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Zhiyong Hu, Qing Liu, Meilian Tan, Hualin Yi, and Xiuxin Deng

In this article, we describe a novel biological phenomenon that the tips of the juice sac of citrus hybrid HRB turned brown and became tough like many “brown thorns”; HRB is a triploid hybrid regenerated from the cross of diploid tangerine BDZ (Citrus reticulata Blanco cv. Huanongbendizao) with an allotetraploid somatic hybrid HR [Hamlin sweet orange (Citrus sinensis Osbeck) + rough lemon (Citrus jambhiri Lush)]. Histochemical analysis indicated that the “brown thorn” of HRB resulted from lignin deposition. The juice sac of HRB had 4.6- and 3.8-fold content of lignin of its parents tangerine and HR, respectively. Microscopy observation of the cross-sections of the juice sac verified that the cell wall of the “brown thorn” was lignified. Moreover, the cell walls near the “brown thorns” were obviously thickened. Analysis of enzyme activity indicated that the peroxidase (POD) activity of HRB was significantly higher than its parents. Quantitative real-time polymerase chain reaction analysis showed that the transcript abundance of the cinnamate 4-hydroxylase (C4H) gene in HRB was 6.5- and 3.4-fold of its parent BDZ and HR, respectively, but the POD gene transcript abundance was lower than its parent with 1/2-fold of BDZ and 1/7-fold of HR. These facts led to the conclusion that POD and C4H were the key regulating factors for lignin biosynthesis in juice sacs of the hybrid HRB. The POD gene, one member of the POD family, with transcript abundance lower than its parent indicated that this POD isoenzyme was not the regulating factor of lignin biosynthesis, and further study should be carried out to determine which POD isoenzyme is the key factor for lignin biosynthesis.

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Zhiyong Hu, Min Zhang, Qigen Wen, Jie Wei, Hualin Yi, Xiuxin Deng, and Xianghua Xu

Seedlessness is of commercial importance in citrus (Citrus L.). Seedless ‘Ougan’ mandarin (C. suavissima) was selected from a bud sport mutation that occurred in ‘Ougan’ mandarin. We analyzed their pollen viability through KI-I2 and FDA staining, and examined the anthers of wild-type (seedy) and seedless mutant ‘Ougan’ mandarin using histological and cytochemical methods to characterize the process of pollen development. No pollen fertility was detected in this mutant. Pollen abortion in anthers of the mutant occurred at the tetrad stage of microspore development, and almost all the tetrads were abnormal. The mutant had heterogeneous microspore populations, including monads, dyads, triads, tetrads, and polyads in the same microsporangium. Pollen grain number per anther of the mutant was 21.9% less than the wild type. Morphology of mature pollen grains using SEM showed that the shape of mature pollen grains from both wild type and mutant is similar, but the microsporangia of the latter contained pollen grains of more variable sizes. At the early mature pollen grain stage, abundant starch grains and lipids appeared in the wild type's pollen, but fewer amounts were observed in the mutant. Moreover, the tapetal cells of the wild type accumulated lipids, but not those of the mutant. Results indicated that the abnormal development of the microspore led to pollen abortion in the mutant, and this could be the reason for its seedlessness. However, the genetic reasons for the aberrant tetrads are not clear and are under investigation.

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Min Zhang, Xiuxin Deng, Changping Qin, Chunli Chen, Hongyan Zhang, Qing Liu, Zhiyong Hu, Linlin Guo, Wenhua Song, Yong Tan, and Shengcai Liao

‘Zaohong’ navel orange [Citrus sinensis (L.) Osbeck + C. unshiu Marc.], a new strain of citrus from a graft chimera, was discovered in China. It was diploid and arose at the junction where a ‘Robertson’ navel orange scion was top-worked onto a Satsuma mandarin (C. unshiu). Some characteristics determined by the L1 cell layer, such as juice sacs of fruit and stoma length, were similar to those of Satsuma mandarin, while others, including leaf index, fruit shape, navel, and color and aroma of the rind, were determined by the L2 cell layer, were similar to ‘Robertson’ navel orange. High-performance liquid chromatography analysis of the carotenoid extracts of the flesh of ‘Zaohong’ navel orange indicated that it had the carotenoids profile of Satsuma mandarin with β-cryptoxanthin as the predominant component in the juice sacs in mature fruit. Simple sequence repeats (SSR) and chloroplast simple sequence repeats (cpSSR) analysis showed that both nuclear and chloroplast genomes of ‘Zaohong’ navel orange were composed of both donor plants. On the basis of these facts, ‘Zaohong’ navel orange was found to be a periclinal chimera consisting of L1 derived from Satsuma mandarin and L2/L3 from ‘Robertson’ navel orange. It combined the valuable traits of both donor plants, matured ≈1 month earlier than the present navel orange cultivars, and therefore had good potential in citrus fresh market.