Rice Genetics IV - IRRI books - International Rice Research Institute

LEA proteins and their effect on stress toleranceLEA proteins were first characterized in cotton as a set of proteins that are highlyaccumulated in embryos at the late stage of seed development (Dure 1981). Subsequently,many LEA proteins or their genes have been characterized from differentplant species (collated by Dure 1992). Based on their common amino acid sequencedomains, LEA proteins have been classified into three major groups. The regions ofhomology among group 3 LEA proteins are composed of tandem repeats of an 11-amino-acid motif that may form an amphiphilic alpha-helix structure (Dure et al 1989,Dure 1993).The correlation between LEA gene expression or LEA protein accumulation andstress tolerance in several nontransgenic plants provides additional evidence supportinga role of LEA proteins in stress tolerance. For example, in severely dehydratedwheat seedlings, the accumulation of high levels of group 3 LEA proteins was correlatedwith tissue dehydration tolerance (Ried and Walker-Simmons 1993). Studies ofseveral indica varieties of rice (Oryza sativa L.) showed that the levels of group 2LEA proteins and group 3 LEA proteins in roots were significantly higher, or inducedby ABA and salt stress, only in naturally occurring salt-tolerant varieties as comparedwith salt-sensitive varieties (Moons et al 1995).A barley group 3 LEA protein, HVA1 (related to the cotton D-7 protein and carrotDc3 protein), accumulates to high levels in the aleurone layer and embryos duringlate seed development, correlating with the seed desiccation stage (Hong et al 1988).Expression of the HVA1 gene is rapidly induced in young seedlings by ABA and byseveral stress conditions, including dehydration, salt, and extreme temperature (Honget al 1992). A 26-kDa group 3 LEA protein was induced by ABA and salt stress in asalt-tolerant indica rice variety (Moons et al 1995).We have taken a transgenic approach to investigating the function of the HVA1protein in stress protection (Xu et al 1996). The HVA1 from barley, joined to the riceAct1 promoter in a suitable plasmid, was introduced into rice suspension cells usingthe biolistic-mediated transformation method, and a large number of independenttransgenic rice plants were generated. Expression of the barley HVA1 gene in transgenicrice led to high-level, constitutive accumulation of the HVA1 protein in both leavesand roots of transgenic rice plants. Second-generation transgenic rice plants showedsignificantly increased tolerance for water deficit and salinity. Transgenic rice plantsmaintained higher growth rates than nontransformed control plants under stress conditions.The increased tolerance was also reflected by delayed development of damagesymptoms caused by stress and by improved recovery upon the removal of stressconditions. Thus, by using a transgenic approach, this study provides direct evidencesupporting the hypothesis that LEA proteins play an important role in the protectionof plants under water- or salt-stress conditions (Xu et al 1996).Group 1 LEA proteins are characterized by their high glycine content and aminoacids with charged R-groups (ca. 40%) (Stacey et al 1995). Group 1 LEA proteinsalso contain a hydrophilic 20-amino-acid sequence that occurs in tandem one to fourtimes in the various proteins (Dure et al 1989). Group 2 LEA proteins, which are alsoreferred to as dehydrins, are characterized by a highly conserved sequenceTransgenic approaches for generating rice . . . 425