Mechanism Of The BIG Gene Regulating Radial Pattern Formation In Arabidopsis Root

In plants,the balance between stem cell self-renewal and differentiation of their progeny is crucial for tissue and organ formation.The growth,development and patterning of Arabidopsis roots depend on tight spatiotemporal control of asymmetric stem cell division.The cortex and endodermis cell layers,collectively referred to as the ground tissue(GT),arise from two consecutive asymmetric divisions of cortex/endodermis initial(CEI)and cortex/endodermis initial daughter cells(CEID)in the stem cell niche(SCN).The radial SHR protein abundance and the longitudinal auxin gradient have been speculated to regulate the spatiotemporal-specific asymmetric divisions of CEI/CEID.However,how does auxin signal integrate into this molecular pathway,regulate the asymmetric division of CEI/CEID and in turn affect root radial patterning remains largely unknown.In addition to participating in the regulation of polar auxin transport,the functional mutation of the BIG gene also led to serious defects in root development,but the exact molecular mechanism remains to be revealed.The purpose of this study is to clarify the possible interaction and regulation mechanism between BIG and the key regulatory factors that mediate root development and auxin signals.The main findings are as follows:1.Mutation in the BIG gene,which is highly expressed in the cortex and endodermis,led to decreased cell division activity in the meristem,resulting in a decrease in the number of meristem cells and root length;the BIG gene deficiency led to decreased quiescent center activity,delayed development of columella stem cells and columella cells;the BIG gene was involved in maintaining the activity of the SCN in a WOX5dependent manner;2.The BIG gene impacted on the polar auxin transport by regulating the transcript and protein levels of several members of the PIN gene family;Enhancing auxin transport capacity or auxin content did not restore the growth and developmental defects of the big mutant roots;mutations in the BIG gene led to decreased transcriptional and protein levels of PLT1 and PLT2,but BIG regulated meristem development and SCN maintenance independently of the Auxin-PLT pathway;3.Mutations in the BIG gene inhibited the expression of SCR and SHR,the big-1 mutant had no additive effects with scr-3 and shr-2 mutants in controlling meristem development,SCN maintenance and CEI/CEID asymmetric division,BIG regulated meristem development,SCN maintenance and CEI/CEID asymmetric division by relying on the SCR/SHR pathway;4.Auxin modulated the BIG gene expression in a concentration-dependent manner,and increasing the auxin transport capacity of the big mutant did not restore the delay in CEI/CEID asymmetric division,and the big mutant was insensitive to the delayed CEI/CEID asymmetric division caused by auxin transporter inhibitor and auxininduced additional asymmetric division of GT;auxin regulated CEI/CEID asymmetric division in a BIG-dependent manner;5.Mutations in the BIG gene led to a significant increase in the transcription level of the CYCD6;1 gene,and mutation of the SHR gene led to the disappearance of the expression of the CYCD6;1 gene in the roots of the big mutants.The big mutants were insensitive to auxin-induced CYCD6;1 gene expression.The big-1 mutant had no additive effects in controlling CEI/CEID asymmetric division with the cycd6;1 mutant,BIG and CYCD6;1 regulated the CEI/CEID asymmetric division in the same pathway;6.Mutations in the BIG gene led to a significant increase in the transcriptional and protein levels and CYCD6;1-mediated protein phosphorylation levels of RBR,and the big mutant was insensitive to auxin and proteasome inhibitor-induced accumulation of RBR protein,changing the expression of RBR could correspondingly change the ratio of CEI/CEID asymmetric division,BIG and CYCD6;1 coordinately regulated the accumulation of RBR to regulate the CEI/CEID asymmetric division.In conclusion,the BIG gene is involved in regulating the development of root meristem and the maintenance of SCN in Arabidopsis,and connects the radial SHR protein abundance and the longitudinal auxin concentration gradient to regulate the asymmetric division of CEI/CEID.