Significance of NatB-mediated N-terminal acetylation of auxin biosynthetic enzymes in maintaining auxin homeostasis in Arabidopsis thaliana.

The auxin IAA (Indole-3-acetic acid) plays key roles in regulating plant growth and development, which depends on an intricate homeostasis that is determined by the balance between its biosynthesis, metabolism and transport. YUC flavin monooxygenases catalyze the rate-limiting step of auxin biosynthesis via IPyA (indole pyruvic acid) and are critical targets in regulating auxin homeostasis. Despite of numerous reports on the transcriptional regulation of YUC genes, little is known about those at the post-translational protein level. Here, we show that loss of function of CKRC3/TCU2, the auxiliary subunit (Naa25) of Arabidopsis NatB, and/or of its catalytic subunit (Naa20), NBC, led to auxin-deficiency in plants. Experimental evidences show that CKRC3/TCU2 can interact with NBC to form a NatB complex, catalyzing the N-terminal acetylation (NTA) of YUC proteins for their intracellular stability to maintain normal auxin homeostasis in plants. Hence, our findings provide significantly new insight into the link between protein NTA and auxin biosynthesis in plants.

Effects of auxin and ethylene on root growth adaptation to different ambient temperatures in Arabidopsis.

As sessile organisms, plants can modify their growth strategy in response to different temperatures, however very little is known about how roots growth responds to ambient temperature change. Here, we found that high temperature-induced root elongation is dependent on light intensity and the root growth of most TAA1 loss-of-function mutants is more sensitive to higher temperatures in Arabidopsis. TAA1 encodes a tryptophan aminotransferase which involved in the indole-3-pyruvic acid (IPA) pathway of indole-3-acetic acid (IAA) biosynthesis. The root elongation in ckrc1-1(one allele mutant of TAA1) is less sensitive to lower temperatures and more sensitive to higher temperatures than that of Col-0. By comparing the regulatory mechanisms of ckrc1-1 root growth at different temperatures (17 °C, 22 °C, and 27 °C), different interactions between signals (auxin and ethylene) and the effects of downstream genes were observed at different ambient temperatures in Arabidopsis. Lower temperature-enhanced ETR1-mediated ethylene signaling did not promote the expression of CKRC1, while higher temperature-enhanced signaling did. CKRC1 had an important role in the ACC inhibition of cell elongation at 22 °C and 27 °C but not at 17 °C. CKRC1-dependent auxin biosynthesis was critical for maintaining PIN1, PIN2, and AUX1 expression at lower temperatures. CKRC1, AUX1, and PIN2 regulated root elongation by affecting different regions of the root at different temperatures in Arabidopsis. Our experimental results suggested that changes in the in vivo signals at different temperatures were multi-layered in Arabidopsis.

Plant molecular responses to the elevated ambient temperatures expected under global climate change.

Environmental temperatures affect plant distribution, growth, and development. The Intergovernmental Panel on Climate Change (IPCC) predicts that global temperatures will rise by at least 1.5°C by the end of this century. Global temperature changes have already had a discernable impact on agriculture, phenology, and ecosystems. At the molecular level, extensive literature exists on the mechanism controlling plant responses to high temperature stress. However, few studies have focused on the molecular mechanisms behind plant responses to mild increases in ambient temperature. Previous research has found that moderately higher ambient temperatures can induce hypocotyl elongation and early flowering. Recent evidence demonstrates roles for the phytohormones auxin and ethylene in adaptive growth of plant roots to slightly higher ambient temperatures.

Adaptation of root growth to increased ambient temperature requires auxin and ethylene coordination in Arabidopsis.

KEY MESSAGE: A fresh look at the roles of auxin, ethylene, and polar auxin transport during the plant root growth response to warmer ambient temperature (AT). The ambient temperature (AT) affects plant growth and development. Plants can sense changes in the AT, but how this change is transduced into a plant root growth response is still relatively unclear. Here, we found that the Arabidopsis ckrc1-1 mutant is sensitive to higher AT. At 27 °C, the ckrc1-1 root length is significantly shortened and the root gravity defect is enhanced, changes that can be restored with addition of 1-naphthaleneacetic acid, but not indole-3-acetic acid (IAA). AUX1, PIN1, and PIN2 are involved in the ckrc1-1 root gravity response under increased AT. Furthermore, CKRC1-dependent auxin biosynthesis was critical for maintaining PIN1, PIN2, and AUX1 expression at elevated temperatures. Ethylene was also involved in this regulation through the ETR1 pathway. Higher AT can promote CKRC1-dependent auxin biosynthesis by enhancing ETR1-mediated ethylene signaling. Our research suggested that the interaction between auxin and ethylene and that the interaction-mediated polar auxin transport play important roles during the plant root growth response to higher AT.