Strigolactone signaling in the endodermis is sufficient to restore root responses and involves SHORT HYPOCOTYL 2 (SHY2) activity

Abstract

SummaryStrigolactones (SLs) are plant hormones and regulators of root development, including lateral root (LR) formation, root hair (RH) elongation and meristem cell number, in a MORE AXILLARY GROWTH 2 (MAX2)-dependent way. However, whether SL signaling is acting cell-autonomously or in a non-cell-autonomous way in roots is unclear.We analyzed root phenotype, hormonal responses and gene expression in multiple lines of Arabidopsis thaliana max2-1 mutants expressing MAX2 under various tissue-specific promoters and shy2 mutants.The results demonstrate for the first time that expression of MAX2 under the SCARECROW (SCR) promoter, expressed mainly in the root endodermis, is sufficient to confer SL sensitivity in the root for RH, LR and meristem cell number. Moreover, loss of function mutation of SHORT HYPOCOTYL 2 (SHY2), a key component in auxin and cytokinin regulation of meristem size, has been found to be insensitive to SLs in relation to LR formation and meristem cell number.Endodermal SL signaling, mediated by MAX2, is sufficient to confer SL sensitivity in root, and SHY2 may participate in SL signaling to regulate meristem size and LR formation. These SL signaling pathways thus may act through modulation of auxin flux in the root tip, and may indicate a root-specific, yet non-cell-autonomous regulatory mode of action.

Strigolactones fine-tune the root system

“…It will be interesting in future studies to determine how/whether strigolactones and cytokinins interact to regulate lateral root development because the strigolactone mutants also contain reduced cytokinin levels. A very novel and exciting observation in that respect comes from a recent study by Koren et al (2013) in which they show that strigolactone signaling in the endodermis is sufficient for mediating lateral root responses towards exogenous GR24. Moreover, shy2 loss-of-function mutants were found to be insensitive to GR24 treatment for both lateral root formation and cell meristem number, revealing a role for this regulator of root meristem size not only during the cytokinin and auxin interplay (Dello Ioio et al 2008), but also during strigolactone action in shaping the root system.…”

Strigolactones are involved in sugar signaling to modulate early seedling development in <i>Arabidopsis</i>

“…Recent research has discovered their key roles as plant hormones in controlling shoot branching (Gomez-Roldan et al 2008;Umehara et al 2008;Xie et al 2010). This finding greatly stimulated SL studies and many new roles for SLs in plant growth and development were reported, including its regulation of stem secondary growth (Agusti et al 2011), leaf senescence (Yamada et al 2014) and root development (Koltai, 2011;Koren et al 2013). In addition to physiological research, important progress in SL biosynthesis and its perception molecular mechanism has been made in the past several years.…”

Strigolactone Involvement in Root Development, Response to Abiotic Stress, and Interactions with the Biotic Soil Environment

“…Notably, expression of MAX2 under the SCARE-CROW (SCR) promoter was sufficient to confer a response to GR24 in an max2-1 mutant background for both lateral root formation and cell number in the primary root meristem (Koren et al, 2013). Because SCR is expressed mainly in the root endodermis and quiescence center (Sabatini et al, 2003), these results point to an important role for the endodermis in strigolactone regulation of root architecture.…”

“…GR24 led to elongation of the primary root and an increase in meristem cell number in an MAX2-dependent manner Koren et al, 2013). Accordingly, under conditions of carbohydrate limitation, a shorter primary root and fewer primary meristem cells were detected in strigolactonedeficient and strigolactone-response mutants compared with the wild type .…”

“…Hence, strigolactones affect root hair length, at least in part, through the auxin and ethylene pathways (Koltai, 2011). Here too, expression of SCR::MAX2 was sufficient to confer root hair elongation in roots in response to GR24 (Koren et al, 2013). Because root hair elongation is regulated in the epidermis, the sufficiency of MAX2 expression under SCR (expressed mainly in the root endodermis and quiescence center) for GR24 sensitivity suggests that strigolactones act noncell autonomously at short range.…”