Genome-Wide Identification of the Hsp20 Family Responding to Heat Stress in Sunflower (Helianthus annuus L.)

Small heat shock proteins (Hsp20s) function as essential molecular chaperones in plant stress responses, yet their genome-wide characterization in sunflower (Helianthus annuus L.) remains lacking and their functional role in heat response is also unknown. In this study, 65 HaHsp20 genes were identified in sunflower through a comprehensive genome-wide analysis based on the conserved ACD (α-crystallin) domain. The expansion of this family was primarily driven by whole-genome duplication (WGD) or segmental duplication events, with the CI subfamily (20 members) representing the most significantly expanded lineage-specific clade. While all HaHsp20 proteins harbor the conserved α-crystallin domain (ACD), they exhibit diverse molecular weights (11.31–53.35 kDa), isoelectric points (4.71–9.75), and subcellular localization patterns. Promoter cis-regulatory element analysis revealed a predominance of ABA and MeJA-responsive elements but only two canonical heat shock elements. Transcriptome and RT-qPCR analyses revealed that most HaHsp20 genes are responsive to heat stress, with seven HaHsp20 genes exhibiting extremely upregulated expression (more than 1000-fold) after 10 h of 45 °C treatment. Among these, HaHsp21.59 and HaHsp25.91 showed an increase of over 4000-fold in expression. These findings provide a comprehensive foundation for understanding the evolutionary history and expression dynamics of the HaHsp20 family in sunflower, and highlight HaHsp21.59 and HaHsp25.91 as promising candidate genes for future functional validation of their potential roles in heat stress tolerance.