The U-Box E3 Ligase SPL11/PUB13 Is a Convergence Point of Defense and Flowering Signaling in Plants

Plants use the ubiquitin-proteasome system (UPS) to regulate nearly every aspect of growth and development and to respond to abiotic and biotic stresses. Among the three major enzymes involved in the UPS, E3 ligases determine substrate specificity and actively participate in many biological processes in plants. Emerging evidence shows that some E3 ligases have multiple functions and serve as a connection node in plant signaling. Here, we review the dual functions of the U-box and armadillo (ARM) repeat domain E3 ligase SPL11 in rice and of its ortholog PUB13 in Arabidopsis in modulating innate immunity and flowering. Both SPL11 and PUB13 negatively regulate programmed cell death (PCD) and defense. Intriguingly, SPL11 promotes flowering under long-day (LD) conditions in rice while PUB13 suppresses flowering under LD conditions in Arabidopsis. SPL11 regulates defense through a putative GAP protein and regulates flowering through an RNA-binding protein. PUB13 modulates defense through FLS2 and may control flowering through HFR1. Moreover, PUB13-mediated defense and flowering depend on the plant hormone salicylic acid (SA). The similar functions of SPL11 and PUB13, and the complementation of the pub13 cell death and flowering phenotypes by Spl11 indicate that Spl11/PUB13 is an ancient, functionally conserved locus in monocot and dicot plants. In the process of speciation, the downstream signaling components have, however, diversified in these two species. We conclude by proposing working models of how SPL11 and PUB13 and their associated proteins modulate both defense and flowering in monocot and dicot plants.