An Abscisic Acid-Induced Protein, HVA22, Inhibits Gibberellin-Mediated Programmed Cell Death in Cereal Aleurone Cells

Woei-Jiun Guo,Tuan-Hua David Ho

doi:10.1104/pp.108.120238

Woei-Jiun Guo, Tuan-Hua David Ho

Open Access

https://doi.org/10.1104/pp.108.120238

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Abstract

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.

Highlights

Plant HVA22 is a unique abscisic acid (ABA)/stress-induced protein first isolated from barley (Hordeum vulgare) aleurone cells
Studies showed that GA treatment induces programmed cell death (PCD) of barley aleurone protoplasts, which is coordinated with distinct vacuolation of protein storage vacuoles (PSVs), but ABA blocks this process
To conveniently observe changes of PSVs in living cells, a red fluorescent protein, DesRed, was transiently expressed in barley aleurone cells driven by the constitutive maize (Zea mays) ubiquitin promoter (Fig. 1A)

Summary

Introduction

Plant HVA22 is a unique abscisic acid (ABA)/stress-induced protein first isolated from barley (Hordeum vulgare) aleurone cells. The regulation of HVA22 expression during plant development is well characterized In both barley and Arabidopsis (Arabidopsis thaliana), transcripts of HVA22 homologs in leaves are highly induced by ABA, drought, cold, and salt stresses (Shen et al, 1993, 2001; Chen et al, 2002). The HVA22 transcript level remains high in dormant grains, in which GA treatment is able to break dormancy with a concomitant decline of HVA22 transcripts These results suggest that turnover of HVA22 gene products in aleurone layers controls seed dormancy and germination (Shen et al, 1993). How plant HVA22 proteins function at the molecular and cellular levels in regulating seed germination is unknown

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