Studies Of Abscisic Acid Regulatory Effects On Auxin Biosynthesis To Inhibit The Primary Root Growth In Rice

The root system is an important organ which supports the aerial part of the plant,and absorbs the water and nutrients required for the growth and development.Root morphology affects the ability of crops to obtain nutrients and resist environmental stresses such as drought and salt stresses,ultimately affecting crop yield.Rice primary root is the basic component of the root system and plays an essential role in seedlings growth.The growth and development of rice primary root are regulated by various plant hormones.Abscisic acid(ABA)and auxin play essential role in root growth and development,while the interaction between ABA and auxin in regulating the growth and development of rice primary root remains unclear.ABA can inhibit the elongation of rice primary root.In this study,the cytological observations found that ABA inhibited the size of meristem and the cells number in the meristem of rice primary root.Ed U staining allows us to detect cell proliferation.The results of Ed U staining observation indicated that ABA inhibited the cell proliferation in rice primary root meristem.These results indicated that ABA affected primary root elongation by inhibiting the cell proliferation in root meristem.Os ABIL2 is the protein phosphatase(PP2Cs),and Os SAPK10 is a member of the protein kinases Sn RK2 family in the ABA signaling pathway of rice.Through analyzing their functions in the elongation process of rice primary root,we found that overexpression of Os SAPK10 increased the sensitivity of the primary root to ABA,and overexpression of Os ABIL2 decreased the sensitivity of the primary root to ABA,proving that ABA inhibited primary root growth through ABA signaling pathway.Auxin biosynthesis plays an essential role in root growth.Os YUCs family genes encode a key enzyme that is a rate?limiting step of the indole?3?pyruvic acid(IPA)pathway in auxin biosynthesis.The results of this study indicated that inhibition of the primary root elongation by ABA can be eliminated by YUC activity inhibitor yucasin,indicating that inhibition of the primary root elongation by ABA is dependent on YUC.ABA significantly induced the expression of auxin biosynthesis genes Os YUC3,Os YUC4,Os YUC6,Os YUC8 and Os YUC9 and auxin(IAA)accumulation in the primary root,indicating that ABA can affect root growth and development by regulating auxin biosynthesis.In our laboratory,we demonstrated that Os YUC8 plays an essential role in the auxin biosynthesis.We further found that the primary roots of osyuc8 mutant showed significantly lower sensitivity to ABA and IAA content in osyuc8 mutant had no significant difference before and after ABA treatment,indicating that Os YUC8 mediated the promotion of auxin biosynthesis and the inhibition of rice primary root elongation by ABA.Osb ZIP46 is an important transcription factor in the ABA signaling pathway,and overexpression of Osb ZIP46 can enhance the sensitivity of rice primary root to ABA,indicating that Osb ZIP46 is involved in regulating the inhibition of rice primary root elongation by ABA.Furthermore,by comparing the effects of the expression of Os YUC8 in wild type,Osb ZIP46 overexpression transgenic material,and osbzip46 mutant by ABA,it was found that there was significant up-regulation in Osb ZIP46 overexpressing transgenic material and no significant difference in osbzip46 mutant,indicating that ABA induced the expression of Os YUC8 is dependent on Osb ZIP46.Further,the tobacco transient expression experiment demonstrated that Osb ZIP46 can activate the activity of Os YUC8 promoter.These results suggest that ABA up-regulates Os YUC8 expression through Osb ZIP46,promotes auxin biosynthesis,and then inhibits primary root elongation.Root growth and development are closely related to the environment,and salt stress can significantly inhibit root growth and development,but the molecular mechanism by which ABA regulates root growth in response to salt stress needs to be further explored.The results of this article showed that salt stress or ABA treatment can inhibit primary root elongation and promote primary root swelling,and cytological observation showed that either salt stress or ABA treatment can inhibit cell proliferation and promote the cortical cell expansion in the root apical meristem,indicating that salt stress or ABA affected primary root elongation by inhibiting cell proliferation in the root apical meristem,and caused primary root swelling by promoting the cortical cell expansion in the root apical meristem.Salt stress significantly induced the expression of ABA?responsive genes Rab21,Rab16 B,Os ZEP1,LEA3,Os BZ8,Os RD22 and TRAB1,and ABA accumulation in the primary root.Overexpression of Os SAPK10 increased the sensitivity of the primary root to salt,while overexpression of Os ABIL2 decreased the sensitivity of the primary root to salt,indicating that salt regulates root growth through ABA signaling pathway.Transcriptome analysis showed that 790 DEGs up?regulated and 1,342 DEGs down?regulated by ABA were involved in the cell periphery,the oxidation reduction process,and the organic acid metabolic process.EXPANSINs are a class of cell wall?loosening proteins that are involved in cell wall extension and cell growth in plants.The q PCR analysis indicated that ABA significantly induced the expression of EXPANSIN genes Os EXPA4,Os EXPA10,Os EXPA18 and Os EXPA25.Overexpression of Os SAPK10 increased the inducing of the expression of Os EXPA4,Os EXPA10,Os EXPA18 and Os EXPA25 by ABA,and overexpression of Os ABIL2 decreased the inducing of the expression of Os EXPA4,Os EXPA10,Os EXPA18 and Os EXPA25 by ABA.These results indicated that salt stress induced the expression of EXPANSIN genes is dependent on ABA signaling pathway,thereby regulates root swelling.Therefore,the above results deepen our understanding of the molecular mechanism by which ABA regulates primary root growth in response to salt stress and provide theoretical support for the genetic improvement of salt tolerance in rice.In conclusion,this study shows that ABA inhibits cell proliferation in root apical meristem by promoting auxin biosynthesis,which in turn affects the elongation of primary roots.Salt stress induces the expression of EXPANSIN gene is dependent on the ABA signaling pathway,promotes the cortical cell expansion in the root apical meristem,thereby leads to root tip swelling.