BBX21 reduces abscisic acid sensitivity, mesophyll conductance and chloroplast electron transport capacity to increase photosynthesis and water use efficiency in potato plants cultivated under moderated drought

SUMMARY The B‐box (BBX) proteins are zinc‐finger transcription factors with a key role in growth and developmental regulatory networks mediated by light. AtBBX21 overexpressing (BBX21‐OE) potato ( Solanum tuberosum ) plants, cultivated in optimal water conditions, have a higher photosynthesis rate and stomatal conductance without penalty in water use efficiency (WUE) and with a higher tuber yield. In this work, we cultivated potato plants in two water regimes: 100 and 35% field capacity of water restriction that imposed leaf water potentials between −0.3 and −1.2 MPa for vegetative and tuber growth during 14 or 28 days, respectively. We found that 42‐day‐old plants of BBX21‐OE were more tolerant to water restriction with higher levels of chlorophylls and tuber yield than wild‐type spunta (WT) plants. In addition, the BBX21‐OE lines showed higher photosynthesis rates and WUE under water restriction during the morning. Mechanistically, we found that BBX21‐OE lines were more tolerant to moderated drought by enhancing mesophyll conductance ( g m ) and maximum capacity of electron transport ( J max ), and by reducing abscisic acid (ABA) sensitivity in plant tissues. By RNA‐seq analysis, we found 204 genes whose expression decreased by drought in WT plants and expressed independently of the water condition in BBX21‐OE lines as SAP12, MYB73, EGYP1, TIP2‐1 and DREB2A , and expressions were confirmed by quantitative polymerase chain reaction. These results suggest that BBX21 interplays with the ABA and growth signaling networks, improving the photosynthetic behavior in suboptimal water conditions with an increase in potato tuber yield.