Phytohormone-Mediated Homeostasis of Root System Architecture

Unlike animals, most of the plants are sessile. This may be a reason why they developed the powerful ability of organ generation throughout their lifetime, which is distinct from the animals, whose generation potential is restricted in a certain period during development. Half part of the plant body, the root system, is hidden under the ground, where there is a competition of resources, for example, water and nutrients or biotic stresses and abiotic stresses surrounding the root system. With its strong regeneration ability, the architecture of the root system is shaped by all of these environmental cues together with the internal developmental signals. In this process, phytohormones work as the regulatory molecules mediating the internal and external developmental signals, thus controlling the morphology and function of the root system architecture. This chapter introduces the development of root system regulated by various phytohormones, like auxin, cytokinin, etc

Arabidopsis RPT2a, 19S Proteasome Subunit, Regulates Gene Silencing via DNA Methylation

The ubiquitin/proteasome pathway plays a crucial role in many biological processes. Here we report a novel role for the Arabidopsis 19S proteasome subunit RPT2a in regulating gene activity at the transcriptional level via DNA methylation. Knockout mutation of the RPT2a gene did not alter global protein levels; however, the transcriptional activities of reporter transgenes were severely reduced compared to those in the wild type. This transcriptional gene silencing (TGS) was observed for transgenes under control of either the constitutive CaMV 35S promoter or the cold-inducible RD29A promoter. Bisulfite sequencing analysis revealed that both the transgene and endogenous RD29A promoter regions were hypermethylated at CG and non-CG contexts in the rpt2a mutant. Moreover, the TGS of transgenes driven by the CaMV 35S promoters was released by treatment with the DNA methylation inhibitor 5-aza-2′-deoxycytidine, but not by application of the inhibitor of histone deacetylase Trichostatin A. Genetic crosses with the DNA methyltransferase met1 single or drm1drm2cmt3 triple mutants also resulted in a release of CaMV 35S transgene TGS in the rpt2a mutant background. Increased methylation was also found at transposon sequences, suggesting that the 19S proteasome containing AtRPT2a negatively regulates TGS at transgenes and at specific endogenous genes through DNA methylation

Differential expression of the auxin primary response gene MASSUGU2/IAA19during tropic responses of Arabidopsis hypocotyls

We have examined the expression pattern of an auxin primary response gene, MSG2/IAA19, during photo- and gravitropic responses of hypocotyls using a transgenic Arabidopsis harboring MSG2/IAA19 promoter::GUS. The upper portion of most etiolated hypocotyls showed uniform b-glucuronidase (GUS) staining with the strongest activity in the pericycle. When hypocotyls were irradiated with unilateral blue light, GUS activity on the concave side of hypocotylswas decreased, resulting in differentialGUSstaining with a stronger signal on the convex side. The number of differentially stained hypocotyls peaked at 24 h after the onset of the phototropic stimuli, while hypocotyl curvature continued to increase for the entire 36-h experimental period. This result suggests that the MSG2/IAA19 expression precedes the phototropic responses. When seedlings were grown under dim white light, their hypocotyls displayed almost no GUS activity. The light-grown hypocotyls also showed differential GUS staining after phototropic stimuli as result of the increase in GUS activity on the convex side of hypocotyls, especially in the epidermis, the outer cortex and pericycle, although GUS activity was much weaker than that observed in etiolated hypocotyls. Similar but less obvious differential staining was obtained for gravitropic response of hypocotyls. Considering the recent finding thatAux/IAA proteins are immediate targets of the auxin F box receptors, MSG2/IAA19 is likely to act as one of master genes for tropic responses

Overexpression of the RADICAL-INDUCED CELL DEATH1 (RCD1) Gene of Arabidopsis Causes Weak rcd1 Phenotype with Compromised Oxidative-Stress Responses

rcd1 is a mutant of Arabidopsis thaliana that is more resistant to methyl viologen, but more sensitive to ozone than the wild type. rcd1-2 is caused by a single nucleotide substitution that results in a premature stop codon at Trp-332. The rcd1-2 mRNA level does not change significantly with the mutation. Since overexpression of rcd1-1 cDNA has been shown to bring about an rcd1-like phenotype, we created and examined the overexpression lines of RCD1 by the use of the cauliflower mosaic virus 35S promoter. The transgenic lines exhibited a weak rcd1-like phenotype, although no resistance to methyl viologen was observed. Further, they fully complemented the aberrant rcd1-2 phenotype. Subcellular localization of RCD1 was examined by transiently expressing green fluorescent protein (GFP) fused with RCD1 in onion epidermal cells. GFP signals are observed as aggregated foci in the inner nuclear matrix-like region

Changes in growth kinetics of stamen filaments cause inefficient pollination in massugu2, an auxin insensitive, dominant mutant of Arabidopsis thaliana

We investigated the physiological and molecular basis of lower fecundity of massugu2 (msg2), which is a dominant mutant of an auxin primary response gene, IAA19, in Arabidopsis thaliana. By measuring the length of all stamens and pistils in inflorescences and the reference growth rate of pistils, we constructed growth curves of pistils and stamens between stages 12 and 15 of flower development. Pistil growth was found to consist of a single exponential growth, while stamen growth consisted of three exponential phases. During the second exponential phase, the growth rate of stamen filaments was ～10 times greater than the growth rates in the other two phases. Consequently, stamens whose growth was initially retarded grew longer than the pistil, putting pollen grains on the stigma. msg2-1 stamens, on the other hand, exhibited a less obvious growth increase, resulting in less frequent contact between anthers and stigma. MSG2 was expressed in the stamen filaments and its expression almost coincided with the second growth phase. Stamen filaments appeared to elongate by cell elongation rather than cell division in the epidermal cell file. Considering that MSG2 is likely to be a direct target of the auxin F-box receptors, MSG2 may be one of the master genes that control the transient growth increase of stamen filaments