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An Abscisic Acid-Induced Protein, HVA22, Inhibits Gibberellin-Mediated Programmed Cell Death in Cereal Aleurone Cells^1,[W],[OA]

Institute of Plant and Microbial Biology, Academia Sinica, Taipei 115, Taiwan

Plant HVA22 is a unique abscisic acid (ABA)/stress-induced protein first isolated from barley (Hordeum vulgare) aleurone cells. Its yeast homolog, Yop1p, functions in vesicular trafficking and in the endoplasmic reticulum (ER) network in vivo. To examine the roles of plant HVA22, barley HVA22 was ectopically expressed in barley aleurone cells. Overexpression of HVA22 proteins inhibited gibberellin (GA)-induced formation of large digestive vacuoles, which is an important aspect of GA-induced programmed cell death in aleurone cells. The effect of HVA22 was specific, because overexpression of green fluorescent protein or another ABA-induced protein, HVA1, did not lead to the same effect. HVA22 acts downstream of the transcription factor GAMyb, which activates programmed cell death and other GA-mediated processes. Moreover, expression of HVA22:green fluorescent protein fusion proteins showed network and punctate fluorescence patterns, which were colocalized with an ER marker, BiP:RFP, and a Golgi marker, ST:mRFP, respectively. In particular, the transmembrane domain 2 was critical for protein localization and stability. Ectopic expression of the most phylogenetically similar Arabidopsis (Arabidopsis thaliana) homolog, AtHVA22D, also resulted in the inhibition of vacuolation to a similar level as HVA22, indicating function conservation between barley HVA22 and some Arabidopsis homologs. Taken together, we show that HVA22 is an ER- and Golgi-localized protein capable of negatively regulating GA-mediated vacuolation/programmed cell death in barley aleurone cells. We propose that ABA induces the accumulation of HVA22 proteins to inhibit vesicular trafficking involved in nutrient mobilization to delay coalescence of protein storage vacuoles as part of its role in regulating seed germination and seedling growth.

² Present address: Department of Biology, Washington University, St. Louis, MO 63017.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Tuan-Hua David Ho (ho{at}biology.wustl.edu).

^[W] The online version of this article contains Web-only data.

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.108.120238

^* Corresponding author; e-mail ho{at}biology.wustl.edu.

Received March 31, 2008; accepted June 19, 2008; published June 26, 2008.