Pleiotropic and Non-redundant Effects of an Auxin Importer in Setaria and Maize¹

Abstract

Directional transport of auxin is critical for inflorescence and floral development in flowering plants, but the role of auxin influx carriers (AUX1 proteins) has been largely overlooked. Taking advantage of available AUX1 mutants in Setaria viridis and maize, we uncover previously unreported aspects of plant development that are affected by auxin influx, including higher order branches in the inflorescence, stigma branch number, and glume (floral bract) development, and plant fertility. However, disruption of auxin flux does not affect all parts of the plant, with little obvious effect on inflorescence meristem size, time to flowering, and anther morphology. In double mutant studies in maize, disruptions of ZmAUX1 also affect vegetative development. A GFP-tagged construct of SvAUX1 under its native promoter showed that the AUX1 protein localizes to the plasma membrane of outer tissue layers in both roots and inflorescences, and accumulates specifically in inflorescence branch meristems, consistent with the mutant phenotype and expected auxin maxima. RNA-seq analysis finds that most gene expression modules are conserved between mutant and wildtype plants, with only a few hundred genes differentially expressed in spp1 inflorescences. Using CRISPR-Cas9 technology, we disrupted SPP1 and the other four AUX1 homologs in S. viridis. SvAUX1/SPP1 has a larger effect on inflorescence development than the others, although all contribute to plant height, tiller formation, leaf, and root development. The AUX1 importers are thus not fully redundant in S. viridis. Our detailed phenotypic characterization plus a stable GFP-tagged line offer tools for future dissection of the function of auxin influx proteins.

Genetic control of branching patterns in grass inflorescences

“…Auxin flux through the epidermis converges on small regions of the IM, a process regulated by the auxin influx carrier ZmAUX1 (SvAUX1 in S. viridis ( Zhu et al, 2021a , 2021b ) and the auxin efflux carrier SISTER OF PINFORMED1 (SoPIN1/ZmPIN1D; O'Connor et al, 2014 ; Matthes et al, 2019 ; Figure 4 ). Loss-of-function mutations in SvAUX1 / SPARSE PANICLE1 reduce all orders of branching in the inflorescence, whether primary, secondary, tertiary, or higher, although the effect in maize is less striking than in S. viridis ( Huang et al, 2017 ; Zhu et al, 2021a , 2021b ). Vein patterning is controlled by PIN-FORMED1a (PIN1a) and PIN1b, which move auxin away from the local maxima and establish the position of vascular tissue ( O'Connor et al, 2014 ).…”

“…In rice, OsMADS5 and OsMADS34 directly regulate RCN4 and accelerate the transition to spikelet production. Double mutants of OsMADS5 OsMADS34 or OsMADS34 RCN4 produce more branches, including secondary, tertiary, and even quaternary branches ( Zhu et al, 2021a , 2021b ). OsMADS34 promoters also contain SPL binding motifs, and OsMADS34 is directly regulated by OsSPL14 ( Wang et al, 2015 ).…”