MicroRNA171c-Targeted SCL6-Ⅱ, SCL6-Ⅲ, and SCL6-Ⅳ Genes Regulate Shoot Branching in Arabidopsis

MicroRNAs （miRNAs） are -21-nucleotide noncoding RNAs that play critical roles in regulating plant growth and development through directing the degradation of target mRNAs. Axillary meristem activity, and hence shoot branching, is influenced by a complicated network that involves phytohormones such as auxin, cytokinin, and strigolactone. GAI, RGA, and SCR （GRAS） family members take part in a variety of developmental processes, including axillary bud growth. Here, we show that the Arabidopsis thaliana microRNA171c （miR171c） acts to negatively regulate shoot branching through targeting GRAS gene family members SCARECROW-LIKE6-Ⅱ （SCL6-Ⅱ）, SCL6-Ⅲ, and SCL6-Ⅳ for cleavage. Transgenic plants overexpressing MIR171c （35Spro-MIR171c） and sd6-Ⅱ scl6-Ⅲ scl6-Ⅳ triple mutant plants exhibit a similar reduced shoot branching phenotype. Expression of any one of the miR171c-resistant versions of SCL6-Ⅱ, SCL6-Ⅲ, and SCL6-Ⅳ in 35Spro- MIR171c plants rescues the reduced shoot branching phenotype. Scl6-Ⅱ scl6-Ⅲ scl6-Ⅳ mutant plants exhibit pleiotropic phenotypes such as increased chlorophyll accumulation, decreased primary root elongation, and abnormal leaf and flower patterning. SCL6-Ⅱ, SCL6-Ⅲ, and SCL6-Ⅳ are located to the nucleus, and show transcriptional activation activity. Our results suggest that miR171c-targeted SCL6-Ⅱ, SCL6-Ⅲ, and SCL6-Ⅳ play an important role in the regulation of shoot branch production.

A Gain-of-Function Mutation in IAA7/AXR2 Confers Late Flowering under Short-day Light in Arabidopsis

Floral initiation is a major step in the life cycle of plants, which is influenced by photoperiod, temperature, and phytohormones, such as gibberellins （GAs）. It is known that GAs promote floral initiation under short-day light conditions （SDs） by regulating the floral meristem-identity gene LEAFY （LFY） and the flowering-time gene SUPPRESSOR OF OVEREXPRESSION OF CO 1 （SOC1）. We have defined the role of the auxin signaling component INDOLE-3-ACETIC ACID 7 （IAA7）/AUXIN RESISTANT 2 （AXR2） in the regulation of flowering time in Arabidopsis thaliana. We demonstrate that the gain-of-function mutant of IAA7/AXR2, axr2-1, flowers late under SDs. The exogenous application of GAs rescued the late flowering phenotype of axr2-1 plants. The expression of the GA20 oxidase （GA2Oox） genes, GA2Ooxl and GA2Oox2, was reduced in axr2-1 plants, and the levels of both LFY and SOC1 transcripts were reduced in axr2-1 mutants under SDs. Furthermore, the overexpression of SOC1 or LFY in axr2-1 mutants rescued the late flowering phenotype under SDs. Our results suggest that IAA7/AXR2 might act to inhibit the timing of floral transition under SDs, at least in part, by negatively regulating the expressions of the GA2Ooxl and GA2Oox2 genes.