Summary NPR1, a master regulator of basal and systemic acquired resistance in plants, confers immunity through a transcriptional cascade, which includes transcription activators (e.g., TGA3) and repressors (e.g., WRKY70), leading to the massive induction of antimicrobial genes. How this single protein orchestrates genome-wide transcriptional reprogramming in response to immune stimulus remains a major question. Paradoxically, while NPR1 is essential for defense gene induction, its turnover appears to be required for this function, suggesting that NPR1 activity and degradation are dynamically regulated. Here we show that sumoylation of NPR1 by SUMO3 activates defense gene expression by switching NPR1's association with the WRKY transcription repressors to TGA transcription activators. Sumoylation also triggers NPR1 degradation, rendering the immune induction transient. SUMO modification of NPR1 is inhibited by phosphorylation at Ser55/Ser59, which keeps NPR1 stable and quiescent. Thus, posttranslational modifications enable dynamic but tight and precise control of plant immune responses.

In multicellular organisms, temporal and spatial regulation of cell proliferation is central for generating organs with defined sizes and morphologies. For establishing and maintaining the postmitotic quiescent state during cell differentiation, it is important to repress genes with mitotic functions. We found that three of the Arabidopsis MYB3R transcription factors synergistically maintain G2/M-specific genes repressed in post-mitotic cells and restrict the time window of mitotic gene expression in proliferating cells. The combined mutants of the three repressor-type MYB3R genes displayed long roots, enlarged leaves, embryos, and seeds. Genome-wide chromatin immunoprecipitation revealed that MYB3R3 binds to the promoters of G2/M-specific genes and to E2F target genes. MYB3R3 associates with the repressor-type E2F, E2FC, and the RETINOBLASTOMA RELATED proteins. In contrast, the activator MYB3R4 was in complex with E2FB in proliferating cells. With mass spectrometry and pairwise interaction assays, we identified some of the other conserved components of the multiprotein complexes, known as DREAM/dREAM in human and flies. In plants, these repressor complexes are important for periodic expression during cell cycle and to establish a post-mitotic quiescent state determining organ size.

DELLA protein is a key negative regulator of gibberellin (GA) signaling. Although how DELLA regulates downstream gene expression remains unclear, DELLA has been proposed to function as a transcriptional activator. However, because DELLA lacks a DNAbinding domain, intermediate protein(s) mediating the DELLA/DNA interaction are believed to be necessary for activating DELLA target genes. Here, using yeast hybrid screenings, we identified five members of INDETERMINATE DOMAIN (IDD) protein family which bind physically to both DELLA and the promoter sequence of the GA-positive regulator SCARECROW-LIKE 3 (SCL3), which previously was characterized as a DELLA direct target gene. Transient assays using Arabidopsis protoplasts demonstrated that a luciferase reporter controlled by the SCL3 promoter was additively transactivated by REPRESSOR of ga1-3 (RGA) and IDDs. Phenotypic analysis of transgenic plants expressing AtIDD3 (one of the 16 IDDs in the Arabidopsis genome) fused with the plant-specific repression domain (SRDX) supported the possibility that AtIDD3 is positively involved in GA signaling. In addition, we found that SCL3 protein also interacts with IDDs, resulting in the suppression of its target gene expression. In this context, DELLA and SCL3 interact competitively with IDD proteins to regulate downstream gene expression. These results suggest that the coregulators DELLA and SCL3, using IDDs as transcriptional scaffolds for DNA binding, antagonistically regulate the expression of their downstream targets to control the GA signaling pathway.transcription factor | gibberellin feedback regulation | coactivator/corepressor exchange regulation system G ibberellins (GAs) are diterpene phytohormones that regulate many cellular and developmental events such as cell elongation, leaf expansion, flowering, pollen maturation, and the transition from vegetative growth to flowering (1-4). Several protein factors involved in GA signaling have been identified. Among these, DELLA protein is a key player in the regulation of GA responses. DELLA proteins are characterized by a DELLA/TVHYNP motif at the N terminus and a GRAS domain [named after its first three members: GA INSENSITIVE (GAI), REPRESSOR of ga1-3 (RGA), and SCARECROW (SCR)] at the C terminus, placing DELLAs within the GRAS family of transcriptional regulators. GRAS-domain transcription factors have diverse functions in growth and development. Recent intensive studies revealed how GA is perceived by the GA receptor GIBBERELLIN INSENSITIVE DWARF1 (GID1) and how the perceived GA signal is transmitted to DELLA. By binding to active GAs, GID1 acquires the ability to interact with DELLA, allowing further interaction with an F box protein, SLEEPY1/GID2. DELLA is polyubiquitinated by E3 ubiquitin-ligase SCF SLY1/GID2 and finally is degraded through the 26S proteasome. However, how DELLA regulates downstream gene expression in GA signaling has remained unclear.In Arabidopsis, five DELLA genes have been identified; GAI, RGA, and three RGA-LIKE proteins (RGL1, RGL2, and RGL3) (1-...

The phytohormone auxin, indole-3-acetic acid (IAA), regulates nearly all aspects of plant growth and development. Despite substantial progress in our understanding of auxin biology, delineating specific auxin response remains as a major challenge. Auxin regulates transcriptional response via its receptors, TIR1/AFB F-box proteins. Here we report an engineered, orthogonal auxin-TIR1 receptor pair, developed through a bump-and-hole strategy, that triggers auxin signaling without interfering with endogenous auxin or TIR1/AFBs. A synthetic, convex IAA (cvxIAA) hijacked the downstream auxin signaling in vivo both at the transcriptomic level and in specific developmental contexts, only in the presence of a complementary, concave TIR1 (ccvTIR1) receptor. Harnessing the cvxIAA-ccvTIR1 system, we provide conclusive evidence for the role of TIR1-mediated pathway in auxin-induced seedling acid growth. The cvxIAA-ccvTIR1 system serves as a powerful tool for solving outstanding questions in auxin biology and for precise manipulation of auxin-mediated processes as a controllable switch.

Reactive oxygen species (ROS) are known to be important signal molecules that are involved in biotic and abiotic stress responses as well as in growth regulation. However, the molecular mechanisms by which ROS act as a growth regulator, as well as how ROS-dependent growth regulation relates to its roles in stress responses, are not well understood. We performed a time-course microarray analysis of root tips upon treatment with hydrogen peroxide, which we named "ROS-map." Using the ROS-map, we identified an MYB transcription factor, MYB30, which showed a strong response to ROS treatment and is the key regulator of a gene network that leads to the hydrogen peroxide-dependent inhibition of root cell elongation. Intriguingly, this network contained multiple genes involved in very-long-chain fatty acid (VLCFA) transport. Finally, we showed that is necessary for root growth regulation during defense responses, thus providing a molecular link between these two ROS-associated processes.