Auxin and Flower Development: A Blossoming Field

Abstract

The establishment of the species-specific floral organ body plan involves many coordinated spatiotemporal processes, which include the perception of positional information that specifies floral meristem and floral organ founder cells, coordinated organ outgrowth coupled with the generation and maintenance of inter-organ and inter-whorl boundaries, and the termination of meristem activity. Auxin is integrated within the gene regulatory networks that control these processes and plays instructive roles at the level of tissue-specific biosynthesis and polar transport to generate local maxima, perception, and signaling. Key features of auxin function in several floral contexts include cell nonautonomy, interaction with cytokinin gradients, and the central role of MONOPTEROS and ETTIN to regulate canonical and noncanonical auxin response pathways, respectively. Arabidopsis flowers are not representative of the enormous angiosperm floral diversity; therefore, comparative studies are required to understand how auxin underlies these developmental differences. It will be of great interest to compare the conservation of auxin pathways among flowering plants and to discuss the evolutionary role of auxin in floral development.

Figure 1.

The gene regulatory networks involving auxin that regulate early floral meristem (FM) initiation and development. Schematic diagram of gene regulatory networks superimposed onto an image of an Arabidopsis FM emerging from the flank of the inflorescence meristem (IM), to illustrate how auxin coordinates FM initiation and identity and floral organ initiation (see text for details). (ANT) AINTEGUMENTA, (AIL6) AINTEGUMENTA-LIKE6, (ARF4) AUXIN RESPONSE FACTOR4, (BDL) BODENLOS, (BP) BREVIPEDICELLUS, (BRM) BRAHMA, (CUC) CUP-SHAPED COTYLEDON, (DRN) DORNRÖSCHEN, (DRNL) DORNRÖSCHEN-LIKE, (ETT) ETTIN, (FIL) FILAMENTOUS FLOWER, (HDA19) HISTONE DEACETYLASE19, (MAB4) MACCHI-BOU4, (PID) PINOID, (PIN1) PIN-FORMED1, (RBE) RABBIT EARS, (PTL) PETAL LOSS, (PYD) SPLAYED, (STM) SHOOTMERISTEMLESS, (SUP) SUPERMAN, (TPL) TOPLESS, (YUC1) YUCCA1, (YUC4) YUCCA4.

Figure 2.

The gene regulatory networks involving auxin that orchestrate development of the different flower organs. A representation of an Arabidopsis flower linked to schematic diagrams to show how auxin integrates into the gene regulatory networks that coordinate gynoecium, stamen, petal, and sepal initiation and growth. An additional diagram shows the gene regulatory network involving auxin that relates to floral meristem (FM) determinacy and termination (see text for details). (AG) AGAMOUS, (ARF8/ARF17) AUXIN RESPONSE FACTOR7/17, (AUX1) AUXIN EFFLUX CARRIER1, (Aux/IAA19) AUX/IAA PROTEIN19, (AXR4) AUXIN-RESISTANT4, (CRC) CRABS CLAW, (DRMY1) DEVELOPMENT-RELATED MYB-LIKE1, (HAN) HANABA TARANU, (HEC1/HEC2/HEC3) HECATE1/2/3, (JAG) JAGGED, (KNU) KNUCKLES, (MYB26/MYB108) MYB-DOMAIN PROTEIN26/08, (SPT) SPATULA, (TMO3) TARGET OF MONOPTEROS3, (TRN) TORNADO, (WUS) WUSCHEL (see additional abbreviations in Fig. 1).