FIN5 positively regulates far-red light responses in Arabidopsis thaliana

We report the characterization of a semi-dominant mutation fin5-1 (far-red insensitive 5-1) of Arabidopsis, which was isolated from genetic screening of phytochrome A (phyA) signaling components. Plants with the fin5-1 mutation exhibited a long hypocotyl phenotype when grown under far-red (FR) light, but not under red light. Physiological analyses implied that FIN5 might be differentially involved in diverse responses that are regulated by phyA under continuous FR light. Anthocyanin accumulation, gravitropic response of hypocotyl growth, and FR light-preconditioned blocking of greening were also impaired in the fin5-1 mutant, whereas photoperiodic floral induction was not, if at all, significantly affected. Moreover, light-regulated expression of the CHS, PORA and PsbS genes was attenuated in fin5-1 mutant plants, while the light-induced expression of CAB was normal. The mutation exhibited semi-dominance regarding control of hypocotyl growth in FR light. We suggest that FIN5 defines a novel branch in the network of phyA signaling in Arabidopsis.     

ZEITLUPE positively regulates hypocotyl elongation at warm temperature under light in Arabidopsis thaliana

Hypocotyl cell elongation has been studied as a model to understand how cellular expansion contributes to plant organ growth. Hypocotyl elongation is affected by multiple environmental factors, including light quantity and light quality. Red light inhibits hypocotyl growth via the phytochrome signaling pathways. Proteins of the FLAVIN-BINDING KELCH REPEAT F-BOX 1 / LOV KELCH PROTEIN 2 / ZEITLUPE family are positive regulators of hypocotyl elongation under red light in Arabidopsis. These proteins were suggested to reduce phytochrome-mediated inhibition of hypocotyl elongation. Here, we show that ZEITLUPE also functions as a positive regulator in warmth-induced hypocotyl elongation under light in Arabidopsis.     

CAPRICE positively regulates stomatal formation in the Arabidopsis hypocotyl

In the Arabidopsis hypocotyl, stomata develop only from a set of epidermal cell files. Previous studies have identified several negative regulators of stomata formation. Such regulators also trigger non-hair cell fate in the root. Here, it is shown that TOO MANY MOUTHS (TMM) positively regulates CAPRICE (CPC) expression in differentiating stomaless-forming cell files, and that the CPC protein might move to the nucleus of neighbouring stoma-forming cells, where it promotes stomata formation in a redundant manner with TRIPTYCHON (TRY). Unexpectedly, the CPC protein was also localized in the nucleus and peripheral cytoplasm of hypocotyl fully differentiated epidermal cells, suggesting that CPC plays an additional role to those related to stomata formation. These results identify CPC and TRY as positive regulators of stomata formation in the embryonic stem, which increases the similarity between the genetic control of root hair and stoma cell fate determination.Key words: arabidopsis, epidermis, CPC, stomata, TMM     

The mitogen-activated protein kinase phosphatase PHS1 regulates flowering in Arabidopsis thaliana

Planta - Arabidopsis PHS1, initially known as an actor of cytoskeleton organization, is a positive regulator of flowering in the photoperiodic and autonomous pathways by modulating both CO and FLC...     

Photoperiod regulates flower meristem development in Arabidopsis thaliana

Photoperiod has been known to regulate flowering time in many plant species. In Arabidopsis, genes in the long day (LD) pathway detect photoperiod and promote flowering under LD. It was previously reported that clavata2 (clv2) mutants grown under short day (SD) conditions showed suppression of the flower meristem defects, namely the accumulation of stem cells and the resulting production of extra floral organs. Detailed analysis of this phenomenon presented here demonstrates that the suppression is a true photoperiodic response mediated by the inactivation of the LD pathway under SD. Inactivation of the LD pathway was sufficient to suppress the clv2 defects under LD, and activation of the LD pathway under SD conditions restored clv2 phenotypes. These results reveal a novel role of photoperiod in flower meristem development in Arabidopsis. Flower meristem defects of clv1 and clv3 mutants are also suppressed under SD, and 35S:CO enhanced the defects of clv3, indicating that the LD pathway works independently from the CLV genes. A model is proposed to explain the interactions between photoperiod and the CLV genes.     

Phytochrome A regulates the intracellular distribution of phototropin 1-green fluorescent protein in Arabidopsis thaliana

It has been known for decades that red light pretreatment has complex effects on subsequent phototropic sensitivity of etiolated seedlings. Here, we demonstrate that brief pulses of red light given 2 h prior to phototropic induction by low fluence rates of blue light prevent the blue light-induced loss of green fluorescent protein-tagged phototropin 1 (PHOT1-GFP) from the plasma membrane of cortical cells of transgenic seedlings of Arabidopsis thaliana expressing PHOT1-GFP in a phot1-5 null mutant background. This red light effect is mediated by phytochrome A and requires approximately 2 h in the dark at room temperature to go to completion. It is fully far red reversible and shows escape from photoreversibility following 30 min of subsequent darkness. Red light-induced inhibition of blue light-inducible changes in the subcellular distribution of PHOT1-GFP is only observed in rapidly elongating regions of the hypocotyl. It is absent in hook tissues and in mature cells below the elongation zone. We hypothesize that red light-induced retention of the PHOT1-GFP on the plasma membrane may account for the red light-induced increase in phototropic sensitivity to low fluence rates of blue light.     

Telomere rapid deletion regulates telomere length in Arabidopsis thaliana

Telomere length is maintained in species-specific equilibrium primarily through a competition between telomerase-mediated elongation and the loss of terminal DNA through the end-replication problem. Recombinational activities are also capable of both lengthening and shortening telomeres. Here we demonstrate that elongated telomeres in Arabidopsis Ku70 mutants reach a new length set point after three generations. Restoration of wild-type Ku70 in these mutants leads to discrete telomere-shortening events consistent with telomere rapid deletion (TRD). These findings imply that the longer telomere length set point is achieved through competition between overactive telomerase and TRD. Surprisingly, in the absence of telomerase, a subset of elongated telomeres was further lengthened, suggesting that in this background a mechanism of telomerase-independent lengthening of telomeres operates. Unexpectedly, we also found that plants possessing wild-type-length telomeres exhibit TRD when telomerase is inactivated. TRD is stochastic, and all chromosome ends appear to be equally susceptible. The frequency of TRD decreases as telomeres shorten; telomeres less than 2 kb in length are rarely subject to TRD. We conclude that TRD functions as a potent force to regulate telomere length in Arabidopsis.     

High boron-induced ubiquitination regulates vacuolar sorting of the BOR1 borate transporter in Arabidopsis thaliana

Boron homeostasis is important for plants, as boron is essential but is toxic in excess. Under high boron conditions, the Arabidopsis thaliana borate transporter BOR1 is trafficked from the plasma membrane (PM) to the vacuole via the endocytic pathway for degradation to avoid excess boron transport. Here, we show that boron-induced ubiquitination is required for vacuolar sorting of BOR1. We found that a substitution of lysine 590 with alanine (K590A) in BOR1 blocked degradation. BOR1 was mono- or diubiquitinated within several minutes after applying a high concentration of boron, whereas the K590A mutant was not. The K590A mutation abolished vacuolar transport of BOR1 but did not apparently affect polar localization to the inner PM domains. Furthermore, brefeldin A and wortmannin treatment suggested that Lys-590 is required for BOR1 translocation from an early endosomal compartment to multivesicular bodies. Our results show that boron-induced ubiquitination of BOR1 is not required for endocytosis from the PM but is crucial for the sorting of internalized BOR1 to multivesicular bodies for subsequent degradation in vacuoles.     

ERK pathway positively regulates the expression of Sprouty genes

Sprouty was originally identified as an inhibitor of Drosophila development-associated receptor tyrosine kinase (RTK) signaling. Although RTK signaling has been shown to induce Sprouty gene expression, the precise induction pathway downstream of RTK remains unclear. As RTK signaling pathway includes activation of extracellular signal-regulated kinases (ERKs), we have examined a correlation between activation of ERKs and induction of Sprouty gene expression. All reagents which induce the activation of ERKs induce Sprouty gene expression; these agents include not only growth factors which bind to RTK but also phorbol 12-myristate-13-acetate and active Raf-1 kinase. Furthermore, the Sprouty gene expression induced by all those agents is totally suppressed when the cells are pretreated with specific inhibitors of ERK kinase (MEK). Human tumor cells which exhibit constitutive activation of ERKs show elevated expression of Sprouty genes, which is abolished by treatment of these cells with MEK inhibitors. All these findings clearly indicate that Sprouty gene expression is positively regulated by the ERK pathway downstream of RTK.