Ethylene-binding activity, gene expression levels, and receptor system output for ethylene receptor family members from Arabidopsis and tomato.

Ethylene signaling in plants is mediated by a family of ethylene receptors related to bacterial two-component regulators. Expression in yeast of ethylene-binding domains from the five receptor isoforms from Arabidopsis thaliana and five-receptor isoforms from tomato confirmed that all members of the family are capable of high-affinity ethylene-binding activity. All receptor isoforms displayed a similar level of ethylene binding on a per unit protein basis, while members of both subfamily I and subfamily II from Arabidopsis showed similar slow-release kinetics for ethylene. Quantification of receptor-isoform mRNA levels in receptor-deficient Arabidopsis lines indicated a direct correlation between total message level and total ethylene-binding activity in planta. Increased expression of remaining receptor isoforms in receptor-deficient lines tended to compensate for missing receptors at the level of mRNA expression and ethylene-binding activity, but not at the level of receptor signaling, consistent with specialized roles for family members in receptor signal output.

Comparison of TRY and the closely related At1g01380 gene in controlling Arabidopsis trichome patterning.

A screen of activation-tagged Arabidopsis lines resulted in the identification of At1g01380, which encodes a small R3 single repeat MYB gene, as a negative regulator of trichome initiation. Plants that overexpress this gene have fewer trichomes. The gene is closely related to the previously identified negative regulator TRY, and has a similar pattern of expression as TRY in developing leaves. As previously shown for TRY, At1g01380 protein can inhibit the interaction between the positive trichome regulators GL1 and GL3, and likely limits trichome initiation via this inhibition. While TRY and At1g01380 are closely related, they are not completely functionally equivalent. When placed under the transcriptional control of the TRY promoter, At1g01380 can only partially rescue the try mutant. Interestingly, Atg01380 is highly expressed in gl3-sst trichomes, while TRY expression is greatly reduced. The mutation in gl3-sst causes a reduced interaction between the GL1 and GL3 proteins and results in fewer leaf trichomes that develop in clusters. The differential expression of TRY and At1g01380 in this mutant can be used to explain how its altered trichome pattern in gl3-sst [corrected] is generated.

A contradictory GLABRA3 allele helps define gene interactions controlling trichome development in Arabidopsis.

Previously characterized Arabidopsis gl3 mutants have trichomes that are smaller, less branched and undergo fewer rounds of endoreplication than wild-type trichomes. A new gl3 mutant, called gl3-sst, has oddly shaped trichomes that over expand during early development, undergo more endoreduplication and that have a striking nuclear morphology. The mutant nuclei consist of many interconnected lobes; however, only a single set of polytene-like chromosomes reside in the mutant nuclei. The predicted gl3-sst polypeptide has a Leu to Phe substitution (codon 78) within a region responsible for protein-protein interaction. Yeast interaction assays comparing GL3 with gl3-sst proteins show that the mutant protein interaction with GL1 and TTG1 is decreased by 75% and 50%, respectively, but there is no difference in its interaction with TRY. Furthermore, TRY has the ability to prevent the GL1 GL3 interaction and the GL1 gl3-sst interaction is even more sensitive to TRY. Analysis of plants expressing functional GFP-tagged versions of GL1, GL3 and TRY show that the proteins are localized in trichome nuclei. These results have been used to model trichome initiation in terms of protein interactions and threshold levels of activator complex.