Poster: Late and Moved Abstracts

Abs # 970: AtNOS1: a plant nitric oxide synthase critical for plant growth and ABA signaling in guard cell

Presenter:	Guo, Fang-Qing , fguo@biomail.ucsd.edu
Authors	Guo, Fang-Qing  (A)   Crawford, Nigel  (A)
Affiliations:	(A): University of California at San Diego

An Arabidopsis gene, which we name AtNOS1, encodes a protein with sequence similarity to a gene involved in nitric oxide (NO) synthesis in Helix pomatia (snail). Neither AtNOS1 nor the snail protein has any similarity to the known NOS isoforms from animals (iNOS, eNOS and nNOS), and the action of the snail protein is not known. To determine the function of AtNOS1 in Arabidopsis, we characterized a homozygous mutant with a T-DNA insertion in the first exon of AtNOS1. The mutants are impaired in NO production measured by in vivo staining of roots and in vitro NOS assays of leaf extracts. Purified AtNOS1 produced in E. coli used arginine and NADPH as substrates and was activated by Ca2+ and calmodulin, properties similar to those found for mammalian eNOS and nNOS. The mutant Atnos1 has many phenotypes: the first true leaves are yellow, and the plants have reduced shoot, root and inflorescence growth and poor fertility. Expression of an AtNOS1 cDNA in the mutant complemented the poor growth and fertility and leaf greening phenotypes as did treatment of the plants with a NO donor. We also examined ABA signaling in stomates of the mutant. In response to drought stress, plants accumulate the phytohormone ABA, which enhances NO synthesis and induces stomatal closure. The Atnos1 mutant is defective in ABA-induced NO generation and stomatal closure. This finding indicates that AtNOS1-dependent NO synthesis occurs downstream of ABA in this signaling pathway, and AtNOS1 is a new component critical for ABA signaling in guard cell.