Silencing of class I small heat shock proteins affects seed-related attributes and thermotolerance in rice seedlings

Silencing of CI-sHsps by RNAi negatively affected the seed germination process and heat stress response of rice seedlings. Seed size of RNAi^CI-sHsp was reduced as compared to wild-type plants. Small heat shock proteins (sHsps) are the ATP-independent chaperones ubiquitously expressed in response to diverse environmental and developmental cues. Cytosolic sHsps constitute the major repertoire of sHsp family. Rice cytosolic class I (CI)-sHsps consists of seven members (Hsp16.9A, Hsp16.9B, Hsp16.9C, Hsp17.4, Hsp17.7, Hsp17.9A and Hsp18). Purified OsHsp17.4 and OsHsp17.9A proteins exhibited chaperone activity by preventing formation of large aggregates with model substrate citrate synthase. OsHsp16.9A and OsHsp17.4 showed nucleo-cytoplasmic localization, while the localization of OsHsp17.9A was preferentially in the nucleus. Transgenic tobacco plants expressing OsHsp17.4 and OsHsp17.9A proteins and Arabidopsis plants ectopically expressing OsHsp17.4 protein showed improved thermotolerance to the respective trans-hosts during the post-stress recovery process. Single hairpin construct was designed to generate all CI-sHsp silenced (RNAi^CI-sHsp) rice lines. The major vegetative and reproductive attributes of the RNAi^CI-sHsp plants were comparable to the wild-type rice plants. Basal and acquired thermotolerance response of RNAi^CI-sHsp seedlings of rice was mildly affected. The seed length of RNAi^CI-sHsp rice plants was significantly reduced. The seed germination process was delayed and seed thermotolerance of RNAi^CI-sHsp was negatively affected than the non-transgenic seeds. We, thus, implicate that sHsp genes are critical in seedling thermotolerance and seed physiology.