PIN6 is required for nectary auxin response and short stamen development

Abstract

SUMMARYThe PIN family of proteins is best known for its involvement in polar auxin transport and tropic responses. PIN6 (At1g77110) is one of the remaining PIN family members in Arabidopsis thaliana to which a biological function has not yet been ascribed. Here we report that PIN6 is a nectary-enriched gene whose expression level is positively correlated with total nectar production in Arabidopsis, and whose function is required for the proper development of short stamens. PIN6 accumulates in internal membranes consistent with the ER, and multiple lines of evidence demonstrate that PIN6 is required for auxin-dependent responses in nectaries. Wild-type plants expressing auxin-responsive DR5:GFP or DR5:GUS reporters displayed intense signal in lateral nectaries, but pin6 lateral nectaries showed little or no signal for these reporters. Further, exogenous auxin treatment increased nectar production more than tenfold in wild-type plants, but nectar production was not increased in pin6 mutants when treated with auxin. Conversely, the auxin transport inhibitor N-1-naphthylphthalamic acid (NPA) reduced nectar production in wild-type plants by more than twofold, but had no significant effect on pin6 lines. Interestingly, a MYB57 transcription factor mutant, myb57-2, closely phenocopied the loss-of-function mutant pin6-2. However, PIN6 expression was not dependent on MYB57, and RNA-seq analyses of pin6-2 and myb57-2 mutant nectaries showed little overlap in terms of differentially expressed genes. Cumulatively, these results demonstrate that PIN6 is required for proper auxin response and nectary function in Arabidopsis. These results also identify auxin as an important factor in the regulation of nectar production, and implicate short stamens in the maturation of lateral nectaries.

PIN-Dependent Auxin Transport: Action, Regulation, and Evolution

“…By contrast, PIN5, PIN6, and PIN8 are characterized by a reduction in the middle hydrophilic loop, partially in PIN6 and more pronounced in PIN5 and PIN8. Surprisingly, localization studies revealed that PIN5, PIN6, and PIN8 predominantly localize to the endoplasmic reticulum (ER; Mravec et al, 2009;Dal Bosco et al, 2012;Ding et al, 2012;Bender et al, 2013;Sawchuk et al, 2013), although instances of plasma membrane localization of PIN5 and PIN8 have been reported as well (Ganguly et al, 2014). Auxin transport and auxin content measurements indicate that PIN5 likely mediates auxin transport into the ER lumen (Mravec et al, 2009), while PIN8 appears to counteract this activity, which is further supported by antagonistic genetic interactions observed between mutants and overexpressors of these two transporters (Ding et al, 2012;Sawchuk et al, 2013).…”

“…PIN8, which is highly expressed in the male gametophyte, has an important role in pollen development and functionality (Dal Bosco et al, 2012;Ding et al, 2012), while PIN6 has been ascribed a function in the production of nectar and proper development of short stamens (Bender et al, 2013) as well as root growth and lateral root development (Cazzonelli et al, 2013). PIN5 appears to be necessary for fine-tuning of auxin function, as a pin5 mutant revealed only minor developmental phenotypes (Mravec et al, 2009).…”

The promoter of theArabidopsisPIN6 auxin transporter enabled strong expression in the vasculature of roots, leaves, floral stems and reproductive organs

“…28 Here the authors showed that PIN6 is required for a proper auxin response as pin6 mutants displayed an abnormal floral and nectary phenotype, exhibiting petals that fail to fully expand and significantly smaller nectaries (stamens) with reduced nectary production. Clearly, auxin is an important factor in the regulation of nectar production and the maturation of lateral nectarines.…”

How can an understanding of plant–pollinator interactions contribute to global food security?

“…Recent studies have identified transcription factors from the MYB family as regulators of nectar production in Arabidopsis (AtMYB57) and Nicotiana (NtMYB305) [58,59]. MYB305 has been shown to regulate nectarin production through NECTARIN genes [60] and starch accumulation during nectary development and thus the amount of nectar secreted [59].…”

“…Nectar secretion is also under the control of plant growth regulators jasmonic acid and auxin [58,61,62]. Downstream, the sugar transporter AtSWEET9 is involved in the export of sugars into the nectary [63**].…”