Napoli et al. show that mRNA splicing variants have tissue- and developmental stage-specific activity in flower development

By Roberta Ghelli and Patrizia Brunetti

Background: Plants that are self-pollinating contain both male (stamen) and female (pistil) organs. To allow self-pollination, three processes in stamen development must take place at the same time: anthers must open, the filament of the stamen must grow in length, and pollen must mature. In Arabidopsis, these three processes are coordinated by the hormone auxin. We know that auxin does that by regulating some genes called ARFs, including ARF8, but the molecular mechanism is not clear. Further, while only one ARF8 gene exists, a process called alternative splicing joins the coding parts of the transcript in different ways to produce different forms (splice variants) of the ARF8 mRNA.

Question: How are the different splice variants are involved in controlling opening of the anther and growth of the stamen filament? To answer this question, we analyzed these processes in mutant flowers lacking ARF8 or expressing each of the ARF8 splice variants.

Findings: We found a new splice variant, ARF8.4, that controls the growth of the stamen filament and the early steps of anther opening, while another variant, ARF8.2, mainly regulates the final steps of anther opening. We show that there are a number reasons for the different action of the two variants. First, during stamen development ARF8.4 and ARF8.2 mRNAs are confined to different tissues. Second, ARF8.4 is more active than ARF8.2 in triggering the expression of the gene AUX/IAA involved in filament growth. Third, ARF8.4 and ARF8.2 act on different genes: ARF8.4 triggers MYB26, required for the early steps of anther opening, while ARF8.2 activates DAD1, which regulates the final steps of this process.

Next Steps: What we have shown is one of the few examples yet of a gene that regulates a developmental process by way of a tissue-specific expression of a splice variant. We still need to clarify the mechanism of this tissue-specific alternative splicing, and find out which other ARF8 splice variant controls pollen maturation.

Roberta Ghelli, Patrizia Brunetti, Nadia Napoli, Angelo De Paolis, Valentina Cecchetti, Tomohiko Tsuge, Giovanna Serino, Minami Matsui, Giovanni Mele, Gianmarco Rinaldi, Gianna Aurora Palumbo, Fabrizio Barozzi, Paolo Costantino, Maura Cardarelli (2018). A Newly Identified Flower-Specific Splice Variant of AUXIN RESPONSE FACTOR8 Regulates Stamen Elongation and Endothecium Lignification in Arabidopsis. Plant Cell 30: 620-637; DOI:

ARF8.4 expression (8.4ox-arf8) rescues the short-stamen phenotype of arf8-7 mutant flowers.

Keywords: flower development, reproduction, auxin, auxin response factors, alternative splicing.