At high ambient temperature, plants display dramatic stem elongation in an adaptive response to heat. This response is mediated by elevated levels of the phytohormone auxin and requires auxin biosynthesis, signaling, and transport pathways. The mechanisms by which higher temperature results in greater auxin accumulation are unknown, however. A basic helix-loop-helix transcription factor, PHYTOCHROME-INTERACTING FACTOR 4 (PIF4), is also required for hypocotyl elongation in response to high temperature. PIF4 also acts redundantly with its homolog, PIF5, to regulate diurnal growth rhythms and elongation responses to the threat of vegetative shade. PIF4 activity is reportedly limited in part by binding to both the basic helix-loop-helix protein LONG HYPO-COTYL IN FAR RED 1 and the DELLA family of growth-repressing proteins. Despite the importance of PIF4 in integrating multiple environmental signals, the mechanisms by which PIF4 controls growth are unknown. Here we demonstrate that PIF4 regulates levels of auxin and the expression of key auxin biosynthesis genes at high temperature. We also identify a family of SMALL AUXIN UP RNA (SAUR) genes that are expressed at high temperature in a PIF4-dependent manner and promote elongation growth. Taken together, our results demonstrate direct molecular links among PIF4, auxin, and elongation growth at high temperature.indole-3-acetic acid | TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS 1 | CYP79B2 T he hormone indole-3-acetic acid (IAA, or auxin) is fundamental to plant growth and development, controlling many key aspects of shoot and root growth (1). When plants are grown at elevated temperatures, IAA levels increase, resulting in increased hypocotyl elongation (2, 3). Although genetic studies in Arabidopsis have demonstrated that this growth response is dependent on auxin biosynthesis, signaling, and transport pathways, precisely how high temperature promotes an increase in auxin levels has not been established. There are multiple pathways for the de novo synthesis of IAA, the major naturally occurring plant auxin, which are often classified according to whether or not they require the precursor tryptophan (4). Although progress has been made in elucidating some of the enzymes involved in IAA biosynthesis, our understanding of these pathways and their regulation remains rudimentary.In addition to auxin, the basic helix-loop-helix transcription factor PHYTOCHROME-INTERACTING FACTOR 4 (PIF4) is also required for temperature-dependent hypocotyl elongation (5, 6). PIF4 has emerged as a key regulator of elongation in response to external signals, such as temperature and light, as well as internal signals, including gibberellin and the circadian clock (7-12). In the present work, we investigated potential mechanistic links between PIF4 and IAA in the control of temperatureinduced hypocotyl elongation. We found that PIF4 promotes IAA biosynthesis, possibly by activating the expression of key IAA biosynthetic genes in a temperature-dependent manner. Results and DiscussionGiven that s...

The plant hormone auxin promotes cell expansion. Forty years ago, the acid growth theory was proposed, whereby auxin promotes proton efflux to acidify the apoplast and facilitate the uptake of solutes and water to drive plant cell expansion. However, the underlying molecular and genetic bases of this process remain unclear. We have previously shown that the SAUR19-24 subfamily of auxin-induced SMALL AUXIN UP-RNA (SAUR) genes promotes cell expansion. Here, we demonstrate that SAUR proteins provide a mechanistic link between auxin and plasma membrane H + -ATPases

SUMMARY The plant hormone auxin controls numerous aspects of plant growth and development by regulating the expression of hundreds of genes. SMALL AUXIN UP RNA (SAUR) genes comprise the largest family of auxin-responsive genes, but their function is unknown. Although prior studies have correlated the expression of some SAUR genes with auxin-mediated cell expansion, genetic evidence implicating SAURs in cell expansion has not been reported. The Arabidopsis SAUR19, SAUR20, SAUR21, SAUR22, SAUR23, and SAUR24 (SAUR19– 24) genes encode a subgroup of closely related SAUR proteins. We demonstrate that these SAUR proteins are highly unstable in Arabidopsis. However, the addition of an N-terminal GFP or epitope tag dramatically increases the stability of SAUR proteins. Expression of these stabilized SAUR fusion proteins in Arabidopsis confers numerous auxin-related phenotypes indicative of increased and/or unregulated cell expansion, including increased hypocotyl and leaf size, defective apical hook maintenance, and altered tropic responses. Furthermore, seedlings expressing an artificial microRNA targeting multiple members of the SAUR19–24 subfamily exhibit short hypocotyls and reduced leaf size. Together, these findings demonstrate that SAUR19– 24 function as positive effectors of cell expansion. This regulation may be achieved through the modulation of auxin transport, as SAUR gain-of-function and loss-of-function seedlings exhibit increased and reduced basipetal indole-3-acetic acid transport, respectively. Consistent with this possibility, SAUR19–24 proteins predominantly localize to the plasma membrane.

The plant hormone auxin regulates numerous growth and developmental processes throughout the plant life cycle. One major function of auxin in plant growth and development is the regulation of cell expansion. Our previous studies have shown that SMALL AUXIN UP RNA (SAUR) proteins promote auxin-induced cell expansion via an acid growth mechanism. These proteins inhibit the PP2C.D family phosphatases to activate plasma membrane (PM) H+-ATPases and thereby promote cell expansion. However, the functions of individual PP2C.D phosphatases are poorly understood. Here, we investigated PP2C.D-mediated control of cell expansion and other aspects of plant growth and development. The nine PP2C.D family members exhibit distinct subcellular localization patterns. Our genetic findings demonstrate that the three plasma membrane-localized members, PP2C.D2, PP2C.D5, and PP2C.D6, are the major regulators of cell expansion. These phosphatases physically interact with SAUR19 and PM H+-ATPases, and inhibit cell expansion by dephosphorylating the penultimate threonine of PM H+-ATPases. PP2C.D genes are broadly expressed and are crucial for diverse plant growth and developmental processes, including apical hook development, phototropism, and organ growth. GFP-SAUR19 overexpression suppresses the growth defects conferred by PP2C.D5 overexpression, indicating that SAUR proteins antagonize the growth inhibition conferred by the plasma membrane-localized PP2C.D phosphatases. Auxin and high temperature upregulate the expression of some PP2C.D family members, which may provide an additional layer of regulation to prevent plant overgrowth. Our findings provide novel insights into auxin-induced cell expansion, and provide crucial loss-of-function genetic support for SAUR-PP2C.D regulatory modules controlling key aspects of plant growth.