Distinct Characteristics of Indole-3-Acetic Acid and Phenylacetic Acid, Two Common Auxins in Plants.

The phytohormone auxin plays a central role in many aspects of plant growth and development. IAA is the most studied natural auxin that possesses the property of polar transport in plants. Phenylacetic acid (PAA) has also been recognized as a natural auxin for >40 years, but its role in plant growth and development remains unclear. In this study, we show that IAA and PAA have overlapping regulatory roles but distinct transport characteristics as auxins in plants. PAA is widely distributed in vascular and non-vascular plants. Although the biological activities of PAA are lower than those of IAA, the endogenous levels of PAA are much higher than those of IAA in various plant tissues in Arabidopsis. PAA and IAA can regulate the same set of auxin-responsive genes through the TIR1/AFB pathway in Arabidopsis. IAA actively forms concentration gradients in maize coleoptiles in response to gravitropic stimulation, whereas PAA does not, indicating that PAA is not actively transported in a polar manner. The induction of the YUCCA (YUC) genes increases PAA metabolite levels in Arabidopsis, indicating that YUC flavin-containing monooxygenases may play a role in PAA biosynthesis. Our results provide new insights into the regulation of plant growth and development by different types of auxins

Distinct Characteristics of Indole-3-Acetic Acid and Phenylacetic Acid, Two Common Auxins in Plants

The phytohormone auxin plays a central role in many as-pects of plant growth and development. IAA is the most studied natural auxin that possesses the property of polar transport in plants. Phenylacetic acid (PAA) has also been recognized as a natural auxin for>40 years, but its role in plant growth and development remains unclear. In this study, we show that IAA and PAA have overlapping regula-tory roles but distinct transport characteristics as auxins in plants. PAA is widely distributed in vascular and non-vascular plants. Although the biological activities of PAA are lower than those of IAA, the endogenous levels of PAA are much higher than those of IAA in various plant tissues in Arabidopsis. PAA and IAA can regulate the same set o

Cytokinin receptors in sporophytes are essential for male and female functions in Arabidopsis thaliana

Arabidopsis has three cytokinin receptors genes: CRE1, AHK2 and AHK3. Availability of plants that are homozygous mutant for these three genes indicates that cytokinin receptors in the haploid cells are dispensable for the development of male and female gametophytes. The triple mutants form a few flowers but never set seed, indicating that reproductive growth is impaired. We investigated which reproductive processes are affected in the triple mutants. Anthers of mutant plants contained fewer pollen grains and did not dehisce. Pollen in the anthers completed the formation of the one vegetative nucleus and the two sperm nuclei, as seen in wild type. The majority of the ovules were abnormal: 78% lacked the embryo sac, 10% carried a female gametophyte that terminated its development before completing three rounds of nuclear division, and about 12% completed three rounds of nuclear division but the gametophytes were smaller than those of the wild type. Reciprocal crosses between the wild type and the triple mutants indicated that pollen from mutant plants did not germinate on wild-type stigmas, and wild-type pollen did not germinate on mutant stigmas. These results suggest that cytokinin receptors in the sporophyte are indispensable for anther dehiscence, pollen maturation, induction of pollen germination by the stigma and female gametophyte formation and maturation