Kyoko Baba

Activity–dormancy transition in the cambial meristem involves stage-specific modulation of auxin response in hybrid aspen

The molecular basis of short-day–induced growth cessation and dormancy in the meristems of perennial plants (e.g., forest trees growing in temperate and high-latitude regions) is poorly understood. Using global transcript profiling, we show distinct stage-specific alterations in auxin responsiveness of the transcriptome in the stem tissues during short-day–induced growth cessation and both the transition to and establishment of dormancy in the cambial meristem of hybrid aspen trees. This stage-specific modulation of auxin signaling appears to be controlled via distinct mechanisms. Whereas the induction of growth cessation in the cambium could involve induction of repressor auxin response factors (ARFs) and down-regulation of activator ARFs, dormancy is associated with perturbation of the activity of the SKP-Cullin-F-boxTIR (SCFTIR) complex, leading to potential stabilization of repressor auxin (AUX)/indole-3-acetic acid (IAA) proteins. Although the role of hormones, such as abscisic acid (ABA) and gibberellic acid (GA), in growth cessation and dormancy is well established, our data now implicate auxin in this process. Importantly, in contrast to most developmental processes in which regulation by auxin involves changes in cellular auxin contents, day-length–regulated induction of cambial growth cessation and dormancy involves changes in auxin responses rather than auxin content.

Characterization of dynamics of the psbD light-induced transcription in mature wheat chloroplasts

Dynamical aspects of three chloroplast promoters responding to change in light condition were examined in mature chloroplasts of wheat (Triticum aestivum) by in vitro transcription. The wheat psbD/C operon has four distinct promoters, two of which named as D/C-3 and D/C-4 promoters dominantly function in mature chloroplasts to produce the mRNAs encoding D2/CP43 and CP43 alone, respectively. Activity of the D/C-3 promoter in mature chloroplasts was reduced to less than 30% by 24 h dark adaptation and recovered by re-illumination to the original level within 30 to 60 min. The activation of the D/C-3 promoter which requires de novo cytoplasmic protein synthesis was induced by low fluence of light (e.g. 16 µE m-2 s-1), but the extent of activation increased with increasing light fluence. The accumulation of mRNAs from the D/C-3 promoter saturated at 2- to 3-fold higher level within 2 h when the dark-adapted seedlings were transferred to the lig at 72 µE m-2 s-1, concomitant with the increase in rate of D2 synthesis, suggesting that synthesis of D2 in mature chloroplasts is controlled via the D/C-3 promoter activity in a light-dependent way. Activity of the D/C-4 promoter slightly increased in the dark and decreased in the light. Effect of light on the psbA promoter activity was not observed at all in mature chloroplasts. In vitro transcriptional analysis of the D/C-3 promoter with 5′ deletion mutations revealed that at least two cis elements which are located within the sequences of -78 to -47 and -46 to -29 of the transcription initiation site, respectively, act as enhancing elements in the D/C-3 promoter. The light-switching element of the transcription, however, was suggested to be located in the core promoter sequence downstream of the -35 element.

Cloning of a Novel Protein Which Binds to the Chloroplast psbD LRP Upstream Sequence by the One-Hybrid System

The chloroplast psbD gene which encodes the D2 protein of PS II has a unique light responsive promoter (LRP). Transcription levels of most chloroplast genes which are involved in photosynthesis increase irreversibly in the early stage of light-dependent development of chloroplasts. However, the psbD LRP of wheat is inactive in darkness and active in light in mature chloroplasts (1). In Arabidopsis, mRNA from the LRP accumulates in a later stage of chloroplast maturation compared to other promoters in the psbD gene cluster or promoters of psbA and rbcL (2). The mechanism which brings about the unique feature of LRP is not fully understood.

Selective Activation of the psb A and psb D Promoters in Matured Wheat Chloroplasts

Chloroplast genes of higher plants are transcribed by at least two types of plastid RNA polymerases, eubacterial RNA polymerase of which core subunits (α, β, β′ and β″)are encoded by the plastid genome (plastid-encoded RNA polymerase; PEP) and phage type nuclear encoded RNA polymerases (NEP). PEP promoters are reminiscent of the E. coli σ70 promoters with -35 and -10 consensus elements that are spaced 17–19 bases apart. Photosynthesis genes have been shown to be transcribed by PEP[1]. Transcription from most of PEP promoters increases during chloroplast development and declines after chloroplast maturation[2]. Exceptions are the promoters for the psbA and psbD genes encoding PSII reaction center proteins (D1 and D2, respectively) which turn over rapidly in a light-dependent manner[3]. Transcriptions of both genes were maintained highly active in the developed wheat chloroplasts under illumination[4], suggesting presence of gene specific mechanism to activate these promoters.