less unmethylesterified pectin content in the transition zone of Al-resistant cultivars,which is the reason of less Al accumulation on cell wall.(2)Under Al stress,the apoplast p H of Altolerant cultivar is higher than that of Al-sensitive cultivar,which further reduces the accumulation of Al in transition zone.The difference in apoplast p H of cultivars is related to H~+ influx,which is regulated by auxin efflux and PM-H~+-ATPase.