Arabidopsis IQM4, a Novel Calmodulin-Binding Protein, Is Involved With Seed Dormancy and Germination in Arabidopsis

Abstract

Seed dormancy and germination are regulated by complex mechanisms controlled by diverse hormones and environmental cues. Abscisic acid (ABA) promotes seed dormancy and inhibits seed germination and post-germination growth. Calmodulin (CaM) signals are involved with the inhibition of ABA during seed germination and seedling growth. In this study, we showed that <i>Arabidopsis thaliana</i> IQM4 could bind with calmodulin 5 (CaM5) both <i>in vitro</i> and <i>in vivo</i>, and that the interaction was the Ca²⁺-independent type. The IQM4 protein was localized in the chloroplast and the <i>IQM4</i> gene was expressed in most tissues, especially the embryo and germinated seedlings. The T-DNA insertion mutants of <i>IQM4</i> exhibited the reduced primary seed dormancy and lower ABA levels compared with wild type seeds. Moreover, <i>IQM4</i> plays key roles in modulating the responses to ABA, salt, and osmotic stress during seed germination and post-germination growth. T-DNA insertion mutants exhibited ABA-insensitive and salt-hypersensitive phenotypes during seed germination and post-germination growth, whereas <i>IQM4</i>-overexpressing lines had ABA- and osmotic-hypersensitive, and salt-insensitive phenotypes. Gene expression analyses showed that mutation of <i>IQM4</i> inhibited the expression of ABA biosynthetic genes <i>NCED6</i> and <i>NCED9</i>, and seed maturation regulators <i>LEC1, LEC2, ABI3</i>, and <i>ABI5</i> during the silique development, as well as promoted the expression of <i>WRKY40</i> and inhibited that of <i>ABI5</i> in ABA-regulated seed germination. These observations suggest that IQM4 is a novel Ca²⁺-independent CaM-binding protein, which is positively involved with seed dormancy and germination in <i>Arabidopsis</i>.