1. Title: A necessity for use of sticky signaling molecules: the rationale for ABC hormone transporters
Speaker: Prof. Angus Murphy, University of Maryland, USA
Day/Date: Friday/June 1st, 2018
Host PI: Jian-kang Zhu
Venue: PSC Auditorium
Polarized auxin streams that function in embryogenesis, organogenesis, and other aspects of plant development are fully described by nonpolar lipophilic and anionic uptake coupled to polarized cellular efflux mediated by PINFORMED carrier proteins. These processes are motivated by plasma membrane chemiosmotic potentials. The presence of the fundamental cellular components that mediate these polar auxin transport streams are sufficient to assure basic development. All other transport processes described to date elaborate these fundamental mechanisms. However, long distance polar auxin transport streams depend on the presence of ATP Binding Cassette Subclass B (ABCB) active transporters generally associated with multisubstrate specificity and exclusion of hydrophobic anions from plasma membrane leaflets. Simple models of transport developed in Arabidopsis seedlings implicate ABCB exclusion mechanisms in limiting polar auxin streams to the central vascular cylinder of hypocotyls and roots. Such a function would require limited substrate specificity for efficacy. Evidence of the importance of these transporters is more evident in mature plants, where vasculature is more diverse and a boundary function is not supported by transcript and protein localization. Instead, the primary function of ABCB transporters in mature tissues appears to be prevention of auxin reuptake in apical and dividing cells adjacent to sites of loading into transport streams. The specific ABCB transporters contributing each of these functions have been isolated and identified in Arabidopsis. An analysis of their distribution and activity suggests that membrane exclusion is a generalized function and that gene duplication allowed for greater auxin substrate specificity in a limited subset of the subfamily. This new functionalization resulted in their emergence as primary factors in maintenance of long distance polar auxin streams, while other isoforms may exhibit more diverse function.
2. Title: IAA oxidation is important for growth and development in Arabidopsis
Speaker: Prof. Wendy Peer, University of Maryland, US
Auxin homeostasis is maintained by coordination of auxin biosynthesis, transport and catabolism. Recently, auxin (indole-3-actic acid, IAA) inactivation via oxidation by DEOXYGENASE OF AUXIN OXIDATION (DAO) pointed to its role in plant growth and development. However, the results have been difficult to interpret because a gene duplication resulted in two copies of DAO, DAO1 and DAO2, in Arabidopsis thaliana. Functional redundancy of DAO has been suggested since DAO2 appears to be expressed at very low levels and dao1 loss-of-function line show 95% reduction in 2-oxindole-3-acetic acid (oxIAA) levels. The outstanding question is if the mild phenotypes in DAO1 loss-of-function alleles are due to redundancy in the auxin catabolic pathway or DAO gene duplication. Since DAO1 and DAO2 are in tandem, a CRIPSR-Cas9 approach was taken to construct several independent null dao2 and dao1 dao2 mutants. Here we report that DAO2 function is independent from DAO1. DAO2plays a role in regulating floral organ number, and auxin inactivation via oxidation is required for normal floral development and auxin homeostasis.