Mature seeds of most of the higher plants harbor dormant embryos and go through the complex process of germination under favorable environmental conditions. The germination process involves dynamic physiological, cellular and metabolic events that are controlled by the interplay of several gene products and different phytohormones. The small non-coding RNAs comprise key regulatory modules in the process of seed dormancy and germination. Recent studies have implicated the small RNAs in plant growth in correlation with various plant physiological processes including hormone signaling and stress response. In this review we provide a brief overview of the regulation of seed germination or dormancy while emphasizing on the current understanding of the role of small RNAs in this regard. We have also highlighted specific examples of stress responsive small RNAs in seed germination and discussed their future potential.

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Seed germination paves the way for the dormant embryo to establish itself as a new plant marking the first critical step in postembryonic plant growth and development. Germination starts with the uptake of water (imbibition), followed by induction of transcription, translation, energy metabolism, and cell division processes. Although small RNAs have been implicated in many developmental processes, their role during seed germination stages and conditions remained elusive. Here we show that seed germination conditions, like imbibition and temperature, dynamically regulate the expression of many developmentally important miRNAs and their targets. We have identified 58 miRNAs belonging to 30 different families at different seed germination conditions. Amongst these, 15 miRNAs and their targets were significantly differentially expressed in Arabidopsis seeds in dry and 12 h, 24 h and 48 h of imbibition. Interestingly, differential expression of miR390, which targets trans-acting siRNA locus (TAS3) derived transcripts, resulted in alteration of tasiR-ARF mediated regulation of expression of target AUXIN RESPONSE FACTORs (ARF2/3/4). Our results suggest that the dynamic expression of several miRNAs, their targets, and a crosstalk between miRNA and ta-siRNA pathways contribute to the regulation of seed germination in Arabidopsis thaliana.

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We studied the floral ecology, floral visitors and breeding system of Citrus × limon in West Bengal, India. We calculated a coefficient of pollination deficit (D) and also estimated the values of 'relative pollinator service (RPS)' to determine primary polli nators of the plant species. The cultivar is fully self-compatible and produces protandric hermaphrodite flowers and male flowers with considerable amount of floral rewards. Several insects like honey bees, solitary bees, carpenter bees, flies and butterflies visited the flowers. Among those, primary pollinators were Halictus sp. and Nomia sp., and important secondary pollinators were Apis dorsata and Xylocopa fenestrata. Considering the visitors' group, the cultivar is principally pollinated by solitary bees. Besides diverse floral visitors, the cultivar showed medium pollination deficit (D = 0.49) and resulting in low fruit-set. Furthermore, premature fruit abortion is also high in all pollination treatments which lead to a low fruit-set of this lemon variety in West Bengal.

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