Cell-specific Auxin Signaling Regulates Plant Root Growth And Growth Response

Roots are considered to be one of the most important plant organs for adaptation to land life.They are mainly involved in water and nutrient acquisition,anchorage,propagation and storage functions.They also act as a major site of interaction with soil microorganisms and diverse pathogens.Therefore,the root system is very important for the basic growth and development of the plant and responses to the environment.In recent years,the effects of genetic factors,plant hormones and nutrients on plant root development have emerged as a central focus of research.Auxin is an endogenous molecule in plants and functions at very low concentrations.It is a master regulator in root development.Auxin gradients,which arise due to auxin polar transport and local auxin biosynthesis,control root growth and development.However,the contributions or specific roles of cell-type auxin signals in root growth and development are not yet fully understood.In order to systematically investigate how cellular auxin signals are involved in root growth and growth response,we constructed UAS-VP16-IAA17m?m?(which simulates high auxin signaling),UAS-HA-IAA17m?m?(which simulates low auxin signaling)and UAS-YUC4(which simulates high auxin synthesis)lines and crossed them with different cellular GAL4 enhancer trap lines(such as activators expressed in epidermis,cortex,endodermis,pericycle,and stele)in Arabidopsis.To this end,genetic analysis,transcriptome analysis,and other cellular or molecular techniques were applied.Through phenotypic analysis of F1 seedlings,we found that both inhibiting and enhancing auxin signaling can reduce primary root growth of seedlings.It suggested that each cell type in the root tip has an appropriate auxin signal maintained by a specific auxin level to regulate the root growth and development.We further confirmed that root growth inhibition caused by specific enhancement or inhibition of cellular auxin signaling was closely related to abnormal cell division and cell elongation.The study also found that although repressing auxin signaling in the epidermis,cortex,endodermis,pericycle,and stele can inhibit root growth,the root growth was completely inhibited in the endodermis.When we examined the embryonic development at various stages,we observed the quiescent center(QC)cells divided abnormally at the torpedo stage.This suggested that auxin signaling in the endodermis was essential for root establishment during embryonic development.Enhancing auxin signaling in all five tissues tested inhibited root growth to a similar extent,whereas increasing local auxin biosynthesis(upregulation of YUC4)in these tissues displayed different effects on root growth.The increased auxin biosynthesis in the epidermis resulted in the most obvious inhibition of root growth,whereas upregulation in the pericycle or stele did not show obvious root growth change.Transcriptome analysis showed the inhibition of root growth in the epidermis,cortex and endodermis with over-synthesized auxin was mainly due to the changed expression of cell wall-related genes.Gravitational treatment results showed that repression of cell-type specific auxin responses inhibited gravitropic response of the root.However,increasing auxin biosynthesis in epidermal cells also inhibited root gravitropic response,suggesting an optimal auxin concentration and signal was necessary for the gravity response process.In conclusion,our study indicated that both inhibiting and enhancing auxin signaling can reduce primary root growth of seedlings.This inhibition was closely related to abnormal cell division and cell elongation.Auxin biosynthesis specifically in the epidermis resulted in an extensive inhibition of root growth,whereas upregulation in the pericycle or stele had almost no effect on root growth.Moreover,we found that an appropriate epidermis auxin signaling was important for gravitropic response.