Distance-based measurement determines the coexistence of B protein hetero- and homodimers in lily tepal and stamen tetrameric complexes.

The floral quartet model proposes that plant MADS box proteins function as higher order tetrameric complexes. However, in planta evidence for MADS box tetramers remains scarce. Here, we applied a strategy using in vivo fluorescence resonance energy transfer (FRET) based on the distance change and distance symmetry of stable tetrameric complexes in tobacco (Nicotiana benthamiana) leaf cells to improve the accuracy of the estimation of heterotetrameric complex formation. This measuring system precisely verified the stable state of Arabidopsis petal (AP3/PI/SEP3/AP1) and stamen (AP3/PI/SEP3/AG) complexes and showed that the lily (Lilium longiflorum) PI co-orthologs LMADS8 and LMADS9 likely formed heterotetrameric petal complexes with Arabidopsis AP3/SEP3/AP1, which rescued petal defects of pi mutants. However, L8/L9 did not form heterotetrameric stamen complexes with Arabidopsis AP3/SEP3/AG to rescue the stamen defects of the pi mutants. Importantly, this system was applied successfully to find complicated tepal and stamen heterotetrameric complexes in lily. We found that heterodimers of B function AP3/PI orthologs (L1/L8) likely coexist with the homodimers of PI orthologs (L8/L8, L9/L9) to form five (two most stable and three stable) tepal- and four (one most stable and three stable) stamen-related heterotetrameric complexes with A/E and C/E function proteins in lily. Among these combinations, L1 preferentially interacted with L8 to form the most stable heterotetrameric complexes, and the importance of the L8/L8 and L9/L9 homodimers in tepal/stamen formation in lily likely decreased to a minor part during evolution. The system provides substantial improvements for successfully estimating the existence of unknown tetrameric complexes in plants.

Rosette core fungal resistance in Arabidopsis thaliana.

MAIN CONCLUSION: Unlike rosette leaves, the mature Arabidopsis rosette core can display full resistance to Botrytis cinerea revealing the importance for spatial and developmental aspects of plant fungal resistance. Arabidopsis thaliana is a model host to investigate plant defense against fungi. However, many of the reports investigating Arabidopsis fungal defense against the necrotrophic fungus, Botrytis cinerea, utilize rosette leaves as host tissue. Here we report organ-dependent differences in B. cinerea resistance of Arabidopsis. Although wild-type Arabidopsis rosette leaves mount a jasmonate-dependent defense that slows fungal growth, this defense is incapable of resisting fungal devastation. In contrast, as the fungus spreads through infected leaf petioles towards the plant center, or rosette core, there is a jasmonate- and age-dependent fungal penetration blockage into the rosette core. We report evidence for induced and preformed resistance in the rosette core, as direct rosette core inoculation can also result in resistance, but at a lower penetrance relative to infections that approach the core from infected leaf petioles. The Arabidopsis rosette core displays a distinct transcriptome relative to other plant organs, and BLADE ON PETIOLE (BOP) transcripts are abundant in the rosette core. The BOP genes, with known roles in abscission zone formation, are required for full Arabidopsis rosette core B. cinerea resistance, suggesting a possible role for BOP-dependent modifications that may help to restrict fungal susceptibility of the rosette core. Finally, we demonstrate that cabbage and cauliflower, common Brassicaceae crops, also display leaf susceptibility and rosette core resistance to B. cinerea that can involve leaf abscission. Thus, spatial and developmental aspects of plant host resistance play critical roles in resistance to necrotrophic fungal pathogens and are important to our understanding of plant defense mechanisms.

The C-Terminal Sequence and PI motif of the Orchid (Oncidium Gower Ramsey) PISTILLATA (PI) Ortholog Determine its Ability to Bind AP3 Orthologs and Enter the Nucleus to Regulate Downstream Genes Controlling Petal and Stamen Formation.

This study focused on the investigation of the effects of the PI motif and C-terminus of the Oncidium Gower Ramsey MADS box gene 8 (OMADS8), a PISTILLATA (PI) ortholog, on floral organ formation. 35S::OMADS8 completely rescued and 35S::OMADS8-PI (with the PI motif deleted) partially rescued petal/stamen formation, whereas these deficiencies were not rescued by 35S::OMADS8-C (C-terminal 29 amino acids deleted) in pi-1 mutants. OMADS8 could interact with Arabidopsis APETALA3 (AP3) and enter the nucleus. The nuclear entry efficiency was reduced for OMADS8-PI/AP3 and OMADS8-C/AP3. OMADS8 could also interact with OMADS5/OMADS9 (the Oncidium AP3 ortholog) and enter the nucleus with an efficiency only slightly affected by the deletion of the C-terminal sequence or PI motif. However, the stability of the OMADS8/OMADS5 and OMADS8/OMADS9 complexes was significantly reduced by deletion of the C-terminal sequence or PI motif. Further analysis indicated that the expression of genes downstream of AP3/PI (BNQ1/BNQ2/GNC/At4g30270) was compensated by 35S::OMADS8 and 35S::OMADS8-PI to a level similar to wild-type plants but was not affected by 35S::OMADS8-C in the pi-1 mutants. A similar FRET (fluorescence resonance energy transfer) efficiency was observed for Arabidopsis AGAMOUS (AG) and the Oncidium AG ortholog OMADS4 for OMADS8, OMADS8-PI and OMADS8-C. These results indicated that the OMADS8 PI motif and C-terminus were valuable for the interaction of OMADS8 with the AP3 orthologs to form higher order heterotetrameric complexes that regulated petal/stamen formation in both Oncidium orchids and transgenic Arabidopsis. However, the C-terminal sequence and PI motif were dispensable for the interaction of OMADS8 with the AG orthologs.

The Oncidium Ethylene Synthesis Gene Oncidium 1-Aminocyclopropane-1 Carboxylic Acid Synthase 12 and Ethylene Receptor Gene Oncidium ETR1 Affect GA-DELLA and Jasmonic Acid Signaling in Regulating Flowering Time, Anther Dehiscence, and Flower Senescence in Arabidopsis.

In plants, the key enzyme in ethylene biosynthesis is 1-aminocyclopropane-1 carboxylic acid (ACC) synthase (ACS), which catalyzes S-adenosyl-L-methionine (SAM) to ACC, the precursor of ethylene. Ethylene binds to its receptors, such as ethylene response 1 (ETR1), to switch on ethylene signal transduction. To understand the function of ACS and ETR1 in orchids, Oncidium ACC synthase 12 (OnACS12) and Oncidium ETR1 (OnETR1) from Oncidium Gower Ramsey were functionally analyzed in Arabidopsis. 35S::OnACS12 caused late flowering and anther indehiscence phenotypes due to its effect on GA-DELLA signaling pathways. 35S::OnACS12 repressed GA biosynthesis genes (CPS, KS, and GA3ox1), which caused the upregulation of DELLA [GA-INSENSITIVE (GAI), RGA-LIKE1 (RGL1), and RGL2] expression. The increase in DELLAs not only suppressed LEAFY (LFY) expression and caused late flowering but also repressed the jasmonic acid (JA) biosynthesis gene DAD1 and caused anther indehiscence by downregulating the endothecium-thickening-related genes MYB26, NST1, and NST2. The ectopic expression of an OnETR1 dominant-negative mutation (OnETR1-C65Y) caused both ethylene and JA insensitivity in Arabidopsis. 35S::OnETR1-C65Y delayed flower/leaf senescence by suppressing downstream genes in ethylene signaling, including EDF1-4 and ERF1, and in JA signaling, including MYC2 and WRKY33. JA signaling repression also resulted in indehiscent anthers via the downregulation of MYB26, NST1, NST2, and MYB85. These results not only provide new insight into the functions of ACS and ETR1 orthologs but also uncover their functional interactions with other hormone signaling pathways, such as GA-DELLA and JA, in plants.

Regulatory network for FOREVER YOUNG FLOWER-like genes in regulating Arabidopsis flower senescence and abscission.

FOREVER YOUNG FLOWER (FYF) has been reported to play an important role in regulating flower senescence/abscission. Here, we functionally analyzed five Arabidopsis FYF-like genes, two in the FYF subgroup (FYL1/AGL71 and FYL2/AGL72) and three in the SOC1 subgroup (SOC1/AGL20, AGL19, and AGL14/XAL2), and showed their involvement in the regulation of flower senescence and/or abscission. We demonstrated that in FYF subgroup, FYF has both functions in suppressing flower senescence and abscission, FYL1 only suppresses flower abscission and FYL2 has been converted as an activator to promote flower senescence. In SOC1 subgroup, AGL19/AGL14/SOC1 have only one function in suppressing flower senescence. We also found that FYF-like proteins can form heterotetrameric complexes with different combinations of A/E functional proteins (such as AGL6 and SEP1) and AGL15/18-like proteins to perform their functions. These findings greatly expand the current knowledge behind the multifunctional evolution of FYF-like genes and uncover their regulatory network in plants.

Multifunctional evolution of B and AGL6 MADS box genes in orchids.

We previously found that B and AGL6 proteins form L (OAP3-2/OAGL6-2/OPI) and SP (OAP3-1/OAGL6-1/OPI) complexes to determine lip/sepal/petal identities in orchids. Here, we show that the functional L' (OAP3-1/OAGL6-2/OPI) and SP' (OAP3-2/OAGL6-1/OPI) complexes likely exist and AP3/PI/AGL6 genes have acquired additional functions during evolution. We demonstrate that the presumed L' complex changes the structure of the lower lateral sepals and helps the lips fit properly in the center of the flower. In addition, we find that OAP3-1/OAGL6-1/OPI in SP along with presumed SP' complexes regulate anthocyanin accumulation and pigmentation, whereas presumed L' along with OAP3-2/OAGL6-2/OPI in L complexes promotes red spot formation in the perianth. Furthermore, the B functional proteins OAP3-1/OPI and OAGL6-1 in the SP complex could function separately to suppress sepal/petal senescence and promote pedicel abscission, respectively. These findings expand the current knowledge behind the multifunctional evolution of the B and AGL6 genes in plants.