Nobuharu Goto

Auxin and Ethylene Response Interactions during Arabidopsis Root Hair Development Dissected by Auxin Influx Modulators

The plant hormones auxin and ethylene have been shown to play important roles during root hair development. However, cross talk between auxin and ethylene makes it difficult to understand the independent role of either hormone. To dissect their respective roles, we examined the effects of two compounds, chromosaponin I (CSI) and 1-naphthoxyacetic acid (1-NOA), on the root hair developmental process in wild-type Arabidopsis, ethylene-insensitive mutantein2-1, and auxin influx mutants aux1-7,aux1-22, and double mutant aux1-7 ein2. β-Glucuronidase (GUS) expression analysis in the BA-GUS transgenic line, consisting of auxin-responsive domains ofPS-IAA4/5 promoter and GUS reporter, revealed that 1-NOA and CSI act as auxin uptake inhibitors in Arabidopsis roots. The frequency of root hairs in ein2-1roots was greatly reduced in the presence of CSI or 1-NOA, suggesting that endogenous auxin plays a critical role for the root hair initiation in the absence of an ethylene response. All of these mutants showed a reduction in root hair length, however, the root hair length could be restored with a variable concentration of 1-naphthaleneacetic acid (NAA). NAA (10 nm) restored the root hair length ofaux1 mutants to wild-type level, whereas 100 nm NAA was needed for ein2-1 andaux1-7 ein2 mutants. Our results suggest that insensitivity in ethylene response affects the auxin-driven root hair elongation. CSI exhibited a similar effect to 1-NOA, reducing root hair growth and the number of root hair-bearing cells in wild-type andein2-1 roots, while stimulating these traits inaux1-7and aux1-7ein2 roots, confirming that CSI is a unique modulator of AUX1.

Arabidopsides A and B, two new oxylipins from Arabidopsis thaliana

Two new oxylipins, arabidopsides A (1) and B (2), were isolated from the aerial parts of Arabidopsis thaliana, and their structures and absolute stereochemistries were elucidated by spectroscopic data and chemical means. Arabidopsides A (1) and B (2) were rare monogalactosyl diacylglycerides containing 12-oxophytodienoic acid and/or dinor-oxophytodienoic acid.

Growth-promoting allelopathic substance exuded from germinating Arabidopsis thaliana seeds

Abstract A potent growth-promoting allelopathic substance was located from the exudates of germinating Arabidopsis thaliana (line Shokei) seeds and identified by analysis of its spectral data as 2-O- l -rahmnopyranosyl-4-deoxy-α- l -threo- hex-4-enopyranosiduronoic acid (lepidimoic acid).

Senescence-promoting effect of arabidopside A

Arabidopside A isolated from Arabidopsis thaliana is a rare oxylipin, containing 12-oxophytodienoic acid (OPDA) and dinor-oxophytodienoic acid (dn-OPDA) which are known as precursors of jasmonic acid (JA) and methyl jasmonate (MeJA). The senescence-promoting effect of arabidopside A was examined by an oat (Avena sativa) leaf assay under dark or continuous light condition. Arabidopside A promoted senescence of oat leaves, and the promoting activity was more effective than for JA and OPDA, and as strong as for MeJA, which was well known to be a senescence promoter. These results suggest that arabidopside A plays important roles in leaf senescence.

Effects of chromosaponin I and brassinolide on the growth of roots in etiolated Arabidopsis seedlings.

Summary The stimulatory effects of chromosaponin I (CSI) on the growth of roots in etiolated Arabidopsis thaliana (ecotype Columbia) seedlings were compared with those of brassinolide (BR) bearing a structural similarity to CSI. The optimum concentrations of CSI and BR were 100 μmol/L and 1 nmol/L, respectively. The roots grew curved on wetted filter paper in the absence of CSI, but elongated straight in the presence of CSI. CSI increased the length of root cells and decreased the diameter of roots. However, BR did not increase the cell length although it decreased the root diameter. Neither CSI- nor BR-induced growth in roots was detected in ethylene-insensitive mutants etrl-1 and ein2-1 , supporting the idea that CSI and BR stimulate the growth of roots by interfering with the action of ethylene. However, the mechanism of CSI action is not the same as that of BR.

Enhanced stomatal conductance by a spontaneous Arabidopsis tetraploid, Me-0, results from increased stomatal size and greater stomatal aperture

The Arabidopsis tetraploid ecotype, Me-0, overcomes the handicap of stomatal opening that is typical for tetraploid plants and achieves a high stomatal conductance. The rate of gas exchange in plants is regulated mainly by stomatal size and density. Generally, higher densities of smaller stomata are advantageous for gas exchange; however, it is unclear what the effect of an extraordinary change in stomatal size might have on a plant’s gas-exchange capacity. We investigated the stomatal responses to CO2 concentration changes among 374 Arabidopsis (Arabidopsis thaliana) ecotypes and discovered that Mechtshausen (Me-0), a natural tetraploid ecotype, has significantly larger stomata and can achieve a high stomatal conductance. We surmised that the cause of the increased stomatal conductance is tetraploidization; however, the stomatal conductance of another tetraploid accession, tetraploid Columbia (Col), was not as high as that in Me-0. One difference between these two accessions was the size of their stomatal apertures. Analyses of abscisic acid sensitivity, ion balance, and gene expression profiles suggested that physiological or genetic factors restrict the stomatal opening in tetraploid Col but not in Me-0. Our results show that Me-0 overcomes the handicap of stomatal opening that is typical for tetraploids and achieves higher stomatal conductance compared with the closely related tetraploid Col on account of larger stomatal apertures. This study provides evidence for whether larger stomatal size in tetraploids of higher plants can improve stomatal conductance.