Characterization of a pigment-deficient mutant of sunflower (Helianthus annuus L.) with abnormal chloroplast biogenesis, reduced PSII activity and low endogenous level of abscisic acid

Nuclear mutation xantha1 (xan1) arose spontaneously in sunflower and it appeared to have many pleiotropic effects. Homozygous xan1/xan1 seedlings, in field conditions, die after depletion of cotyledonary reserves and can be only recovered by selfing normal heterozygous plants (XAN1/xan1). xan1 mutant showed aberrant development of chloroplasts, deficient pigment content and reduced CO2 assimilation rate. Also, xan1 mutation negatively affected endogenous ABA levels both in non-stressed and dehydrated leaves. Photodestruction of both chlorophyll a and -carotene in xan1 leaves directly correlated with light intensity. The activation of the xanthophyll cycle under low light (165 PPFD) and the appreciable amounts of zeaxanthin and antheraxanthin retained by xan1 mutant after a 24 h dark period were clear signals of a strong photoinhibition state. The unusual patterns of thylakoid proteins composition detected in seedlings grown under 165 PPFD, by using both denaturing and native electrophoresis analyses, were also indicative of high sensitivity of the thylakoid polypeptide complexes to light stress. Moreover, immunoblot analysis of psbA gene product (D1 protein) indicated that even at 165 PPFD the structure of photosystem (PS) II reaction centerwas severely damaged. Interestingly, xan1 seedlings grown in dim light (3.5 PPFD) revealed a severely altered chloroplast ultrastructure, despite the lack of photooxidative effects on leaf pigments and the lack of alterations in the electrophoretic profiles of thylakoid proteins. Electron microscopy indicated that XAN1 product is required for normal organization and distribution of thylakoids in the stroma. Chlorophyll a fluorescence analysis pointed to xan1 incompetence to fully express and retain PS II activity, even in the absence of photooxidation.