ABA Receptors PYL8 And PYL9 Regulate Plant Lateral Root Growth Via MYB Transcription Factors

Root architecture is vital for plants in mineral absorption and stress responses. Auxin is one important phytohormone and regulates various metabolic processes and has a role in promoting root growth. Abscisic acid is another plant phytohormone reg-ulating plant growth, development, senescence and stress responses. Previous studies have revealed the role of ABA in inhibiting root growth. PYR1/PYLs/RCARs pro-tein family is widely accepted as ABA receptors and they interact with clade-A protein phosphatases type 2Cs, forcing them to dissociate from SnRK2 kinases (Sucrose non-fermenting-1-Related protein Kinase 2). Then SnRK2 kinases autophosphorylate and become active and phosphorylate downstream genes, like ABF2 transcription factor. So the signals are passed down to trigger a series of physiological reactions. Some of the ABA receptors have other roles in cellular processes besides being ABA receptors. For example, PYL13 inhibits clade A PP2Cs in an ABA-independent manner and also interacts with other PYLs and antagonizes their function as ABA receptors. Overex-pression of PYL5 in Arabidopsis and rice, like PYL13 in Arabidopsis, leads to an in-creased drought tolerance. Among all the mutants impaired only in one ABA receptor, only pyl8 single mutant shows an observable elongated primary root on ABA medium, indicating a nonredundant role of PYL8.Salinity stress and exogenous ABA could cause a quiescence phase in lateral root growth and lacking one of ABA receptors PYL8 prolongs the quiescence. On the con-trary, quadruple mutant pyrlpyl 1/2/4 and triple mutant snrk2.2/3/6 display shortened quiescence on ABA medium, suggesting a distinguishable role of PYL8 in lateral root quiescence. This prolonged quiescent phase of pyl8 can be rescued by application of IAA in the medium. Among the 14 members of ABA receptors, PYL9 has a high amino acid similarity to PYL8. We found that knockout of PYL9 and PYL8 together resulted in a longer quiescence phase. The prolonged quiescence phase of pyl8-lpyl9 required more exogenous IAA for recovery than pyl8. Although single mutant of pyl9 showed no obvious phenotype like pyl8, induced overexpression of PYL9 promoted the lateral root growth, suggesting the overlapping function of PYL8 and PYL9 with PYL8 play-ing the dominant role. Yeast-two-hybrid assay demonstrated that both PYL8 and PYL9 interacted with transcription factor MYB77. In vivo studies showed that this interac-tion transduced the signal down to auxin pathway. MYB77 belongs to R2R3 MYB transcription factor family. In the subgroup where MYB77 belongs to (subgroup 22), there are three other members:AtMYB44/AtMYBR1, AtMYB70 and AtMYB73. PYL8 and PYL9, together with MYB77 and MYB44, regulate auxin-responsive genes in vivo. In conclusion, PYL8 and PYL9 are responsible for recovery of lateral root from ABA medium via auxin signaling pathway components MYB transcription factors. Thus, crosstalk is established between two phytohormones, ABA and IAA.