Plants adapt to heterogeneous soil conditions by altering their root architecture. For example, roots branch when in contact with water by using the hydropatterning response. We report that hydropatterning is dependent on auxin response factor ARF7. This transcription factor induces asymmetric expression of its target gene LBD16 in lateral root founder cells. This differential expression pattern is regulated by posttranslational modification of ARF7 with the small ubiquitin-like modifier (SUMO) protein. SUMOylation negatively regulates ARF7 DNA binding activity. ARF7 SUMOylation is required to recruit the Aux/IAA (indole-3-acetic acid) repressor protein IAA3. Blocking ARF7 SUMOylation disrupts IAA3 recruitment and hydropatterning. We conclude that SUMO-dependent regulation of auxin response controls root branching pattern in response to water availability.

Detection of conserved microbial patterns by host cell surface pattern recognition receptors (PRRs) activates innate immunity. The FLAGELLIN-SENSITIVE 2 (FLS2) receptor perceives bacterial flagellin and recruits another PRR, BAK1 and the cytoplasmic-kinase BIK1 to form an active co-receptor complex that initiates antibacterial immunity in Arabidopsis. Molecular mechanisms that transmit flagellin perception from the plasma-membrane FLS2-associated receptor complex to intracellular events are less well understood. Here, we show that flagellin induces the conjugation of the SMALL UBIQUITIN-LIKE MODIFIER (SUMO) protein to FLS2 to trigger release of BIK1. Disruption of FLS2 SUMOylation can abolish immune responses, resulting in susceptibility to bacterial pathogens in Arabidopsis. We also identify the molecular machinery that regulates FLS2 SUMOylation and demonstrate a role for the deSUMOylating enzyme, Desi3a in innate immunity. Flagellin induces the degradation of Desi3a and enhances FLS2 SUMOylation to promote BIK1 dissociation and trigger intracellular immune signalling.

The red/far red light absorbing photoreceptor phytochrome-B (phyB) cycles between the biologically inactive (Pr, λ max , 660 nm) and active (Pfr; λ max , 730 nm) forms and functions as a light quality and quantity controlled switch to regulate photomorphogenesis in Arabidopsis. At the molecular level, phyB interacts in a conformation-dependent fashion with a battery of downstream regulatory proteins, including PHYTOCHROME INTERACTING FACTOR transcription factors, and by modulating their activity/abundance, it alters expression patterns of genes underlying photomorphogenesis. Here we report that the small ubiquitin-like modifier (SUMO) is conjugated (SUMOylation) to the C terminus of phyB; the accumulation of SUMOylated phyB is enhanced by red light and displays a diurnal pattern in plants grown under light/dark cycles. Our data demonstrate that (i) transgenic plants expressing the mutant phyB Lys996Arg -YFP photoreceptor are hypersensitive to red light, (ii) light-induced SUMOylation of the mutant phyB is drastically decreased compared with phyB-YFP, and (iii) SUMOylation of phyB inhibits binding of PHYTOCHROME INTERACTING FACTOR 5 to phyB Pfr. In addition, we show that OVERLY TOLERANT TO SALT 1 (OTS1) de-SUMOylates phyB in vitro, it interacts with phyB in vivo, and the ots1/ots2 mutant is hyposensitive to red light. Taken together, we conclude that SUMOylation of phyB negatively regulates light signaling and it is mediated, at least partly, by the action of OTS SUMO proteases.photoreceptor | phytochrome | sumoylation | signaling | photomorphogenesis P lants are sessile organisms; thus, they have to adapt to the ever-changing environment by modifying their growth and developmental programs. To respond to changes in ambient light conditions, plants evolved a battery of photoreceptors including the blue/UVA absorbing cryptochromes and phototropins (1, 2), the red/far red absorbing phytochromes (phys) (3), and the UVBspecific photoreceptor UVR8 (4). The red light/far red light (RL/FRL) absorbing phys exist as dimers, and each monomer has a covalently linked open tetrapyrrole chain as chromophore. They are synthesized in their biologically inactive Pr form (RL-absorbing state; λ max , 660 nm) and converted into the biologically active Pfr (FRL-absorbing state; λ max , 730 nm) by RL. Subsequent FRL treatment converts the Pfr form back into Pr. The Pr and Pfr conformers have partially overlapping absorption spectra; thus, phys cycle between their Pfr/Pr forms, and the ratio of Pr/Pfr forms is determined by the RL/FRL content of the incipient sunlight (5). In Arabidopsis five phys have been identified (phyA, phyB, phyC, phyD, and phyE), and among these, phyB has been shown to be especially important after seedling establishment (6).The overwhelming majority of molecular events underlying phyB-controlled photomorphogenesis take place in the nucleus. Light in a quality-and quantity-dependent fashion induces translocation of phyB Pfr in the nuclei (7,8), where it interacts with downstream acting regulatory proteins includ...

Highlights d BZR1 SUMOylation allows brassinosteroids to shape plant growth to its environment d SUMOylation stabilizes BZR1 by inhibiting BIN2 interaction, promoting plant growth d Salinity stimulates BZR1 deSUMOylation via ULP1a SUMO protease to suppress growth d BRs destabilize ULP1a, allowing SUMOylated BZR1 to accumulate and promote growth

SUMMARYThe SnRK1 protein kinase balances cellular energy levels in accordance with extracellular conditions and is thereby key for plant stress tolerance. In addition, SnRK1 has been implicated in numerous growth and developmental processes from seed filling and maturation to flowering and senescence. Despite its importance, the mechanisms that regulate SnRK1 activity are poorly understood. Here, we demonstrate that the SnRK1 complex is SUMOylated on multiple subunits and identify SIZ1 as the E3 Small Ubiquitin-like Modifier (SUMO) ligase responsible for this modification. We further show that SnRK1 is ubiquitinated in a SIZ1-dependent manner, causing its degradation through the proteasome. In consequence, SnRK1 degradation is deficient in siz1-2 mutants, leading to its accumulation and hyperactivation of SnRK1 signaling. Finally, SnRK1 degradation is strictly dependent on its activity, as inactive SnRK1 variants are aberrantly stable but recover normal degradation when expressed as SUMO mimetics. Altogether, our data suggest that active SnRK1 triggers its own SUMOylation and degradation, establishing a negative feedback loop that attenuates SnRK1 signaling and prevents detrimental hyperactivation of stress responses.