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Auxin Steers Root Cell Expansion Via Apoplastic Ph Regulation in Arabidopsis Thaliana

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Auxin Steers Root Cell Expansion Via Apoplastic Ph Regulation in Arabidopsis Thaliana Auxin steers root cell expansion via apoplastic pH regulation in Arabidopsis thaliana Elke Barbeza,b,1, Kai Dünserb, Angelika Gaidoraa, Thomas Lendla, and Wolfgang Buscha,c,1 aGregor Mendel Institute, Austrian Academy of Sciences, Vienna Biocenter, 1030 Vienna, Austria; bDepartment of Applied Genetics and Cell Biology, University of Natural Resources and Life Sciences, 1190 Vienna, Austria; and cPlant Biology Laboratory, Salk Institute for Biological Studies, La Jolla, CA 92037 Edited by Mark Estelle, University of California at San Diego, La Jolla, CA, and approved May 8, 2017 (received for review August 12, 2016) Plant cells are embedded within cell walls, which provide structural stimulating effect of apoplast acidification on cell expansion in + integrity, but also spatially constrain cells, and must therefore be roots, as well as the requirement of functional PM H -ATPases for modified to allow cellular expansion. The long-standing acid growth root growth (14–16). On the other hand, high auxin concentrations theory postulates that auxin triggers apoplast acidification, thereby are known to inhibit root cell expansion and overall root growth (8, activating cell wall-loosening enzymes that enable cell expansion in 17). Moreover, exogenous auxin application has been described to shoots. Interestingly, this model remains heavily debated in roots, trigger apoplast alkalization in roots, which is the opposite effect as because of both the complex role of auxin in plant development as in shoots (18–20). Notably, a recent study provides substantial well as technical limitations in investigating apoplastic pH at cellular transcriptomic insight into auxin-triggered cell wall modification resolution. Here, we introduce 8-hydroxypyrene-1,3,6-trisulfonic acid and cell expansion in Brachipodium distachion roots (21). However, trisodium salt (HPTS) as a suitable fluorescent pH indicator for the authors also observed that medium acidification does not cor- assessing apoplastic pH, and thus acid growth, at a cellular resolution relate with root cell elongation (21). Notably, most of the afore- in Arabidopsis thaliana roots. Using HPTS, we demonstrate that cell mentioned studies indirectly investigated apoplast acidification by wall acidification triggers cellular expansion, which is correlated with measuring pH alterations in the medium, thereby failing to directly a preceding increase of auxin signaling. Reduction in auxin levels, assess the apoplastic pH at cellular resolution. The discrepancies in perception, or signaling abolishes both the extracellular acidification the current literature point to a complex role for auxin in apoplastic and cellular expansion. These findings jointly suggest that endogenous pH homeostasis and highlight the need to reassess the acid growth auxin controls apoplastic acidification and the onset of cellular theory at the cellular level. elongation in roots. In contrast, an endogenous or exogenous in- Here, we introduce 8-hydroxypyrene-1,3,6-trisulfonic acid tri- crease in auxin levels induces a transient alkalinization of the ex- sodium salt (HPTS) as a suitable fluorescent pH indicator for tracellular matrix, reducing cellular elongation. The receptor-like assessing apoplastic pH at a cellular resolution. Using HPTS, we kinase FERONIA is required for this physiological process, which dissected the apoplastic pH dynamics in A. thaliana roots and affects cellular root expansion during the gravitropic response. show that root cell expansion correlates with its acidification and These findings pinpoint a complex, presumably concentration- increased nuclear auxin signaling. In agreement, interference with dependent role for auxin in apoplastic pH regulation, steering the endogenous auxin levels or signaling abolishes acidification and rate of root cell expansion and gravitropic response. elongation. However, we also find that exogenous and endogenous increases in cellular auxin accumulation lead to a transient alka- apoplastic pH | auxin | cellular expansion | root growth | root gravitropism lization of the apoplast, correlating with the inhibition of root cell expansion. A significant proportion of this transient alkalization lant cells are surrounded by a rigid cell wall, which provides Pform and stability, enabling plants to grow to extreme heights Significance despite the absence of a skeleton. However, these advantages come with the price that plant cells are encased within the stiff Cellular growth in plants is constrained by cell walls; hence, cell wall matrix, which must be remodeled to allow for cellular loosening these structures is required for growth. The long- elongation. How cell walls are modified to enable cellular expan- standing acid growth theory links auxin signaling, apoplastic sion has been of scientific interest since the 1930s, as insight into pH homeostasis, and cellular expansion, providing a conceptual this physiological process would provide a wealth of knowledge on framework for cell expansion in plant shoots. Intriguingly, this how plants grow (1). In the early 1970s, a physiological mechanism model remains heavily debated for roots. Here, we present a explaining cell expansion, the acid growth theory, was proposed fluorescent dye that allows for the correlation of cell size and (2–4). This theory postulates that the plant hormone auxin triggers apoplastic pH at a cellular resolution in Arabidopsis thaliana. + the activation of plasma membrane (PM)-localized H -ATPases This enabled us to elucidate a complex involvement of auxin in (proton pumps), resulting in acidification of the intercellular space root apoplastic pH homeostasis, which is important for root cell (apoplast). The reduction in apoplastic pH activates cell wall- expansion and gravitropic response. These findings shed light loosening enzymes, which, in concert with turgor pressure, en- on the poorly understood acid growth mechanism in roots. ables cellular expansion (1). Auxin was the first plant hormone shown to be involved in processes important for plant growth and Author contributions: E.B. and W.B. designed research; E.B., K.D., and A.G. performed research; T.L. contributed a new analytic tool; E.B., K.D., and A.G. analyzed data; and E.B. development, including tissue growth, apical dominance, wound and W.B. wrote the paper. response, flowering, and tropisms, such as the gravitropic response The authors declare no conflict of interest. (5). Auxin is known to play a complex role in plant growth regu- This article is a PNAS Direct Submission. lation, as it can both stimulate and inhibit tissue expansion, Freely available online through the PNAS open access option. depending on the tissue and its concentration (6–8). A positive Data deposition: The raw data files of the presented experiments have been uploaded to effect of auxin on growth was hypothesized by the acid growth the Dryad database, datadryad.org (doi: 10.5061/dryad.sq7s3). theory (1). Subsequent literature provided significant insight into 1To whom correspondence may be addressed. Email: [email protected] or elkebarbez@ the molecular mechanisms of auxin-triggered acid growth in gmail.com. – shoots (9 13). However, in roots, the acid growth theory remains This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. the subject of debate. On one hand, several studies report the 1073/pnas.1613499114/-/DCSupplemental. E4884–E4893 | PNAS | Published online May 30, 2017 www.pnas.org/cgi/doi/10.1073/pnas.1613499114 Downloaded by guest on September 23, 2021 is dependent on the receptor-like kinase FERONIA. Taken capacity of the root (33) affects our measurements, these short- PNAS PLUS together, our data suggest a complex role of auxin in apoplastic term treatments allowed us to approximate an absolute pH value pH regulation, which is important for root organ growth and of 5.4 in the cell wall of meristematic cells (Fig. 1 E and F). Ac- gravitropic response. cordingly, we conclude that HPTS is a suitable dye for assessing apoplastic pH in Arabidopsis roots at a cellular resolution. Results HPTS Enables the Assessment of Apoplastic pH at a Cellular Resolution. Apoplastic pH Determines Cellular Expansion in A. thaliana Roots. Hav- To efficiently dissect acid growth in A. thaliana roots, we aimed to ing established an accurate method for assessing apoplastic pH, our identify a fluorescent dye that would enable the assessment of next step in dissecting acid growth in A. thaliana roots was to de- apoplastic pH with a cellular resolution. We screened the literature termine the relationship between apoplastic pH and root cell ex- for nontoxic, fluorescent, pH-sensitive dyes that are also water pansion. Several studies have reported a stimulating effect of soluble so they would easily penetrate the root apoplast, but not apoplast acidification on root cell expansion in maize, pea, and bean enter the root cells (22). Our search identified HPTS as a suitable (14, 15). To assess apoplastic pH dynamics in A. thaliana roots, we candidate to assess apoplastic pH in A. thaliana roots. HPTS is a measured the epidermal cell length of root cells, beginning at the water-soluble fluorescent dye displaying pH-dependent spectral height of the quiescent center, and recorded their corresponding characteristics (23). This fluorescent pH indicator has been pre- apoplastic pH (Fig. 1 D–G). Under our growth and experimental viously described to be suitable for the pH assessment of neuronal conditions, cells in the meristematic zone harbor an approximate
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